Institute of Astronomy


Ask an Astronomer - Solar System

Length of a day

Published on 08/02/2012 

Hello my name is Sam and I am 11. I have recently got into Science and I done some research and showed my teacher that there wasnt 24 hours in a day. My teacher said that I was wrong and there was 24 hour in a day. could you help me and tell me the real answer to how many hours, minutes and second there are in a day please?

The answer is that you are both right, the problem is what you mean by 'day', and there are two ways of thinking about it.
One way is the length of time between the Sun appearing in the same place in the sky (overhead for example), this is what people usually think of as a 'day' and is what our clocks measure.  For this way of defining the length of a day there are exactly 24 hours in a day, and in scientific terms this is called a 'solar day'.
The other way of thinking about the length of a day is the time it takes for the Earth to rotate once about it's axis.  This is slightly shorter at only 23 hours 56 minutes and 4 seconds and is called a 'sidereal day'.

You can see why there is a difference between the two in the diagram below (which I admit we borrowed from Wikipedia!).  As the Earth rotates it is also moving around the Sun, so if you are living where the little red arrow is on the diagram by the time the Earth has rotated once (the curved arrows next to Earth show the direction of rotation) it has also moved along its orbit from position 1 to position 2, and although the Sun was directly above the red arrow at position 1 it isn't quite overhead at position 2.  For the Sun to be directly overhead again you have to wait until the Earth has moved and rotated a little further, to position 3, so the usual way people think of a 'day' is slightly longer than the time it takes Earth to spin once.

Stars on the Moon

Published on 02/05/2011 

Is it possible to see stars from the surface of the moon? I thought it was but in an interview in 1970 ( Neil Armstrong said you can only see the Sun and Earth.

When US astronauts visited the moon as part of the apollo program, people often comment in images that you don't see the stars. This is the same phenomenon Neil Armstrong described in the interview on the BBC in 1970.

The stars are actually visible from the surface of the moon but they are particularly difficult to observe. If on Earth you compare a night where there is no moon with a night with a full moon, it is much easier to see stars on the dark night with no moon. The same phenomena happens on the moon with the Earth reflecting the Sun's light as well as the astronauts only working in sunlight. And as you know from experience, during the day time we don't see any stars (unless you know exactly where to look using a telescope!) even though they're still shining.

So although the stars are there, the light conditions when the astronauts were working meant that they were unable to see them.

What are the Northern lights?

Published on 01/05/2011 

What are the Northern lights?

The Northern lights (or 'aurora') are caused by energetic particles that perpetually rain down onto the upper part of Earth's atmosphere - most of these particles stream from the sun in the 'solar wind',  although some originate from deeper out in space. Many are guided along the Earth's magnetic field down deeper into our atmosphere, where they collide with atoms and molecules of air. In such a collision, the particle gives up some of its energy to the atom, which later gives off this energy in the form of light. So if the sun is active there's a strong solar wind, and many millions of these collisions occur high up in the atmosphere to produce the luminous glow that is the northern lights. The actual colour observed depends on what kind of atom or molecule is giving off the light, and how much energy has been given to it by the collision. The northern lights are emitted from a region from about 100-300km above the Earth's surface.

Amateur detection of Near Earth Objects

Published on 04/04/2011 

Is it possible for an amateur astronomer to assist with detecting near-Earth asteroids or comets? If so, what would be the minimum telescopic aperture and type of photographic equipment required to conduct this kind of research?

Amateur astronomers can and do play an important part in detecting near-Earth objects. Today more than 5% of all near-Earth objects are discovered by amateurs and this proportion is on the increase. The Minor Planets Centre ( based at Harvard University is the organisation responsible for cataloguing and archiving all discoveries of small bodies in the solar system and have a wealth of information to help potential amateur astronomers. The professional search programs typically use telescopes with a diameter of ~1m; LINEAR, one of the longest running and most successful programs currently has two 1m telescopes and a 0.5m telescope. When conducting searches smaller telescopes are to some extent preferred since they have a larger field of view and can image a larger area of the sky at once.

When you have your telescope, discovering asteroids is still not trivial! Because these objects are particularly faint (due to their size), the detectors being used (usually a CCD) need to be sensitive enough to be able to distinguish them from the background noise from the device. For CCDs, when they are cooled the background noise is reduced and because of this, most of the CCD detectors you can purchase for amateur astronomy today use fans to cool the CCD well below the ambient temperature.

Even with both of these, the main requirement to successfully discover asteroids and other small (and faint) objects is a dark site with good seeing (clear, still skies). Combining all of these things together means you could potentially discover some new asteroids!

Moon causing earthquakes

Published on 04/04/2011 

I happened to notice that the recent close pass of the moon (supermoon) coincided with the earth quake in Japan. Has any study been undertaken of the effects of the gravitational field of the moon on the tectonic plates on earth? Is there a correlation between earth quake activity and the proximity of the moon to earth?

The particular 'super Moon' phenomenon is a coincidence of the Moon being at perigee (the closest point on its orbit) and a new or full moon.  The Moon completes an orbit and passes through perigee once a month so this close approach of the Moon is not an unusual phenomenon.  The eccentricity of the Moon's orbit (how elliptical it is) is quite small so the difference in the tidal influence of the Moon between apogee (the furthest point in its orbit) and perigee is also quite small.  The influence of the Sun is larger and is responsible for the 'spring' (slightly higher) and 'neap' (slightly lower) tides that occur on a two week cycle with spring tides occuring around new moon and full moon and neap tides occuring around first quarter and last quarter.  By far the largest variation in the tidal effect of the Moon however, and what sets the primary rythm of the tides, is the daily rotation of the Earth.

There have been a number of studies over the years of the effect of the tidal influence of the Moon on earthquakes.  Since the largest variation in the tidal influence of the Moon is the roughly twice daily pattern of high and low tides any effect of the Moon on earthquakes should also show this pattern.  There is some evidence that it is statistically more likely for earthquakes to occur at a time near high tide, when the Moon is overhead or on the opposite side of the Earth, but the link is not very strong.  There is no evidence linking the rather lower level modulations of the tides from the Sun, the eccentricity of the Moon's orbit and the eccentricity of the Earth's orbit to the occurence of earthquakes.  The Earth is a seismically active planet and there are thousands of small earthquakes occuring all the time, which would tend to blot out any signal from the gravitational effects of the Moon.  The Moon on the other hand is a rather seismically inactive body and there is definite evidence from seismometers left on the Moon by the Apollo astronauts that the Moon experiences weak 'moonquakes' as a result of the Earth's tidal influence.  The gravitational influence of the Earth on the Moon is around 100 times larger than that of the Moon on the Earth as well which makes it easier to observe.

To give you a sense of scale the variation in gravity at the Earth's surface due to the main tidal rythm compared to the Earth's gravity is about 1 part in 10 million, in comparison the variation in gravity on the surface of the Moon due to the tidal influence of the Earth is about 1 part in 100,000.