Astronomy From Ancient Times To 1975

Compiled from articles by Joe Walsh, Roy Gooding, Roy Adams and Mike Harlow in the OASI Newsletter

Throughout history Mankind has been interested in the appearance and movement of objects in the sky: the Sun by day, the Moon, stars and planets by night. All cultures have held beliefs about the sky and some gained empirical knowledge of the sky through patient observation, for example knowledge of the patterns with which particular objects appear in the heavens. Some early civilisations regarded the Sun and Moon as gods. All advanced civilisations needed to develop an accurate calendar to determine when to hold religious festivals and to regulate agriculture and trade, and this provided one of the first important practical benefits of studying the sky.

Very early in the history of astronomy, pure science was linked with astrology; indeed, in early times, astronomy and astrology were so intertwined that it is impossible to separate them. Copernicus was one of the first to adopt a purely scientific approach to the subject. Later Kepler and Newton enhanced his approach, and astronomy and astrology were at last separated. The development of the telescope in the late sixteenth century opened a vast potential for new astronomical discoveries, and subsequent inventions extended this enormously. Scientists determined the distances of the stars and their chemical and physical properties and determined the overall distribution of stars, the shape of the galaxy and our position within it. The work of Hubble and Baade revealed the existence of countless millions of other galaxies, of all shapes and sizes.

Since the end of the Second World War, many new branches of astronomy have been developed. Radio astronomy has provided a completely new outlook on the universe, and together with gamma-ray, X-ray, infra-red and ultra-violet observations has provided vast quantities of information on all manner of celestial objects.

More recently, quasars, pulsars and Active Galactic Nuclei (AGN) have been discovered. Such objects radiate vast amounts of energy by mechanisms which are not yet understood.

This page provides an overview of key elements of the development of astronomy from ancient times up to 1975.

Assyrian Astronomy

Historians have learned about Assyrian astronomy mainly by studying thousands of fragments of clay tablets dug up around the plains of Mesopotamia in the Middle East. The tablets also provided information on on the customs, business life, culture and religion of the Assyrians. The majority of the tablets are now in the British Museum.

During the Assyrian period, around 1000 BC, astronomy was, as with all early civilisations, largely astrological in nature and all movements in the sky were believed to have significance for individual people, and in particular for kings and empires. For this reason, the Assyrians studied the motions of the Sun, Moon and planets very carefully, looking for omens, good or bad. The Assyrians regarded the Sun, Moon and planets as gods. The priests were therefore the people who took most interest in the sky. Of all objects in the sky, the planets were the most inexplicable: their motions took them all around a band of the sky, and they would change direction, approach and recede from one another and change brightness. The Assyrians named the heavenly bodies as follows:

The Assyrians considered eclipses important omens. They predicted lunar eclipses with reasonable accuracy, and accounted for effects like the invisibility of a lunar eclipse occurring in daylight.

Babylonian Astronomy

Babylonian astronomy came to prominence after the Assyrian Empire began to weaken through wars. The Babylonians were the first to undertake astronomy as a science. Many of the names of the groupings of stars and constellations used today originated with the Babylonians, for example Taurus, Aquila and Leo. However, the boundaries of the constellations known to the Babylonians were different from those in use today. In Babylonian times, many bright stars were associated with days of the year.

Through very careful observation, Babylonian astronomers constructed accurate calendars, based on two systems: an eight year cycle and a 19 year cycle. In each system, there were some years with 12 months and some with an extra 13th month. In the eight year cycle, the 13th month occurred at intervals of three, three and two years. In the 19 year cycle, the 13th month was inserted at intervals of three, three, two, three, three, three and two years. Writings discovered by archaeologists show that the Babylonians used the eight year system early during their civilisation, around 530 BC. A century later, around 430 BC, the 19 year system was in use, presumably because it was more accurate in regard of the motions of the Sun and Moon. It is not known how many days the Babylonians assigned to each month.

Babylonian astronomers collected a large amount of data about the positions and motions of the planets and Moon. From this, they were able to compile an ephemeris for each planet and the Moon, and were able to predict positions of the bodies and rise and set times over current, previous and future time periods. The Babylonians were able to predict both lunar and solar eclipses with great accuracy. The Babylonians showed far greater understanding of the theory of eclipse predictions than did the Assyrians.

For both the Assyrians and the Babylonians, the basis of predicting eclipses was understanding the reasons for the interval between successive eclipses. For an eclipse to occur, the Moon has to be in a certain position in its orbit. A new series of eclipses occurs at intervals of either 41 or 47 months. Each series of eclipses consists of two or three total eclipses in the middle, with one or two partial eclipses before and after. Each series is separated from the last by a period of two or three times six months minus one month without eclipses. The Moon's orbital plane crosses the Earth's equatorial plane at a slightly different point in the Moon's orbit each month. After five series of eclipses, the Moon's orbital plane crosses the Earth's equatorial plane at approximately the same point once more. Predictions of eclipses thus run in series of fives, contained within series of 18 years.

The Babylonians certainly knew about the 18 year series, but it has not been proved that the Assyrians understood it. The Babylonians formulated a table which they used to make eclipse predictions for previous, current and future times. The British Museum holds a fragment of this table. Archaeologists Stassmaier and Epping studied the table, and named it the saros canon.

Egyptian Astronomy

Very little is known about Egyptian astronomy. The Egyptians left no trace of development of astronomy as a science. The Egyptians had sun dials and water clocks to tell the time, so perhaps they had little need to develop astronomy for this purpose. The only significant astronomical sign which the Egyptians relied upon was the rising of Sirius at a particular point each year, signifying the coming of the Nile floods upon which Egyptian agriculture depended.

Chinese Astronomy

Astronomy in early China, as in many other countries, started from a need to measure time. The beginning of seasons was denoted by certain constellations being in known positions at sunset. For example, midsummer was when Antares was due South at sunset and Winter began when the Great Bear's tail pointed downwards.

Astrology played a major part in astronomy in early China, and as with the Assyrians, priests were the only people to study the sky in detail. The priests constantly looked for omens in the sky which concerned the emperor and the state, and they assigned names to the brighter stars and groupings of stars which were connected with the emperor and state, for example Palace, Emperor and names of members of the royal household.

Early Chinese astronomy exhibited no real understanding of the motions of the planets and Moon. However, the early Chinese astronomers compiled a star chart, fragments of which have survived today. The astronomer Shih-Shen compiled a list of stars containing descriptions of 122 constellations and 809 stars; this pre-dated the catalogue of the Greek, Hipparchus.

At the time when the Babylonians and Greeks were making progress in understanding the motions of the Moon, Sun and planets, the direction of Chinese astronomy changed and deepened in its understanding of the motions of heavenly bodies. This lead to a belief that there were links between the two cultures. Chinese astronomy made major progress during the first centuries AD. China had previously been isolated from the Middle East and Mediterranean, but during the Roman conquest of Central Asia, in the first century AD there were scientific contacts between the Roman Empire, Iran and Greece by way of India. By the middle of the first century AD, Chinese astronomers had worked out the length of the year to within 11 minutes, and the length of the synodic month to within 23 seconds. The Chinese astronomers took a different approach to the Assyrians and Babylonians to the prediction of eclipses.

Even though early Chinese astronomers did achieve high standards of astronomical knowledge, over a period of many centuries their astronomical calendars became inaccurate. The Chinese did not resolve this problem until the Jesuits came to China in the 16th Century and were commissioned to construct a more accurate calendar.

The astronomical data compiled by the early Chinese astronomers is of great value as no other records exist from around this period. For example, the early Chinese astronomers recorded the appearance of comets in the years 989, 1066, 1145 and 1301. These apparitions are now known to be early appearances of Halley's Comet, although the Chinese astronomers did not know that it was the same comet on each occasion.

Arguably the most important observation that the Chinese made was of the appearance of a supernova in 1054. During June 1054 the object was visible in the daytime. The remnant of this supernova is the famous Crab Nebula in Taurus, one of the most interesting objects for observation, by both amateurs and professionals alike.

Greek Astronomy

Fragmentary writings have survived from the classical Greek period, and it is possible to piece together the main ideas of the time on the subject of astronomy. All the Greek philosophers contributed to ideas on astronomy. Each philosopher had his own idea on the motions of the planets, Moon and stars. Many philosophers tried to have their ideas accepted by refuting or adapting previous writings by earlier philosophers. This resulted in the original ideas and opinions of many philosophers being either scanty or contradictory. However, the works of the Greek philosophers Plato and Aristotle have been preserved well, and both were highly esteemed in later epochs.

Astronomy in Greece proceeded, as with other cultures, from the need for accurate timekeeping. Greek civilisation developed a high standard of mathematics, particularly in the field of geometry, and an accurate calendar. In common with the Babylonians, the Greeks discovered the inequality of the seasons, in that the Sun moves at different speeds at different times of the year, resulting in the periods between solstices and equinoxes differing by several days.

The Greeks were one of the most mathematically-orientated cultures of ancient times. The Greeks were the first to use mathematics to describe the motions of the planets. However, their passion for mathematics overshadowed observational astronomy to the extent that most Greek astronomers were, in fact, much better mathematicians than astronomers, and Greek notions of planetary motion were rather primitive. The famous mathematician Eudoxus was the first to formulate a mathematical model of planetary motion. In simple terms, Eudoxus, in keeping with the ideas current at the time, fixed every planet to a sphere which rotated around the Earth. In order to explain the irregular motions of the planets he proposed the use of more spheres, all of which rotated in a regular manner around a common centre. This theory was reasonably accurate for Jupiter and Saturn, but not for Mars, Venus or Mercury. However, Eudoxus did not have available sufficiently many accurate observations of the motions of the planets to realise this fact. Eudoxus' theory is known as the homocentric spheres of Eudoxus. Calippus added another sphere to the model for the planets Mercury, Venus and Mars. Two more sphere for the Sun and Moon were subsequently added.

There were many conflicting ideas in Greek philosophy about the interpretation of planetary motions and with earlier models, the diurnal motion of the Earth and its orbit around the Sun were unknown. The most original ideas came from Heraclides and Aristarchus. Using the Greek mathematics of geometry, Aristarchus attempted to find the distance of the Sun from the Earth. Aristarchus' estimate was very inaccurate: he found the Sun-Earth distance to be 19 times the Moon-Earth distance (the accepted modern value is 390 times). Aristarchus then estimated the relative volumes of the Sun and the Earth: his estimate was a factor between 254 and 368 (the accepted modern value is 1.3 million).

Aristarchus along with Heraclides proposed a heliocentric system with all the planets orbiting the Sun in circular orbits. However, the minds of other astronomers and philosophers of the period were unable to recognise that such a system would account for many of the difficult aspects of planetary motion, and this system was then abandoned for some seventeen centuries before coming to prominence once more.

During the second century BC, Hipparcus invented the epicyclic system, which utilised a set of circular motions to represent the paths of the heavenly bodies in the sky. Although incorrect, the epicyclic system could represent the motions of the planets and Moon to within an error of less than one arcmin. Ptolemy, one of the greatest astronomers of ancient times, who lived in Alexandria during the time of the Roman emperor Hadrian in the second century AD, favoured the epicyclic system of Hipparcus to the heliocentric system of Aristarchus. Ptolemy refined the epicyclic system, and ever since it has been known by his name. Subsequently, for over 16 centuries the epicyclic system was accepted as an explanation of the heavens - the accuracy of the system, combined with its positioning of the Earth at the centre of the cosmos, were the main reasons why the system gained such widespread and long-lived acceptance. No one questioned that the Earth was stationary and all the other heavenly bodies revolved around it. During the period when the Ptolemaic system was accepted, the Church had sufficient power and influence to condemn any person who dissented against it and anyone expressing belief in a heliocentric theory was liable to be burned at the stake as a heretic. (Note also that Ptolemy was the first observational astronomer as the term is understood in the modern sense; he compiled the most accurate star catalogue of the time.)

Nicolaus Copernicus, 1473 - 1543

Nicolaus Copernicus was born in Thorn, on the river Vistula, in Poland, on 19 February 1473. His father was a merchant who had moved from Crakow in 1458. When Copernicus was ten, his father died and his uncle, Lucas Watzelode, took charge of his upbringing. In 1849 Watzelode became bishop of the diocese of Ermland.

In 1491 Copernicus entered the University of Crakow, renowned for its flourishing schools of mathematics and astronomy. In Crakow, Copernicus first became acquainted with astronomy, through astrology. In 1496 he finished his course at Crakow and moved to Italy to study canon law and medicine. In 1500 he started lecturing on astronomy at Rome. In 1503 he took his doctoral degree in canon law and in 1505 he went to stay at Heilsburg as companion and physician to his uncle, Watzelode, who was still bishop of Ermland.

While at university, Copernicus studied Greek, among other things. This enabled him to read the works of the Greek philosophers in the original text. At the end of the 15th and beginning of the 16th centuries, there was a great quest for knowledge about the accuracy of the works of the Greek philosophers. One result of this quest was the discovery of many inaccuracies in the Alfonsine Tables of the positions of the heavenly bodies, which were based on a geocentric model of the universe. Copernicus, on noticing the inaccuracies of the Alfonsine Tables, began to formulate a new theory of the position of the Sun, Moon and planets. Copernicus' work was prompted by some ideas from the Greek philosophers: Nicetas, who according to Cicero, perceived the motion of the Earth; Philoleus who perceived that the Earth made a daily orbit around a central fire and Heraclides who, with Euphantus, gave the Earth a rotation about its axis once a day. Copernicus formulated several hypotheses:

  1. The objects in the Solar System have no single centre.

  2. The Earth is not the centre of the Universe but only the centre of the orbit of the Moon.

  3. All objects orbit the centre of the Sun.

  4. The Earth rotates once a day on its axis.

  5. Retrograde motion of a planet in its orbit is due to the motion of the Earth around the Sun.

The few years from 1505 at Heilsburg were very difficult, as Watzelode was a very firm administrator and Copernicus had many administrative duties to perform. However, in 1512 Watzelode died and Copernicus went to live in Frauenburg, becoming a canon to Frauenburg Cathedral. Copernicus stayed at Frauenburg for about 30 years, living all this time in a few rooms in one of the turrets in the wall that surrounded the cathedral. The turret also served Copernicus as an observatory, While in Frauenburg, in 1514 Copernicus circulated a manuscript summarising his new ideas among his friends. For the next several years Copernicus made observations of the positions of the planets and stars, and used them to develop his theories of astronomy.

It took Copernicus some 36 years to fully develop a new theory to replace the Ptolemaic system. He finally evolved a theory whereby the Sun and background stars were fixed, the planets including the Earth orbited the Sun, and the Moon orbited the Earth. Copernicus assumed that all the orbits were circular, and in order to make the theory fit observations accurately, he had to introduce a system of deferents and epicycles, similar to those used in the Ptolemaic system. Copernicus worked on a book explaining his ideas and conclusions, De Revolutionibus Orbium Coelestium. By about 1529, Copernicus finalised the manuscript, but realising that the work would create a storm he did not publish it.

In 1539 Georg Joachim, known as Rheticus, a young professor from Wittenburg, went to Frauenburg to find out more about Copernicus' new theory. He was so impressed that he wrote an account of the theory and had it published in 1540; the account was known as Narratio Prima. It received such a good reception that Copernicus agreed to publish De Revolutionibus. The publisher was Andreas Osiandet from Nerenburg. He also realised what trouble publication could cause and to defuse any potential difficulties inserted a preface at the beginning of the publication (without Copernicus knowing) to the effect that the theories were merely hypothetical.

The completed publication reached Nicolaus on 22 May 1543 as he lay in a coma, apparently suffering from a stroke, just hours before he died. He never read the publication. At first, De Revolutionibus caused little excitement, but later, when its full implications were appreciated,  the Church, as predicted, gave a strong and prolonged hostile reaction.

However, Copernicus' theory marked the start of modern theories of the Solar System. Astronomers realised that Copernicus had solved the major problem of the movement of the planets, and there was a resurgence in observing in order to test the new heliocentric theory. Tycho Brahe was one of the astronomers who made many observations of the positions of the stars and planets. Kepler later used Brahe's detailed observations in his formulation of his famous planetary laws of motion. It was Galileo's telescopic observations of Venus which finally confirmed the heliocentric nature of the Solar System, and later Newton provided the theoretical underpinning of Kepler's laws.

Tycho Brahe, 1546 - 1601

Tycho Brahe was born in Denmark in 1546, the son of a Danish nobleman. His family sent him to study law at Leipzig University, but Brahe was an enthusiastic astronomer and made secret nightly observations and studies of the science. Brahe was convinced of the truth of astrological predictions and would often compile horoscopes. He believed that the stars and planets had influences over events on Earth which were dominated by eternal laws, and in order to produce a reliable science of astrology a better understanding of the motions of the stars and planets was required: Brahe decided to undertake the necessary research.

On 11 November 1572, the appearance of a bright 'new star' or nova in the constellation Cassiopeia caught Brahe's attention, and stirred the interest of a great many people. It proved that the old idea that the heavens were unchanging was not true. Brahe immediately began observing the star and took many measurements of its brightness in comparison to that of other nearby stars in Cassiopeia. After a short period the nova began to fade, and after about two years it had disappeared from sight. The nova was, in fact, a supernova similar to that seen by the Chinese in 1054.

The general interest in the supernova gave many people the idea of building an observatory. King Frederic of Denmark offered Brahe the island of Hven, near Copenhagen, as the sight for an observatory. Brahe built an observatory and designed his own instruments for use in it. His designs were a considerable improvement on previous designs. His instruments ranged from small, hand-held quadrants to large devices with radii in excess of two metres which were able to measure positions to within 10 arcsec.

Brahe used the equipment at his observatory to compile the most accurate star catalogue yet produced. It superseded the catalogues of Hipparcus and Ptolemy that had lasted well over 1000 years. Brahe was not convinced by Copernicus' heliocentric model of the Solar System, and with his numerous measurements of the positions of the planets, he hoped to prove Copernicus wrong. Ironically, it was the accuracy of his observations that, in the hands of Kepler, revealed the truth of the heliocentric approach and showed the true structure of the Solar System.

Johannes Kepler, 1571 - 1630

Johannes Kepler was born at Weil-der-Stadt, Wurttemburg on 27 December 1571. He entered university at Tubingen where he studied mathematics, astronomy and theology. While a student at Tubingen, he heard a lecture on the heliocentric approach to the Solar System given by Maestlin (1550 - 1631). In 1594 he was appointed Professor of Mathematics at Gratz. In 1596, Kepler had his ideas on planetary orbits and distances published, his explanation involving superimposing five regular polyhedra within the planetary orbits. Kepler assumed that each planetary orbit could be represented as a sphere: between each pair of successive spheres he placed one of the regular polyhedra with its edges touching the exterior sphere and its faces tangential to the interior sphere. The ratios of the inner and outer spheres gave the relative distances of the planetary orbits. This model, although in vogue at the time, has no physical basis and is now discredited.

In about 1594 Kepler began writing to fellow astronomer Tycho Brahe. In 1599 Brahe went to work in Prague, having been forced out of Denmark because of differences with the Danish court and noblemen. In 1600, Kepler, who by this time was working with Brahe in Prague, was dismissed from his post at Gratz due to Catholic persecution of Lutherans, and he therefore had to stay with Brahe. In 1601 Brahe and Kepler met with Emperor Rudolph II, and Kepler became acknowledged as Brahe's assistant. Brahe died in 1601 and Kepler was appointed Mathematician to Emperor Rudolph II in his place. But Kepler received no salary for this post and lived a life of poverty even after gaining other appointments at Lintz and and as Astrologer to Wallenstein.

Kepler's most important contribution to astronomy was his discovery of the three laws of planetary motion, which he developed on the basis of Brahe's voluminous and accurate observations:

  1. Every planet moves in an ellipse with the Sun at one focus. This law ousted the ancient belief that the orbits of the planets were circular.

  2. The radius vector joining the planet to the Sun sweeps out equal areas in equal times.

  3. The squares of the siderial periods of two planets are proportional to the cube of their mean distances from the Sun.

Kepler published the first two laws in 1609 in an article in the book Astronomia Nova - Commentaries on the Motions of Mars. He published the third law ten years later in a publication called Harmonices Mundi.

Kepler wrote other books. In 1621 he published The Epitome of the Copernican Astronomer. In this book he detailed the movements of the planets and Moon in great depth. His last book, published in 1627, The Rudolphine Tables (named after Kepler's patron, Emperor Rudolf II), gave the positions of the Sun, planets and Moon based on his work and that of Tycho Brahe. So accurate was the work that it provided the basis of standard astronomical tables for the next one hundred years.

In October 1604 a supernova erupted in the constellation Ophiuchus. Astronomers in Europe, China and Korea studied it. In Europe, Kepler and Fabricius identified its position so accurately that in 1943 it was possible to identify a small patch of nebulosity with the original supernova remnant. The supernova came to be called Kepler's Star in his honour.

Kepler's laws of planetary motion gave a correct description of planetary motion but the physical description of planetary motion was not provided until Isaac Newton's discovery of gravity, which he published in his book Philosophiae Naturalis Principia Mathematica in 1687.

Galileo Galilei, 1564 - 1642

Galileo Galilei was born in Pisa, Italy on 15 February 1564 (the date was actually recorded at the time as 1563, since New Year's Day fell on 25 March, the feast of the Annunciation). He did well at school studying Medicine and Aristotelian Philosophy and in September 1581 he entered the University of Pisa. It was while at Pisa that he made the important discovery that oscillations of a pendulum are of constant period, which led him many years later to greatly improve clocks and other timekeeping equipment which relied on the pendulum.

Galileo became professor of mathematics at Pisa in 1589 at the age of 25. He propounded the novel theory that all falling bodies, heavy or light, fall to the Earth with equal velocity. Legend has it that Galileo proved the point experimentally by dropping a hundred pound ball and a one pound ball simultaneously from the top of the leaning Tower of Pisa. No contemporaneous account of the demonstration exists, but Galileo recounted the story in his old age.

In 1591 Galileo resigned his chair and moved to Florence. In the following year he was nominated Professor of Mathematics at the University of Padua. Galileo's time at Padua was a happy one, and he was very successful as a lecturer, attracting students from all over Europe. Galileo made several discoveries and inventions during his time at Padua, among them a type of thermometer, the proportional or sector compass, and most famously of all great improvements to the design of the refracting telescope.

At the end of the seventeenth century, the spectacle maker, Hans Lippershey of Middleburg, found that two lenses held in line had the ability to provide a magnified view of distant objects. News of the discovery soon spread across Europe. When Galileo heard about the telescope, he proceeded to construct small instruments for himself. In 1610, Galileo began serious astronomical investigations with his new telescopes, the most powerful of which provided a magnification of x32. Among many findings he discovered that the Moon, instead of being self illuminating and smooth, had an uneven surface marked with hills, valleys and mountains which cast shadows in the direction away from the Sun. This provided conclusive proof that the Moon shone from light reflected from the Sun. He could see the Milky Way as countless thousands of stars. He also began a series of observations of Jupiter which resulted in the discovery of the planet's four large satellites (Io, Europa, Ganymede and Callisto) on 07 January 1610. The four satellites are referred to collectively as the Galileans in his honour. Galileo discovered sunspots on the face of the Sun and by timing their progress across the face of the Sun he estimated that the rotation period of the Sun was about twenty seven days.

Through his astronomical discoveries, Galileo came to believe in the heliocentric or Copernican view that the Sun was at the centre of the Solar System. It was especially his observations of Venus, which in the telescope showed phases like the Moon, which confirmed this belief. In the Ptolomaic or geocentric view it was impossible for Venus to exhibit phases.

The ecclesiastical authorities supported the geocentric view of the heavens and were enraged by Galileo's support for the Copernican view. On 26 February 1616, Cardinal Bellarmino, representative of Pope Paul V, admonished Galileo against further support for the Copernican view. In 1633, the ecclesiastical authorities under Pope Urban tried Galileo for his beliefs and he was forced to speak aloud on his knees an abjuration of his beliefs in front of an Ecclesiastical Tribunal. Galileo was not imprisoned, for by then he was in his late sixties and not a well man. But he had to live the remaining nine years of his life in retreat at a tiny village called Arcetri near Florence.

Galileo died in 1642 and is buried at the church of Santa Croce in Florence.

Giovanni Cassini, 1625 - 1712

Giovanni Cassini was born at Perinaldo, Italy on 08 June 1625. He was the founder of five successive generations of astronomers. Cassini studied mathematics and astronomy at a Jesuit college - his original reason for studying astronomy was said to be a desire to prove that astrology had no scientific validity. In 1650 Cassini was offered the post of Professor of Astronomy at Bologna University; he accepted the offer and held the position until 1669.

While at Bologna, Cassini's main area of study was the Solar System. In 1665 and 1666 he determined the rotation periods of Jupiter and Mars. Two years later he published a table of the motions of Jupiter's Galilean satellites: this table was later to help Roemer in his estimate of the speed of light (Roemer presented his estimate, close to the currently accepted figure, to the French Academy of Sciences in 1675). Cassini was one of the first astronomers to study the zodiacal light.

By 1669 Cassini's reputation was known throughout Europe to such an extent that Louis XIV invited him to become director of the Paris Observatory. Cassini redesigned much of the observatory and removed ornamentation that would be a hindrance to his observations. At Paris, Cassini worked with Christian Huyghens and together they made many important advances in astronomy.

Cassini's telescopes were very cumbersome to use: they were refractors that had very long focal lengths, over 30m in some cases. However, this did not appear to compromise his observational abilities. He discovered four satellites of Saturn between 1671 and 1684 and discovered a division in Saturn's ring system which still bears his name. Cassini believed that the rings were composed of large numbers of small particles, but few other astronomers of the time were in agreement. Cassini's guess as to the nature of the rings remained in doubt until 1857 when Maxwell proved that they could only be stable if they comprised a myriad of small particles.

Cassini's most important work was the determination of the parallax of Mars in 1672. Mars was observed from two locations: Paris and French Guiana. The parallax of Mars was used to calculate the distance of Mars and since the relative distances of all the planets from the Sun is known from Kepler's laws it was possible to calculate the size of the orbit of Mars.

Cassini died in 1712.

John Flamsteed, 1646 - 1719

John Flamsteed was born at Danby in Derbyshire on 19 August 1646. He suffered bad health all throughout his life. At the age of 15, his poor health caused him to leave school and led to the pursuit of his hobby, astronomy. He began to construct his own astronomical instruments, and by 1670 received attention through the publication of several papers on astronomical topics. This in turn led Flamsteed to become acquainted with Newton and to gain entrance to Cambridge University.

During the mid-seventeenth century, sea exploration and trading overseas were becoming very widespread. However, the inability to determine longitude accurately remained a severe limitation to the accuracy of navigation at sea. King Charles II established a committee, which included Flamsteed, to investigate possible methods for determining longitude at sea. Flamsteed concluded that no method would work until there was an accurate star map available, so he petitioned the King to establish an observatory for this purpose. Charles II agreed to Flamsteed's request, appointing him Astronomer Royal in 1675.

An observatory was built for Flamsteed, on a hill at Greenwich, to the design of Christopher Wren. The King imposed a limit of only £500 for the building. Money was raised by selling off military gunpowder to merchants who were able to re-treat it for less onerous use than as ordnance. Building materials were obtained from a variety of establishments that were being demolished, including wood from a gate house at the Tower, and bricks, iron and lead from a fort at Tilbury.

Unfortunately the King had not made any provision for the purchase of observing instruments so when the observatory was complete, Flamsteed had to equip it with instruments paid for with money from his own pocket assisted by whatever he could raise from friends and well-wishers. Flamsteed's annual income was only £100 and by 1683 he had to supplement this by taking private pupils in astronomy and mathematics.

Flamsteed had to supervise all the observing work himself until his father's death, when he was able to employ Abraham Sharp. Sharp was an instrument maker and a tireless worker at processing observational results. He built a large mural instrument for the observatory which speeded up the work of measuring stellar altitudes. Flamsteed was one of the first observers to use a telescope in combination with a graduated arc for measuring angles. By 1703 he had completed more than 30,000 stellar positional measurements with a greater accuracy than had been achieved before.

As Flamsteed had to provide his own instruments he regarded the results of his observations as his own property. Contemporary astronomers, notably Newton and Halley, took the different view that as Flamsteed received a salary from public funds, he should make his results public and publish them as quickly as possible. However, Flamsteed was not prepared to publish his data until it had all been corrected for observational errors and reduced to a standard form. The resulting delays caused much bad feeling. In 1708 Halley obtained several of Flamsteed's observations and published them. Flamsteed's reply was to burn as many of the publications as he could find - in total over 300!

The final version of Flamsteed's star catalogue did not appear until 1725, six years after his death. It consisted of three volumes, the second two being completed by Abraham Sharp and Joseph Crossthwait. The catalogue gave Greenwich an international reputation for precise observations that it has held ever since.

Isaac Newton, 1642 - 1726

Isaac Newton was born in 1642, the year of Galileo's death. He was the son of a Lincolnshire farmer. He went to Cambridge University in 1661 to study mathematics. He made many contributions to mathematics and science, and his main contribution to astronomy was a mathematical understanding of the force of gravity that attracts all objects in the universe.

The idea that there must be some form of attractive force between matter in the universe was introduced much earlier. Copernicus had spoken of a mutual attraction between parts of the Earth as the cause of its spherical shape, and he assumed that a similar force was present between astronomical bodies. Kepler had spoken of gravity as a force that tended to attract bodies to one another. He understood that the Moon exerted an attractive force upon the Earth which produced the tides: to Kepler this was proof that an attraction existed between the bodies.

On leaving university, Newton returned to his home village of Woolsthorpe in 1665 and started work on the laws governing the motion of objects. He eventually turned his attention to the motion of the Moon around the Earth. The word gravity had been in use for some time, meaning a mutual affection between bodies that tended to draw them together. Newton wanted to define gravity in an exact manner using mathematics. He wanted to understand how the force of gravity reduced with increasing distance between bodies. His ultimate aim was to use an understanding of gravity to make comprehensible the observational facts of lunar and planetary orbits.

Around this time, other scientists were also trying to understand lunar and planetary motions in terms of a force of gravity, but the mathematics involved proved to be the main stumbling block. Robert Hooke was one of Newton's main rivals. Hooke, like Newton, suggested that the force of gravity decreases inversely to the square of the distance between two objects. Hooke and Newton exchanged much correspondence on the subject until Newton realised that Hooke was getting near to a solution. Spurred on by the competition, Newton continued to work on the problem which he solved during the next five years.

Hooke often met with Christopher Wren and Edmund Halley to talk about astronomy. After discussing gravitation, Wren offered a valuable book as a prize to whoever found a mathematical description of the force. Hooke declared that he had a solution but was slow to produce it. Meanwhile, Halley visited Newton in Cambridge and discovered that Newton had already worked out the solution. Halley persuaded Newton to present his work to the Royal Society. Newton presented his first paper in December 1684. Through the efforts of Halley, Newton's complete works were published as a book in July 1687 after much haggling with Hooke before it appeared in print.

Newton also contributed two other important innovations to astronomy. He discovered that white light was composed of colours that became visible when the light was passed through a prism. Early refracting telescopes were hampered by chromatic aberration associated with their lenses; Newton invented the reflecting telescope which relied on a mirror rather than an objective lens and thus avoided chromatic aberration.

Edmond Halley, 1656 - 1742

Edmond Halley is known by most people nowadays for the periodic comet named after him, although he had a wide range of scientific interests and held posts varying from sea captain to Astronomer Royal. His contribution to astronomy was considerable, but he was largely overshadowed by his contemporary, Sir Isaac Newton.

There is some doubt as to Halley's exact date of birth. Some sources give it as 08 November 1656, but Halley's own account gives it as 29 October 1656. As there was no baptism certificate in his local parish near Shoreditch, the exact date has never been satisfactorily determined. Whatever the exact date, Halley was born into a financially secure home at Haggerston, just outside London. He went to school at Saint Pauls, and then went on to Queen's College, Oxford where his interest in astronomy grew. At the age of 20 and only halfway through his university degree, and influenced by John Flamsteed's Star Catalogue of the Northern Skies, Halley decided to catalogue stars in the southern hemisphere. Halley and a friend set off in November 1676 to sail to St. Helena (an island in the South Atlantic) with free passage granted by the King with the East India Company. During Halley's stay in St Helena, he observed a transit of Mercury on 07 November 1677 and catalogued the positions of some 360 stars. On returning to England in 1679 he had his observations published as the Catalogus Stellarum Australium, which contained detailed positions of 341 stars. The Catalogus received wide acclaim. Not only was it the first catalogue of the southern hemisphere stars but also the first mapping of stars compiled using a telescope. It established Halley's scientific reputation, prompted Cambridge University to award him an honorary degree, and the Royal Society to elect him a Fellow.

In 1679 Halley suggested that observations of a transit of Mercury or Venus across the Sun's disk could be used to measure the size of the Sun and the scale of the Universe. Because of the small size of Mercury and the difficulty of observing its transit across the Sun, Halley decided to concentrate on Venus. He calculated the dates of the transits of Venus and predicted two of these, one in 1761 and the other in 1769. Although he died before these dates the two transits of Venus were observed throughout the world by astronomers.

In 1684 Halley worked with Sir Isaac Newton on the laws of planetary motion. In 1685, Halley was elected Clerk to the Royal Society and started editing its journal, Philosophical Transactions. Halley asked the Royal Society if it would finance publication of Newton's Theory of Gravitation but it refused, as it had just financed the publication of a book which was not selling very well, by Francis Willoughby, entitled The History of Fishes. (In fact the Royal Society owed Halley fifty pounds salary but could not afford to pay him so it sent him fifty copies of The History of Fishes instead. Halley was renowned for having a strong sense of humour but whether he saw the joke in this matter was uncertain!) As Halley could not get persuade the Royal Society to finance the publication of Newton's work he financed the printing of the most famous scientific book of all time, Newton's Principia, himself. The book was published in 1687.

During his term as Clerk, Halley wrote many scientific papers on subjects ranging from trade winds to astronomy. Among other things Halley discovered at this time were the differences between the orbits of Jupiter and Saturn and the slow secular acceleration of the mean motion of the Moon. In 1686 Halley published an account of the trade winds and monsoons for mariners. He also investigated the salinity of the oceans and he was able to estimate fairly accurately the age of the Earth by measuring the rate at which salinity increased. 

At this time, there was considerable interest in finding a quick and reliable way of determining longitude at sea. Halley proposed a method based on an understanding of variations in the Earth's magnetic field. He contacted the Admiralty, which took a great interest in the proposal and commissioned him to the rank of captain, giving him a small ship, the Palamour, for his research. Halley's first voyage in the Palamour began in November 1698 at Portsmouth, but was cut short by a mutiny. His second trip, completed in mid-1700, was more rewarding, and covered the Atlantic as far south as the Falkland Islands. Halley also discovered that the Aurora Borealis was related to the Earth's magnetic field.

Halley was one of the first astronomers to apply Newton's laws of motion to comets. In 1705 he published Astronomiae Cometicae Synopsis which gave observations of 24 bright comets that had appeared between 1397 and 1698. He also articulated his theory that the comets of 1456, 1531, 1607 and 1682 were the same, and that the comet in question would appear again in 1758. As Halley knew that he would not be alive to see the return of the comet, he wrote in his diary: If the Comet should return according to my prediction, about the year 1758, impartial posterity will not refuse to acknowledge that this was discovered by an Englishman. The comet was first seen again on Christmas Day 1758 and it was given the name Halley's Comet in recognition of his outstanding work in astronomy. It indeed returns every 76 years.

During 1718 Halley observed Sirius, Aldebaran and Arcturus and compared their positions in the sky with Ptolomey's Star Atlas; noticing that the positions did not agree Halley thus discovered the phenomenon of stellar proper motion.

Halley also did much work in the field of geometry and in 1704 became Savilian Professor of Geometry at Oxford.

The highlight of Halley's career was his appointment to the post of Astronomer Royal in 1719, at the age of 64, succeeding Flamsteed. Halley occupied the post for 20 years. From as early as 1684, Halley had observed regular deviations of the Moon from its predicted motion, and as Astronomer Royal he continued his work on the Moon's motion. He observed the Moon through one entire saros cycle of eighteen years. Halley died before he could analyse fully his observations of the Moon but they proved to be of great value to later astronomers in calculating the complex nature of the Moon's motion. The results of his observations were published in 1749 and included tables of the Moon and planets which he had prepared as far back as 1719.

Halley died at the age of 86 in January 1742.

Charles Messier, 1730 - 1817

Charles Messier was born on 26 June 1730, in the small French town of Radonviller. Messier was born into a large family, being the tenth of twelve children. He received only a simple education, which he completed whilst still at an early age. Finding very few prospects of employment at home, Messier, at the age of 21, left to seek work in Paris.

Messier's sole skills at this time were neat handwriting and a little knowledge of draughtsmanship! He eventually found employment with Nicholas Delisle, who bad established an observatory in the Hotel du Cluny under the auspices of the French Navy. After a few years at the observatory, Messier's position earned him an official title, this being Clerk of the Depot of the Navy, which included a small salary. Though his main duties wore keeping the observatory's records, Messier found night-time observing more to his liking. He had been interested in astronomy since his early teens, especially in the discovery of comets.

In 1705, HaIley predicted that the comet which he had observed in 1682 would return in late 1758 or early 1759. Many observers started searching for the comet more than a year before its predicted return. Delisle had drawn up a chart for Messier to use in searching for the comet. Messier actually started to look for the comet nearly two years before its expected reappearance: he found the comet on 21 January 1759, but was not the first to sight it. A German astronomer, Patitzch, had seen the comet some four weeks earlier on Christmas Evening 1758.

About a year later, Delisle retired, leaving Messier the observatory and equipment for his own use. Messier devoted his work almost exclusively to the discovery and observation of comets. He discovered a comet on 03 January 1764 and was fortunate enough to discover another one two years later by a chance naked-eye observation. Between 1760 and 1798 Messier discovered 15 comets.

Although in his work Messier concentrated on comets to the almost total exclusion of all else, today he is remembered for his catalogue of nebulae and star clusters. In 1758, whilst observing a comet in Taurus, Messier discovered a nebulous object that resembled a comet. He made a note of its position for future reference. In 1760 he discovered a second nebulous object, in Aquarius. By May 1764 he decided to make a list of as many such objects as he could, so as to avoid any possible confusion with comets. Several earlier astronomers had discovered nebulous objects so Messier observed these too, noting their positions accurately. By the end of 1764 he had compiled a list of 40 objects, 22 of which he had discovered. He discovered the 41st object in January 1765. The list, with a few additions, was published in 1769. The final version of the catalogue was printed in 1781, containing 103 objects. Messier's nebulae and star clusters are simply known today with their catalogue number prefixed with the letter 'M'.

For his services to astronomy, Messier was elected to many societies and academies all over Europe, including the Royal Society, London, the Academy of Stockholm, and the Academy of Sciences, Paris.

Messier lived until he was 86, dying in April 1817.

William Herschel, 1738 - 1822

William Herschel was born in Hanover, Germany, in 1738. He was the son of a Hanoverian bandsman and his family contained many musicians. He entered the Hanoverian Foot Guards as an oboe player. In 1755, Herschel visited England with the Guards, and decided that he would like to stay there. He returned to the Continent, but after the defeat of the German army at Hastenbeck, Herschel resigned from the Guards and in 1757 he left Germany for England, where he settled in Leeds as a music teacher. He later became an organist in Halifax, and in 1766 became organist at the Octagon Chapel in Bath.

In Bath, Herschel taught music. It was while in Bath that his early interest in astronomy was reawakened when he read Ferguson's book on the subject, and soon he was devoting all his spare time to reading and studying astronomy and mathematics. Not being able to afford a telescope, Herschel made one, a Newtonian reflector, and in 1774 he began serious astronomical observations. His interest in astronomy began to occupy so much of his time that he reduced the number of students in his music classes to about seven a day.

On Tuesday 13 March 1781, while Herschel was observing the night sky with a 5.7 inch reflector of his own construction, he found an object in Taurus, near the border with Gemini, which showed a definite disk and which he considered to be either a nebulous star or perhaps a comet. On observing the object four nights later, he commented: I looked for the Comet or Nebulous Star and found that it is a Comet for it has changed its place. Herschel continued observing the object until he had captured sufficiently many positions to permit accurate calculation of its orbit. During April 1781 he announced his discovery to the Royal Society. The Astronomer Royal at the time was Nevil Maskelyne who, after observing the object for some time concluded that it was behaving more like a planet than a comet.

When sufficiently many measurements of the position of the object were available, three continental mathematicians, Simon Laplace and Jean Bochart de Sarron in France and Anders Lexell in Russia, calculated the orbit of the object. They determined that the object had an almost circular orbit at a distance approximately double that of Saturn from the Sun. At first Herschel called the new object Georgium Sidus in honour of George III, but today we know it as Uranus, named after a classical god. Subsequent analysis of historical records showed that in fact Uranus had been observed before its true nature was known. John Flamsteed had recorded it as a star of magnitude six, and during 1768-79, Lemonnier had made eight observations of it. Tobias Meyer had recorded its position accurately at the beginning of 1756.

Herschel's discovery of Uranus brought his name to the attention of other astronomers. George III, who was himself interested in astronomy and had had an observatory built at Kew, asked to meet Herschel and requested the latter to bring his telescope to Greenwich. The instrument with which Herschel discovered Uranus aroused much interest among other astronomers when it was set up at Greenwich. It had a much greater magnification than other telescopes, and everyone familiar with telescopes thought that it was the highest quality instrument that they had ever seen. Herschel was elected a Fellow of the Royal Society and awarded the Copley Medal for his discovery. When Herschel took the telescope to Windsor for examination by the Royal Family, George III appointed him King's Astronomer and made him independent of his musical profession by giving him an annual salary of £250 and an observatory site at Datchet. The salary enabled Herschel to live quite comfortably: he gave up music altogether and moved to Slough where he became a full time astronomer and telescope maker. In 1788 he married Mary Pitt, a lady of considerable wealth, enabling him to devote his life to astronomy without further worries about his income.

Herschel was a prodigious maker of astronomical telescopes. He constructed approximately 400 mirrors of various sizes and sold some 69 telescopes. One of his mirrors was the largest constructed at the time. After beginning his full-time astronomical work at Datchet, Herschel succeeded in obtaining £4000 from the privy purse for construction of his observatory. He had a telescope built with a mirror 48 inches in diameter. The telescope was constructed at Slough, and though it was used for the first two years after completion, it proved to be too cumbersome for constant use and gradually became obsolete.

Using smaller instruments, Herschel made many important discoveries through the next 30 years. He produced a catalogue of approximately 2500 nebulae and similar objects and made many series of measurements on double and variable stars. After his discovery of Uranus, his most important work was on the distribution of stars in, and shape of, the galaxy. For this he undertook star counts in approximately 3400 selected areas of the sky. Herschel thought that the galaxy was shaped like a rectangular box that was split open at one end.  From a point (the Earth) within this box, the stars would appear as a luminous band across the sky, the Milky Way.

In 1787 Herschel found two of the moons of Uranus, Oberon and Titania, and in 1789 he discovered two moons of Saturn, Enceladus and Mimas. The huge crater which dominates the face of Mimas is named after Herschel. He also discovered infra red radiation using a thermometer held at the red end of a spectrum produced by a prism. In 1816, five year before he died, he was knighted.

After Herschel died in 1822, his sister Caroline Lucretia (1750-1848), who had for several years before been his assistant, carried on his work. She constructed a catalogue of star clusters and nebula that Herschel had discovered. Herschel was also outlived by his son, Sir John Herschel (1792-1871), who went on developing and updating his father's catalogue work. In time, John Herschel went on to discover more clusters and nebula than his father had done, doing much of his work in the Southern Hemisphere.

Johann Bode, 1748 - 1826

Johann Bode was born in Hamburg on 19 January 1747. He became interested in astronomy at an early age. Throughout his life Bode wrote many books on astronomy, the first being published in 1766 when he was aged only 19.

Between 1774 and 1779 Bode discovered several nebulae of various types. At the end of December 1774, he found two nebulae in Ursa Major, now known as the galaxies M81 and M82, though at the time of discovery their true nature was unknown. He discovered more objects in 1775 (including M53) and 1777 (including M92). Bode published a catalogue of some 75 objects during 1777. However, many of the entries were asterisms and non-existent objects obtained from early catalogues compiled by Hevelius and others. Only about 50 of the entries were in fact nebulae or star clusters, several of which had positional errors. At around this time, Messier used Bode's catalogue and those of early observers to compile the most comprehensive and accurate catalogue of nebulae and star clusters yet achieved. The final version of Messier's catalogue appeared in 1784.

In 1781 William Herschel discovered the planet Uranus. After the discovery, Herschel wanted to name the planet Georgium Sidus in honour of George III. It was Bode, who became greatly interested in the new discovery, who proposed the name finally adopted. During the subsequent months Bode started a search for any earlier observations of the object. Tobias Mayer in 1756 had observed Uranus and had unwittingly included it in his star catalogue of 1775. In 1785 Bode found that the earliest observation of the planet had been made in 1690 by Flamsteed. These early observations of Uranus helped in calculating the planet's orbit accurately.

Bode is primarily known for his law of relative planetary distances from the Sun. This law is now of only historical interest, its importance being lost after the discovery of Neptune. The law was originally proposed by Johann Titius, who published it in 1772. Bode popularised it in his book, The Knowledge of the Starry Heavens, and he received the most credit for it.

In 1786 Bode became Director of the Berlin Observatory. He held this post for nearly 40 years, retiring in 1825. During 1801 he published a comprehensive star atlas with the title Uranographia. This atlas proved popular and contained many new constellations, none of which however were ever officially adopted. The only remnant of Bode's new constellations today is the Quadrantid Meteors of early January. The shower radiant is in the northern part of Bootes: Bode had given this area the name of Quadrans Muralis.

Bode died on 23 November 1826.

Friedrich Bessel, 1784 - 1846

Friedrich Bessel was born on 22 July 1784, in Prussia. He started working life as an accountant. Bessel was an entirely self-taught astronomer. His first significant astronomical achievement came in 1804 when he recalculated the orbit of Halley's Comet and sent his conclusions to Olbers. Olbers was impressed with Bessel's work and obtained a post for him at an observatory.

During the next six years, Bessel gained much fame from his work, both in astronomical and court circles. Amongst his admirers was King Frederick William III of Prussia, who offered him a position in which he was in control of construction of a new observatory at Konigsberg. After the observatory was completed, Bessel remained director until his death.

Soon after the observatory at Konigsberg became operational, Bessel began to make accurate positional measurements of stars for inclusion in a new star catalogue. The completed catalogue was an extension and revision of an early one compiled by James Bradley. On its completion in 1818, Bessel had recorded some 63,000 accurate star positions. His measurements were precise enough to reveal irregularities in the proper motions of Sirius and Procyon, from which he surmised that they must each have an object in orbit around them. This surmise proved correct and in the second half of the nineteenth century, the companion stars were discovered by Clark in 1862 and Schaeberle in 1892 respectively. Bessel is best known today for being the first astronomer to determine a star's distance from parallax measurements. He chose a star that had a large proper motion and and hence was likely to be relatively close to Earth. He reported his results in 1838, quoting a parallax of 0.31 arcsec for the star 61 Cygnii. (The modern, accepted value is 0.3 arcsec.) However Bessel was not alone in measuring stellar parallax: Struve and Henderson were also attempting measurements, independently of each other.

Towards the end of his life Bessel started researching the anomalous motion of Uranus. He calculated the masses of Jupiter and Saturn with greater accuracy than had been achieved previously, and was thereby able to eliminate their gravitational influence as the cause of the irregularities of Uranus.

Bessel died in March 1846, some six months before Neptune was discovered.

Caroline Herschel, 1750 - 1848

Caroline Herschel was born on 16 March 1750, eleven years after her more famous brother William. William came to England to settle in 1757. In England, he took up various musical posts, as organist and teacher of repute, and in 1772 he was able to bring Caroline to England from the family home in Hanover. Caroline was probably appreciative of this move as at home she had had to run the family household alone!

During Caroline's first few years in England, she participated in her brother's musical career, having been trained as a concert singer. However, William had much wider interests than music, and his predominant hobby was astronomy, especially the construction of telescopes. Caroline enthusiastically joined him in this interest. After many failures the Herschels became expert grinders of lenses and mirrors, producing ultimately the best telescopes of the day. When grinding some of the larger mirrors, William would remain at work for up to 16 hours; during these lengthy sessions, Caroline would physically feed him and occasionally would read aloud from books such as Don Quixote and Arabian Nights.

In 1782 William was able to give up his music career, taking up astronomy full time, having received a pension from King George III. William undertook systematic sweeps of the sky, being greatly assisted by Caroline, who acted among her many other duties as note-taker. If Caroline had not been willing to help her brother as much as she did, William would probably never have achieved the immense number of observations and discoveries that he achieved.

In order to give Caroline more independence for her own astronomical research, William built her a telescope, but she was able to use it only when William did not require her assistance. In 1786 William went on a visit to Hanover. This gave Caroline a chance to do come sky sweeps of her own. On 01 August 1786, she discovered a comet; this discovery established her place in the astronomical community. Between 1788 and 1797 she discovered an additional seven comets. Unfortunately, three of the comets ware either simultaneously or previously discovered by other observers. The sixth and seventh were found by Messier and Mechain respectively. Caroline's eighth comet in 1797 was discovered by Stephen Lee on the same night.

One of Caroline's most important contributions to astronomy was the indexing of the star catalogue compiled by the first Astronomer Royal, John Flamsteed, together with a list of omissions. This work was later published. Caroline returned to Hanover after her brother's death in 1822. In Hanover she commenced to prepare a catalogue of nebulae and star clusters discovered by William during his sky sweeps. The catalogue was never published but was of use to her nephew, John, in his astronomical researches. The Royal Astronomical Society awarded Caroline its Gold Medal in recognition of the work involved in compiling the catalogue. While in Hanover, Caroline also took a great interest in the astronomical career of her nephew John.

Caroline Herschel lived to the age of 98, dying in 1848.

Friedrich Struve, 1793 - 1864

Friedrich Struve was born on 05 April 1793, in Altona, Germany. In 1808, at the age of 15, Struve was forced to decide whether to stay in Germany and risk being conscripted into Napoleon's army, which was then in occupation, or to flee the country. Struve decided on the latter course, leaving Germany to stay for a short time in Denmark before moving to Russia where he settled for the remainder of his life.

Once in Russia, Struve enrolled at the University of Dorpat (now known as Tartu). In 1815 he became the director of the Dorpat Observatory. This observatory was very well equipped for the period. In 1824, a ten-inch refractor made by Fraunhofer was installed at Dorpat. It was equatorially mounted and driven by one of the first clock-drives ever used. When the Fraunhofer telescope was installed, Struve commenced the research for which he became famous, the study of double stars.

Struve started a complete survey of the sky, as far south as -15° declination. At the end of his survey he had catalogued about 120,000 stars including some 2200 doubles. He published the catalogue in 1827. During the years 1825-1827, Struve constructed a travelling wire micrometer. He then used this instrument to measure accurately the positions of the various components of the double and multiple stars which he discovered. After publication of his first catalogue, Struve wrote two books on double and multiple stars, published in 1837 and 1852. The 1837 book included additional double and multiple stars, increasing the total to 3112.

Between 1834 and 1837 Struve determined the parallax of Vega, arriving at a parallax angle of 0.26 arcsec. (The modern value is under half this, 0.12 arcsec.) Bessel is usually credited with determination of the first stellar parallax, of 61 Cygni in 1837; in fact it is probable that Struve preceded this date by a year or so. However, Bessel's results gained a quicker acceptance by the astronomical community than did Struve's.

After holding the post of Director of the Dorpat Observatory for 24 years, Tsar Nicholas I of Russia invited Struve to take over the directorship of a new observatory at Pulkovo. The observatory was situated about ten miles south of St. Petersburg, being built and equipped to Struve's own specifications. Struve worked at Pulkovo Observatory for over 20 years, concentrating on more double star studies. Struve's son, Otto, assisted with the observations. In 1861, Struve retired from the Directorship of Pulkovo and was succeeded by Otto.

Friedrich Struve lived to the age of 71, dying in November 1864.

Pietro Secchi, 1818 - 1878

Pietro Secchi was born on 29 June 1818 in the Italian town of Reggio. At the age of 15 he entered the Society of Jesus, the Jesuit Order, where he continued to study his principle interest, astronomy. As a member of the Jesuits he taught in many of the schools of the Order up to 1848 when due to religious persecution then prevalent on the Continent he was forced to leave Italy. He settled briefly in the UK before going on to the US where he taught at Georgetown University, Washington DC.

When the political climate changed in Europe, Secchi returned to Italy taking up the post of Director of Rome Observatory. His principle interest was the new field of astronomical spectroscopy. In this he was a pioneer and contemporary of Huggins in England. Secchi carried out a systematic observational programme, recording the spectra of approximately 4000 stars between 1864 and 1868.

Up to the time of Secchi's work, the only information known about stars was their position, brightness and colour. Secchi noticed that stellar spectra had great diversity, discovering that stars had different chemical compositions. During 1867 Secchi suggested that stars could be classified by their spectra. He divided stars into four spectral classes: this first attempt at spectral classification has subsequently been expanded to ten spectral classes.

Secchi was also a pioneer in the use of photography for astronomical research. During an eclipse in 1851 he took photographs of the Sun, recording the various phases. By 1859 he had photographed the entirety of the Moon's visible surface.

Secchi died on 26 February 1878.

William Huggins, 1824 - 1910

William Huggins was born in London on 07 February 1824. He had no scientific education, joining the family business of clothes and fabric merchants on leaving school. His first interest was microscopy, but his astronomical pursuits slowly took precedence. In 1856 Huggins had built an 8-inch refractor, obtaining the lens from a leading American lens maker, Alvan Clark. Three years after acquiring his telescope, Huggins sold the family business to concentrate exclusively on astronomy.

During the first half of the nineteenth century several famous physicists and chemists (Fraunhofer, Kirchoff, Bunsen and others) had experimented with prisms and the dispersion of light. The work led to modern spectroscopy. The pioneer scientists only dabbled in astronomical spectroscopy, this not being their prime interest, and they left it to others to discover the importance of this new astronomical tool. During the year 1859, Huggins attended a lecture held by the Pharmaceutical Society, which included a demonstration of the new spectroscopic techniques. Huggins considered that the new tool of spectroscopy was just what he was looking for - a new astronomical research method. He approached a leading English spectroscopist present at the lecture, Professor Miller, who was sceptical about his intentions. Miller was aware of the immense technical problems of constructing spectroscopic equipment sensitive enough for astronomical work (no such equipment was available at the time). Undaunted, Huggins set about the task of building an astronomical spectroscope. Miller provided encouragement but was unable to give much of his own time to the project. After much difficulty Huggins finally succeeded in pioneering this new branch of astronomy.

After four years of work (1863) Huggins had obtained sufficient data to present a joint paper with Miller at the Royal Society on the spectral lines of several of the brighter stars. He presented a more complete report the following year, containing a major discovery. Both of the Herschels and Rosse had observed many nebulae that could not be resolved into separate stars: all three concluded that this was due to the equipment they had at their disposal not being of sufficient aperture. At the end of August, Huggins obtained the spectrum of a planetary nebula in Draco. The spectrum was unexpected, having only one bright line. This showed that some nebulae were luminous gas clouds, and not comprised of stars.

About seven-and-a-half years after he started his research, Huggins presented a paper on his findings to the British Association. The paper contained results that took astronomy a major stop forward: he showed that all the stars that he had observed contained the same elements that are to be found on Earth and in the Sun. Some variable stars showed spectral changes, indicating that their changes in magnitude were due to physical processes. He also showed that comets were gaseous.

Huggins married in 1875 and was greatly assisted in his work by his wife. About this time he succeeded in obtaining stellar spectra by photographic methods. This enabled him to investigate the spectra of faint stars. In succeeding years, the Huggins were able to determine the radial velocities of some thirty stars.

Huggins was knighted in 1897 in recognition of his work. He continued observing until 1908 and died two years later. 

Giovanni Schiaparelli, 1835 - 1910

Giovanni Schiaparelli was born on 14 March 1835 at Savigliano, Italy. He gained a degree from the University of Turin in 1854 and on leaving university he went to study under Encke at Berlin Observatory and then under Struve at Pulkovo Observatory in Russia in 1859.

Five years later Schiaparelli was appointed Director of Milan Observatory, a position which he retained until his retirement. The majority of his work was associated with the Solar System. The English astronomer John Adams had calculated the orbit of the Leonid meteor swarm, demonstrating that it was comet-like. During the 1860s, Schiaparelli discovered a connection between Comet 1862 III and the Perseid meteor shower.

In 1877, Mars was at a favourable opposition, being at its minimum distance from Earth of 56 million kilometres. (This was the opposition at which Asaph Hall, observing from the US Naval Observatory in Washington, discovered the satellites of Mars, Phobos and Diemos.) From observations at this opposition and subsequent ones, Schiapparelli inadvertently instigated one of the biggest astronomical hoaxes of all time. Through careful micrometer measurements Schiaparelli convinced himself that some of the features on Mars were straight lines, arranged in a complicated pattern. He published reports referring to these lines in Italian as canali, which was should have been translated into English as channels but in fact was inevitably mis-translated as canals, implying artificial structures which had been created by intelligent life on the planet. The popular press of the time gave the latter notion much publicity.

Several astronomers carried the idea of canals on Mars far beyond Schiaparelli's original drawings. The most notable exponent of Martian canals was Percival Lowell, who claimed to have observed over 500 of them! Lowell's canals were probably the result of optical illusions combined with an over-imaginative mind. Many of Lowell's contemporaries reported seeing no such features.

Schiaparelli retired from Milan Observatory in 1900. Up to his death in 1910 he compiled an extensive survey of early astronomical history, concentrating in particular on Babylonian astronomy.

Joseph Norman Lockyer, 1836 - 1920

Joseph Lockyer wan born in Rugby on 17 May 1836. In 1857 he started a career as a clerk at the War Office. His employment included editing 'Army Regulations', a job that held no inspiration for him. Lockyer directed his attention towards astronomy: this began as a hobby but subsequently was to become his full-time profession. He started his observations with a 6.25 inch refractor by Thomas Cooke of York.

For the first few years Lockyer observed the planets, singling out Mars for special study. When Lockyer heard of the work being done by Kirchhoff and Bunsen he decided to take up spectroscopy, as Huggins was doing. He conducted his spectroscopic observations quite independently of Huggins. Lockyer concentrated on observing the Sun, leaving the studying of more remote stellar spectra to others.

Lockyer was the first to study the spectra of sunspots. By 1866 he had amassed sufficient information to to conclude that sunspots appeared dark against the Sun's disk for two principal reasons:

  1. Sunspots emitted much less light than their surroundings (previously widely accepted).

  2. Sunspots absorb more sunlight than their surroundings.

Two years later he discovered that prominences could be observed in daylight by widening the slit at the front of the spectroscope and directing the light through the edge of the prism. Before this time prominences had only been seen fleetingly during total solar eclipses. Pierre Janssen, and a little later Huggins, also independently discovered this method of observing prominences in daylight.

In 1868 a total solar eclipse was visible from India. Janssen vent to India to observe the phenomenon having agreed to share observational results with Lockyer. Janssen obtained a spectrum of the Sun and noticed a previously unidentified spectral line. He forwarded his results to Locker, who was able to identify the line, concluding that it belonged to a new element that had not yet been discovered on Earth. Locker named the element Helium: it was not identified on Earth until Ramsey found it at the turn of the 20th Century.

During 1869, Locker founded the science magazine Nature, taking up the editorship as a spare-time job, as officially he still worked for the War Office. The following year he was appointed secretary to a Royal Commission on Scientific Instruction and the Advancement of Science. It was six years before the Commission reached its conclusions. One recommendation was to set up an observatory for solar study. The observatory was built at the Science and Art Department of the Royal College of Science in South Kensington. (This centre has since been renamed, now being the Imperial College of London University.) Locker was transferred from the War Office to direct the new observatory. Now able to pursue astronomy on a full time basis, Locker extended his work from solar research to stellar spectra.

Lockyer received a knighthood in 1897 for services to astronomy. Four years later he retired, moving to Sidmouth, where he quickly established a new observatory even though he was in his seventies. He continued observing until his death in August 1920.

Edward Barnard, 1851 - 1923

Edward Barnard was born in Nashville, Tennessee on 16 December 1851. He received only a mediocre education, following which he became interested in photography, and for a few years made it his profession. From an early age Barnard was interested in astronomy, being a serious sky observer in his leisure time. In August 1877, the American Association for the Advancement of Science (AAAS) held a meeting in Nashville. Barnard had recently acquired a 5-inch refractor, and he attended the AAAS meeting to seek advice on how best to use it. Barnard returned from the meeting having talked to Professor Newcomb who suggested that he should search for comets.

The meeting with Newcomb was the beginning of Barnard's world fame as a comet discoverer. After about four years of searching he discovered his first comet on 12 May 1881, near Alpha Pegasi. The discovery was short-lived, as the object was visible for only two nights before being lost from view. There was, however, only a four month gap until Barnard found his first comet that was to bring his name to prominence (1881 VI). The following years proved even more fruitful, as Barnard discovered 1882 III, 1884 II and six more comets during the next three years. Barnard discovered every comet discovered in 1891! John Isaac Plummer, Colonel Tomline's astronomer at Orwell Park Observatory, observed several comets discovered by Barnard during the latter half of the nineteenth century.

During 1883, Vanderbilt University awarded Barnard a fellowship in astronomy. After only a short period at Vanderbilt, Barnard was given charge of the University Observatory. Later in the year, whilst observing an occultation of Beta Capricorni he noticed that the star flickered prior to disappearance instead of disappearing instantly. As a result of this observation, Barnard proposed that the star could be a binary. The Dearborn Observatory, Chicago, which housed a larger telescope than Barnard had access to, subsequently confirmed this suspicion. Also in 1883, Barnard re-discovered the Gegenschein (sunlight scattered by minute dust particles in the interplanetary medium in the inner Solar System), which had first been reported in 1854, but had attracted little attention.

After completing his University course, Barnard was offered, and accepted, the post of assistant astronomer at the new Lick Observatory on Mount Hamilton, California. While at Lick Observatory he made three important discoveries in 1892:

In 1895 Barnard moved, and began working at the Yerkes Observatory where he had the use of the 40 inch refractor. This move necessitated him taking up the post of Professor of Astronomy at Chicago University. At Yerkes, Barnard continued his program of photographing the Milky Way. Along with Wolf, he was one of the first to realize that the dark patches in the Milky Way were in fact clouds of gas and dust that obscured the stars behind. In 1917, the Carnegie Institute in Washington published a photographic atlas of selected Milky Way regions based on Barnard's photographs.

During 1916 Barnard discovered a star that had a very fast proper motion in Ophiuchus. This star moves about 1/2° in a period of only 18 years. This object has become generally known as Barnard's Star, holding the record for the greatest proper motion of any known star.

Barnard lived to the age of 65, dying in February 1923.

Bernard Schmidt, 1879 - 1935

Bernhard Schmidt was born on 30 March 1879 on the small island of Nargen off the coast of Estonia. Only 5 miles long by 2 miles wide and 12 miles from the mainland, life on Nargen was dominated by the Lutheran Church and farming. It is remarkable that from such an isolated environment such an influential figure should emerge. Despite his parents' emphasis on a strict Lutheran upbringing his instinctive interest in science soon became apparent. When he was eleven he was already experimenting with gunpowder, and as a result he almost lost his life. In one experiment while packing powder into a metal tube it exploded and he lost his right hand and forearm. Despite this devastating accident he maintained and developed his interest in mathematics and physics. In the years that followed he became interested in optics and working from drawings of a camera in a book he ground a lens from the bottom of a bottle, mounted it in a cigar box, and with some photographic plates from his friend the village chemist he succeeded in taking photos. This was a sign of greater things to come!

In Schmidt's late teens he enrolled as an engineering student in Gothenburg, Sweden where he specialised in optics. While studying there he came across the work of the German optician Stehl, and, after completing his studies, Schmidt left for Germany to seek him out. Stehl had worked at Mittweida but when Schmidt arrived Stehl had gone elsewhere. Schmidt however liked the place and stayed on. His interest in optics had developed to such a degree that he could support himself financially by making mirrors and selling them to local amateur astronomers. Initially he made mirrors only for amateurs but soon orders came in from professionals too when they realised how good the mirrors were. Beginning in 1900 Schmidt made mirrors up to about 200mm in diameter. In 1905 he made a 400mm mirror which far surpassed anything then available and as his skill developed he worked on figuring 300mm, 500mm and 600mm objectives for Leipzig, Potsdam and Hamburg observatories. It is remarkable that Schmidt carried out all his work with his left hand and that he never used machines.

Schmidt's reputation spread rapidly and he was offered several jobs by the great German optical companies of the day. However, despite these offers Schmidt wished to maintain his independence. A man who disliked regimentation, he worked only as the mood took him.

By 1920 Schmidt had made several mirrors for Hamburg observatory at Bergedorf and in 1926 the director of the observatory, R Schorr, eventually persuaded Schmidt to join the staff, albeit as a 'voluntary colleague' as Schorr described him. Schmidt maintained his irregular, independent style of work, often roaming off into the nearby woods instead of being in the optical workshop.

From the beginning at Bergedorf, Schmidt was set on overcoming the limitations of conventional telescopes. In 1929 he went on an eclipse expedition to the Philippines with the great astronomer Walter Baade and during the trip Schmidt told Baade that he had at last solved, in principle, the problem of producing a reflecting telescope that not only had a large aperture but also had a wide field of view. Schmidt's design has a large mirror at the bottom of the tube, a thin glass plate at the top end and the film is held on a curved surface facing the mirror in the middle. (Click here for more details on the configuration of the Schmidt telescope.) Baade, realising the importance of this new design, urged Schmidt to build one as soon as possible as did Schorr on hearing the details. Despite this however, Schmidt continued his apparently aimless walks in the woods insisting that he had to solve the problem of how to grind the complex curves involved in his design. In late 1929, Schmidt announced that the problem was solved and he began work.

Schmidt's ability to work was phenomenal, once he started. On one occasion Baade visited him to find him sleeping after 36 hours continuous work. Schmidt completed his first camera in early 1930 and soon used it to produce fine photographs. His first camera had a 350mm glass plate and a 430mm mirror and with a focal length of 635mm had a photographic speed of f/1.7 - incredibly fast for such a large instrument. It seems that Schmidt had to work very close to the limits imposed by the breaking strain of glass in order to produce his first instrument.

Photos from the camera initially failed to impress European astronomers but as soon as Edwin Hubble of Mount Wilson saw them he immediately asked Schmidt what was the largest camera that could be built. The answer turned out to be 1.22m diameter for the glass plate and 1.83m diameter for the mirror. The two large cameras of Mount Palomar and Siding Springs observatories are of this size: anything bigger would run into technical problems.

Schmidt continued work until his death on 01 December 1935. The 1.22m Schmidt camera at Mount Palomar was completed in the late 1940's and continues to be of immense value to astronomers; a great tribute to Schmidt's insight and optical genius.

Ejnar Hertzsprung, 1873 - 1967 and Henry Russell, 1877 - 1957

Ejnar Hertzsprung was born on 08 October 1873 and Henry Russell was born on 25 October 1877. Both men independently discovered the relationship between the absolute magnitude and the colour of stars. The result was the Hertzsprung-Russell, or H-R diagram. The H-R diagram, in both its original and modern forms, has greatly assisted astronomers to understand the evolution of stars.

Hertzsprung was educated as a chemical engineer, working for two years in St. Petersburg until 1901. During 1902 he returned to his native Copenhagen with a great interest in astronomy. After some seven years he was appointed as an astrophysical lecturer at Gottingen. Hertzsprung was one of the first to advance the idea of absolute magnitude. (The absolute magnitude of a star is the magnitude that it would have if it were situated at a standard distance from the Earth, this distance being 10 parsecs or 32.6 light-years. This enables stars of differing luminosities to be directly compared.)

Hertzsprung specialized in stellar photography, particularly of double stars and estimates of stellar magnitude from photographs. This led to him publish in a semi-popular manner, in a photographic journal, his ideas about stellar colour and absolute magnitude. The article went unnoticed for nearly ten years.

During 1911 Hertzsprung discovered that the Pole Star was a Cepheid variable, varying by 0.2 magnitude in a period of about four days.

Hertzsprung became a professor at Leiden in Holland in 1935, and upon his retirement, he returned to his native Denmark, dying on 21 October 1967.

Russell was educated at Princeton University, New Jersey, receiving his doctorate in 1900. He worked for a short time in England before returning to teach at Princeton. Russell's research led him to the discovery of the luminosity-colour-spectral class relationships of stars. Russell presented his results at a meeting of the American Association for the Advancement of Science in December 1913. He published his work in 1914, some nine years after Hertzsprung.

Russell was one of the first to analyse the composition of the Sun from its spectrum in great detail, during 1929. He was rather surprised to find that its composition was mostly hydrogen, with helium, oxygen and nitrogen being the most important trace elements present. Russell died in February 1957.

Chronological List Of Important Astronomical Discoveries And Events

 

ca. 3000 BC Earliest recorded Babylonian observations of eclipses, planets and stars.
ca. 2500 BC Egyptian pyramids constructed, oriented north-south by the stars.
ca. 2000 BC Babylonian story of creation. Enuma Elish. Stonehenge built in southern England, aligned with the stars.
ca. 1000 BC Beginnings of Chinese and Hindu astronomical observations.
ca. 700 - 400 BC Greek story of creation. Hesiod's Theogeny: Hebrew story of creation. Greek philosophers Thales, Pythagoras and Meton note the regularity of celestial motions.
ca. 400 - 300 BC Greek philosophers Plato, Eudoxus and Calippus develop the concept of celestial motion on spheres. Aristotle develops the idea of four elements and the concept that heavy things fall, light things rise.
ca. 300 - 100 BC Aristarchus proposes that the Earth moves. Eratosthenes measures the size of the Earth. Hipparchus makes accurate positions of the position of stars.
ca. 150 AD Ptolemy's Almagest summarises the geocentric theory: the motion of the planets is explained by the circular motion of deferents and epicycles.
ca. 1400 Ulugh-Beg in Samarkand re-observes star positions. 
1530 Copernicus in Poland postulates that the Earth and planets move around the Sun because this involves fewer circular motions. This revolutionary idea later provokes strong opposition.
ca. 1600 Tycho Brahe measures the motions of the planets accurately. Kepler uses these measurements to show that the orbits of the planets are ellipses rather than combinations of circles. Galileo uses one of the first telescopes to observe the satellites of Jupiter and the crescent shape of Venus, supplying strong support for the model of Copernicus. Galileo also establishes that falling weights would all be accelerated to the same degree if there were no air resistance to hold the larger ones back.
ca. 1680 Newton combines findings from Kepler and Galileo, together with observations of the Moon and comets, to formulate the fundamental laws of mechanics and gravitation. He also studies light, its colour and spectrum. By this time, accurate pendulum clocks are in use.
1690 Edmund Halley notes periodic reports of a bright comet every 76 years and concludes that the reports relate to a single object moving around the Sun in a highly eccentric, elliptical orbit. He predicts the return of the comet in 1758. The comet duly reappears (16 years after Halley's death) and it is subsequently named Halley's Comet, in his honour. In the modern era, Halley's Comet had an apparition in 1985-86.
1755 Kant postulates that the Sun and planets formed from the coalescing of a cloud of gas like a spiral nebula.
1780 William Herschel builds large telescopes, discovers the planet Uranus and explains the Milky Way as a flat disk of stars surrounding the Sun.
1700 - 1800 Mathematical astronomy flourishes, involving many Europeans - Cassini, Bradley, d'Alembert, Laplace, Legrange and others - who apply Newton's mechanics to celestial motion with remarkable precision.
1814 Joseph Fraunhoffer discovers dark lines in the spectrum of the Sun, paving the way for the invention of spectroscopy.
1822 Sir William Herschel dies.
1826 Johann Bode dies. He is best known for popularising a mathematical relationship between the distances of planets from the Sun.
1830 Friedrich Bessel determines the distance of the star 61 Cygni.
1834 Sir John Herschel, son of Sir William Herschel, travels to Cape Town and produces the first extensive catalogue of stars in the southern hemisphere.
1840 J W Draper takes the first astronomical photograph; his subject is the Moon. By 1905 astronomical photography is well established and used with telescopes ranging in size up to 40" in aperture, recording stars of brightness only 0.000 01 that visible to the naked eye.
1843 Doppler explains the effect of motion on the spectrum of light.
1845 William Parsons, 3rd Earl of Rosse, discovers the spiral shape of galaxies.
1846 Jean Couch Adams and Urbain Jean-Joseph Leverrier predict the position of a new planet beyond the orbit of Uranus. Johann Gottfried Galle and a student assistant, Heinrich d'Arrest, working at the Berlin Observatory in Germany, find the planet. It is named Neptune.
1848 William Parsons gives the name Crab Nebula to the remnants of the supernova of 1054 (the supernova was observed by Chinese astronomers).
1850 William Bond is first to photograph a star.
1851 Pietro Secchi is one of the first to photograph the Sun during an eclipse.
1854 Gustav Kirchhoff begins developing the discoveries of Fraunhoffer and invents the spectroscope.
1856 Sir William Huggins uses a spectroscope to analyse starlight.
1857 George Bond is first to photograph the double star Mizar (in the constellation Ursa Major).
1862 After studying stellar spectra, Pietro Secchi suggests testablishing spectral classes for stars.
1868 Sir William Huggins discovers that the star Sirius is receding from the Earth.
1870 Hermann Vogel uses spectroscopy to analyse planetary atmospheres. William Huggins, though, had started such observations some time earlier.
1877 The Italian astronomer Giovanni V Schiaparelli reported on observations of Martian canali, which was mis-translated into English as canals, implying an artificial construction. This initiated huge interest in the possibility of intelligent life on Mars utilising advanced engineering to transport scarce water around the planetary globe. 
1800 - 1900 Navigation is now a precise and important practical application of astronomy. Accurate observations of stellar positions reveal that annual parallax is due to the Earth's motion around the Sun, confirming the Copernican model of the Solar System and providing a method of estimating the distance of the nearer stars. Other precise measurements show that the stars are themselves moving through space.
1850 - 1900 The laboratory study of light together with physical theory shows that spectral analysis can be used to determine the temperature and chemical composition of a light source (e.g. a star).
1889 George Hale invents the spectroheliograph for observing the distribution of specific elements in the Sun.
1892 George Hale initiates his idea for building a 40" refracting telescope.
1897 George Hale's telescope is completed. It was, and still is, the largest refracting telescope in the world. It was named after its chief financier, Charles Yerkes.
1900 Chaimberlin and Moulton speculate that the planets were formed after another star passed close to the Sun and pulled some of the material of the latter into orbit around the Sun.
1904 Henrietta Leavitt discovers the period-luminosity law for Cepheid variables, which can be used to estimate the distance of such stars.
1902 - 1920 Einstein establishes the theory of relativity. Large reflecting telescopes are built at the Mount Wilson Observatory in California.
1912 Vesto Slipher is the first to observe that the galaxy M31 in Andromeda is approaching the Earth, whereas most others are receding.
1914 Henry Russell publishes his discovery of the relationship between a star's colour and its luminosity. He arrives at the same conclusion as Hertzsprung, but independently. The resulting Hertzsprung-Russell diagram has profound influence in the understanding of stellar evolution.
1915 - 1920 Albert Einstein develops his General Theory of Relativity.

Harlow Shapley studies globular star clusters and identifies Cepheid variables in them to estimate their distances. Shapley goes on to estimate the dimensions of the galaxy. Just as Copernicus moved the centre of the universe from the Earth to the Sun, so Shapley moved the Earth a long way  from the centre of the galaxy.
1930 Clyde Tombaugh discovers Pluto.
1910 - 1940 Slipher and Hubble find that most other galaxies are receding from the Milky Way. De Sitter, Eddington, Lemaitre and others apply relativity theory to explain this recession.
1930 - 1960 Bethe, Gamow and others in the US apply the results of nuclear physics to explain the source of stars' energy. Many astrophysicists work on theories of the formation of stars from giant clouds of interstellar gas and their subsequent evolution. Von Weizsacker, Kuiper, Uray and others develop a theory of the origin of the solar system from a giant gas cloud.
1931 Karl Jansky discovers radio waves emanated from various sources in the galaxy.
1942 Walter Baade discovers over 300 Cepheid Variables in the Andromeda Galaxy and uses them to derive a more accurate estimate of the distance of the galaxy.
1947 - 1960 The first instruments are launched above the atmosphere in the US for astronomical observations.
1952 Baade derives a new period-luminosity law for Cepheid Variables. This effectively doubles the size of the known universe. Baade finds that our own galaxy, the Milky Way, is of only average size, thus de-throning it from holding any special significance due to its size.
1957 USSR launches the first artificial satellite, Sputnik, into Earth orbit.
1959 Soviet scientists launch the first space probe to hit the Moon.
1961 Soviet cosmonaut Yuri Gagarin makes the first manned spaceflight around the Earth.
1961 - 1966 Radiotelescopes locate quasi-stellar radio sources (quasars) which are found to have large optical red-shifts, like the most distant galaxies.
1962 - 1967 Rocket-borne instruments flying above the Earth's atmosphere detect cosmic X-ray sources.
1964 - 1965 US space probes Ranger 7 and Ranger 8 obtain first close-up photographs of the lunar surface.
1965 Mariner 4 takes photographs of the surface of Mars from a distance of approximately 17,000 km. The photographs show a cratered surface.
1966 Soviet space probe Luna 9 makes the first soft-landing on the Moon.
1967 Surveyor 3 carries out the first physical analysis of the lunar surface.
1969 First optical discovery of a pulsar (in the Crab Nebula).

Neil Armstrong becomes the first man to walk on the Moon.
1971 Mariner 9 enters orbit around Mars.
1973 Pioneer 10 makes a successful fly-by of Jupiter.
1974 Pioneer 11 makes a successful fly-by of Jupiter.

Mariner 10 makes a successful fly-by of Venus and Mercury.

 

J Appleton
Original: Newsletters September, October, November, December 1975; January, February, March, April, May, July, August, September, October 1976; October, November, December 1978; January, February, March, April, May, June, August 1979; February, April, June 1980; June, August 1986; May, July, September, November 1992 and February 1993.
Updated 29 July 2009