Institute of Astronomy


Ask an Astronomer - Miscellaneous

Looking back in time

Published on 22/01/2013 

When a picture is taken of deep space and it is said that it is from when the universe was 500,000,000 years old.  Mainly saying that you're looking into the past.  That doesn't make sense to me for the fact that you're able to capture a picture.  Distance and time can coincide but in this case i dont get how this theory works with space?  I understand at such a distance it takes time for light to reach us, the point I'm trying to make is that how can it be said that what we view from deep space is the past not the present?

The effects of large distances and time in astronomy can be a little confusing.  Take as an example Proxima Centauri, the nearest star to our Solar System.  This is 4.2 light years away, which means that it takes light 4.2 years to get from Proxima Centauri to us.  Now since the only way we can see something that has happened at Proxima Centauri is through light, this means anything we see at Proxima Centauri actually happened 4.2 years ago.  If there were a person on Proxima Centauri and they had an exceptionally powerful torch, which the flashed at Earth, it would take 4.2 years for the torch flash to reach us, so by the time we saw it the person would actually have flashed the torch 4.2 years ago.  Now as I said Proxima Centauri is very nearby, when we look at objects in the distant universe they are much farther away, billions of light years, so when we see them we are seeing light that left them billions of years ago, when the universe was much younger.  As a result we can in a way think of looking at objects that are very far away in the distant universe as looking back in time, because the light has taken so long to reach us that the universe has changed a lot in the time it has taken the light to get here.

Using astronomy to date historical events

Published on 03/01/2013 

I cannot get any clear answers to what should be a simple question. "What percentage accuracy do the ancient astronomers have in fixing dates for the first millennium BCE?"

I am researching the period 1000BCE to 00BCE.
There is a wealth of information about Sumerian and Babylonian astronomical dating.
The "Kings lists" are constantly mentioned.
I simply want to have a reasoned answer to the following:

Allowing for human error, scribal miscopying etc - did the ancients in 1000BCE to 00 accurately observe and plot the planets so that we can retrospectively today recognise that which they recorded and afford it absolutely certain dates (to the nearest five years or so)
Do we know, from the positions that they give a planet in the sky that they called X, that it was that which we call, say, Mars.

Are there examples which clearly establish this e.g.
Carved in stone –
“In the 4th year of King X reign Planet V appeared in the west (whatever) and then moved behind the full moon to disappear in the northeast" - this today our computers tell us is exactly a description of Venus on 4th December 560BCE
Carved in stone –
In the 56th year of the reign of King W planet H did rise in the east and set at midnight one step into the quarter west(whatever) - this is an exact description of Mars on 4th January 500BCE"
Carved in stone –
King X was replaced by King W in his eighth year.

That is - we now know these astronomical events were indeed 60 years apart, they could be seen exactly like that from Babylon and it is clear that this was indeed the 60 years from King X 's 4th year to King W's 56th year.

There is much discussion of accuracy and more of eclipses. The only solid facts must surely come from verifying planet observation and then tracking those dates and comparing them to those given by the ancients.

Or as usual am I being simple minded?

As you have found astronomical observations can indeed be used to help provide support for chronologies of the ancient world, no method is perfect however and there are problems with this.

All astronomical observations one can make related to the Solar system contain many layers of cycles.  Everything of course varies on the cycle of a year as the Earth orbits the Sun, however due to the motion of other bodies and slow periodic changes in Earth's orbit there are also longer period cycles.  For example the times at which Venus rises and sets has a cycle of about 8 years (as well as other longer ones).  In general the shorter cycles are the most prominent while distguishing where an observation falls in a longer term cycle is more difficult.

The end result is that for your example stone carving there might be a match with 560BCE, 552BCE, 544BCE, 536BCE or 528BCE.  Sometimes knowing that it must be one of those dates might be sufficient to combine with other data and pin down an exact match, and sometimes even being able to pin it down within 5 cycles or whatever the case may be might be superior to what was known before, but gaps and inaccuracies in ancient records make it difficult to pin down exact dates from historical astronomical observations.

The most accurate records tend to be those of eclipses (because they are hard to miss and have a short duration), which helps with placing individual eclipses within the longest eclipse cycles and so providing more precise dates.  There are still problems with gaps simply because the further back you go the fewer records have survived whether or not they were originally taken, and scribal errors accumulate over time and are difficult to account for.

An additional problem is that in early history (and even comparatively recently) the calendars were not constant.  Many ancient calendars, such as that used by the Babylonians, were lunisolar with months based on lunar phases plus leap months as required to keep reasonable synchrony with the solar year.  That is fine provided that one knows how the scheme on which the leap months are added, but therein lies the difficulty in that they tend to be adjusted on a more ad-hoc basis or systematically over the medium term but with unknown larger jumps in the long term (think about the break that occurred in England in 1752 with the switch from the Julian to Gregorian calendars).  In fact even today this problem exists in the form of 'leap seconds' added by the international body which supervises global time standards, as they are irregular and unpredictable.

Sorry I can't really give you a simple answer, but I hope this goes some way to at least explaining why it isn't a simple question!

Gravitational waves from the Sun and Moon

Published on 17/12/2012 

Having attended an outreach talk on gravitational waves, we had a couple of questions:

1) The moon is a massive orbiting object near to the Earth. Does it produce measurable gravitational waves? If so, do these waves affect the measurements taken by LIGO?

2) The sun is changing mass due to solar wind, flares and nuclear reactions. Is it possible to measure this effect with current (or near future) gravitational wave detectors? We appreciate that it won't be an oscillation but it will be changing the curvature of space-time as the Earth orbits it over time.

Glad that you enjoyed the lecture. In answer to your questions:

  1. Yes, the Earth-Moon system will produce gravitational waves! Because the Earth and Moon aren't that massive (compared to black holes say), they won't be too big; you might think they are still important because we are right there... As it is, the waves are at too low a frequency for detection with LIGO: the Moon orbits about one every 28 days, and the ground based detectors are not sensitive to frequencies low than about 1 Hz (one cycle per second).

    However, more directly, the Moon does affect LIGO though its gravitational pull. This is a direct force that can pull at the mirrors at the end of the arms and move them slightly. Fortunately, since we know where the Moon is, we can take this effect into account. In fact, the detectors are so sensitive, that not only must we take into account the Moon, but also the tides. The tides are the movement of water in response the the gravitational pull of the Moon and the Sun. At different points in the cycle there is a different distribution of water across the Earth, and the pull from having extra water in regions close to detectors can be measured.

    In conclusion, the Moon orbitting will produce gravitational waves, but these are not of the right frequency to be detected. However, the Moon's gravity still needs to be taken into account.

  2. You are quite right that the Sun is changing mass. This change is quite small, at least over human time-scales (the Sun will live for billions of years, so only changes by a tiny amount over a few years, or even centuries). I don't believe that we are currently able to measure the mass of the Sun precisely enough to observe the change; however, this may be possible in the future. Certainly the technology developed for gravitational wave detectors could be adapted to precisely map the Sun's gravitational field. I don't think anyone has come up for a mission design for this yet, although we have recently had missions that have mapped the Earth's and the Moon's gravitational fields (GRACE, GOCE and GRAIL; a GRACE II mission is planned, and will hopefully include technology originally designed for LISA).

    Regarding gravitational wave measurement, you've hit a very important point. There shouldn't be any (significant) oscillation. To excite gravitational waves you must have something like a binary (technically where the mass quadrupole is changing) and not something spherically symmetric. The Sun's mass loss is very nearly perfectly spherical, and so there shouldn't be any real gravitational wave emission. 

Definition of Twilight

Published on 07/11/2012 

As you may know the Holy month of Ramadan is approaching and as a Muslim I am going to observe the fasts in this one month. However Similar to last year there is many conflicting opinions on when the fast should close.
For this reason I have decided to make my own enquiries and get external information.

We have to close our fast when there is some light in the sky. Some Scholars are saying that is when the sun is below the horizon at 18degrees and others are saying 12 or 15degrees. This is causing great confusion
Can you please clarify this
Also During the summer months in the UK,(May-July) is it very difficult to differentiate between the two twilights? Meaning the twilight of the night, and the twilight of the morning?

The confusion I'm afraid is because unlike sunrise and sunset it is difficult to properly define 'twilight'.  There are three rough 'bands' of twilight that are generally agreed upon, civil twilight - the sun is less than 6 degrees below the horizon, nautical twilight - the sun is between 6 and 12 degrees below the horizon, and astronomical twilight - the sun is between 12 and 18 degrees below the horizon.  Civil twilight is what most people would think of as 'twilight', and at the start of which is when you would see the characteristic reflected red-orange glow from high clouds, there is generally still enough light to see by and the horizon is clearly visible.  Most people would likely think of nautical twilight as being 'darkness', in that bright stars will be visible and it would be difficult or impossible to tell which direction is East or West simply by looking for the glow of the sun below the horizon, at the start/end of nautical twilight when the sun is 12 degrees below the horizon even the horizon itself will be indistinct and for almost all intents it is completely dark.  Even after this though there will still be enough scattered sunlight around that it is not ideal for observing faint, diffuse, objects like nebulae until the sun is more than 18 degrees below the horizon.

I expect that for your purposes nautical twilight would be quite sufficient and that civil twilight is probably adequate, but that is something that you should decide for yourself.  Wikipedia has well written, detailed, articles about twilight and the brightness of the night sky which have a variety of other links that you can check.

You are also quite correct that in a similar way to the fact that there are days when the sun does not rise or set within the Arctic and Antarctic circles there are concentric regions around the poles where, although the sun does rise and set, it will for a period around the summer solstice (21st June in the Northern hemisphere), never be darker than civil twilight, nautical twilight, or astronomical twilight.  It is easy to work out the lowest the sun will be below the horizon at any given location on the summer solstice from Earth's axial tilt of 23.4 degrees.  If you are at a latitude of X degrees then at the summer solstice the sun will be no more than 66.6-N degrees below the horizon, so in London (51.5 degrees North) on 21st June the sun was never more than 16.1 degrees below the horizon, so it was never darker than astronomical twilight.  In Edinburgh however at 56 degrees North the sun was never more than 10.6 degrees below the horizon and so it was never darker than nautical twilight.  It will always be darkest at midnight however (though note that since British Summer Time is one hour ahead midnight is actually 1am).

It is also worth noting that aside from the effects of the tilt of the Earth light pollution in large cities means that it may never seem completely dark.

Gravitatitional attraction

Published on 15/12/2011 

If gravity is the force that attracts bodies with mass together, how does this force of attraction work? Why should two bodies be attracted to each other?

Gravity is a familiar force: it is what keeps us on the surface of the Earth, and what keep planets in orbit about the Sun. Despite being such an everyday force, it has taken scientists a long time to unravel how it works, and there still remain some unanswered questions.

Our best theory of gravitation is Einstein's theory of general relativity (GR for short). In GR space is bent by the presence of mass (or energy, as Einstein's earlier theory of special relativity showed they are equivalent, yielding the infamous E = mc2 which allows you to convert between the two). The bending of space is commonly explained using the analogy of a rubber sheet. The sheet is flat when there is no mass, but if you were to place something heavy, say a bowling ball, in the middle, the sheet would stretch. Now imagine rolling a table tennis ball across the sheet. If the sheet were flat the table tennis ball would move in a straight line, but with the bowling ball there the table tennis ball moves on a curved path. If you couldn't see the rubber sheet it would appear as there was some force that is pulling the table tennis ball towards the bowling ball. This is the force of gravity in our analogy.

The real universe works in a similar way. An objects want to travel in a straight line, but when mass bends space, this line gets bent too, so the object follows a deflected trajectory. We say that this deflection is due to gravity. The physicist John Archibald Wheeler summarised this as "Space acts on matter, telling it how to move. In turn, matter reacts back on space, telling it how to curve".

Two bodies are attracted to each other as they curve space in a way that makes them want to move together. Imagine putting two bowling balls on our rubber sheet: they will fall towards each other. That is how the force of attraction works.