# Ask an Astronomer at the IoA

## Length of a day

Published on 08/02/2012
Question:

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.

## The expansion of the universe

Published on 08/02/2012
Question:

I'm a 13 year old, and i have been thinking about the big bang and how it happened. I came up with the theory that dark matter is the force that is making the universe expand and dark energy is trying to send it backwards, as far as i know. And if dark energy wins and pulls everything back and smashes everything into atoms. Like the suns made of hydrogen, carbon, and other materials. And that the whole universe just is engulfed and destoryed. then when it gets to the smallest it gets to, everything just explodes backwards and starts the whole universe all over again. Its like a recycling system for the universe.

It's great that you are thinking about these big questions! The accelerated expansion of the universe is a very difficult concept to grasp.

First off, I think it's important we distinguish between Dark Matter and Dark Energy.  While astronomers don't know exactly what either of these two things are, we do know that they behave in very different ways.  Dark Matter feels an attractive force of gravity, just like the normal matter that we can see.  In other words, Dark Matter gets pulled in. On the other hand, Dark Energy expands space out and is responsible for the expansion of the universe.

So with that in mind, if we think of a universe that has a lot of matter in it (both Dark Matter and normal matter) and no Dark Energy, then the Universe would eventually collapse due to the force of gravity pulling all the matter inwards.  This scenario is appropriately named the "Big Crunch," and as you say, everything would be destroyed in a kind of backwards explosion.

However, we believe our universe does have Dark Energy in it, in addition to Dark Matter and normal matter. This being the case, we can think of two other scenarios for the fate of the Universe:  Either Dark Energy fills more and more space, and the expansion of the universe gets faster and faster, until everything is essentially torn apart (called the "Big Rip"). Or the matter and the Dark Energy find a point of equilibrium, and space just keeps coasting outwards (called the "Big Chill").  As of right now, we don't have enough observational evidence to say which scenario is correct.

There are many questions that we cannot yet answer about the expansion of the universe.  There is lots of room for new ideas and new theories, so keep thinking about it! Hope this background information helps!

## Gravitatitional attraction

Published on 15/12/2011
Question:

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.

## J1950 and J2000 Epochs

Published on 23/08/2011
Question:

What are the epochs J1950 and J2000 when looking at objects in the sky? And what do I have to put in my telescope program?

Astronomers use different epochs to give coordinates of objects in the sky due to changes in motion due to primarily the precession of the Earth on its rotation axis. Much like a spinning top, as the Earth rotates, it's rotation axis gradually rotates as well although much slower than the daily rotation we see. Because of this precession, the positions of the stars change over time with a small motion every day. On small timescales this motion isn't noticeable however over decades it is. For this reason, astronomers update their coordinates every 50 years to make it simpler when finding objects.

Although for an object you can find coordinates in either J1950 or J2000 (the two most recent epochs), this won't actually be the correct location in the sky today. However, what your telescope program will do will take the coordinates from those epochs and then calculate what they should be today i.e. J2011 and then move your telescope there. This saves people doing things by hand and so means you can find objects with the standard coordinates quickly and easily.

As such, all you should need to do is find the coordinates of the object in either J2000 or J1950 and can then put those into your program. The program will then do the rest.

## What are the drifting stars that we see at night?

Published on 09/05/2011
Question:

What are the drifting stars that we see at night?

If by 'drifting stars' you mean stars that appear to cross the sky in a matter of only around 4 minutes or so, then they're not stars, but artificial satellites. They reflect the sunlight down to the Earth's surface as they orbit around it. Some of these can appear very bright indeed - such as the 'iridium' network of satellites which have large, very reflective antennae. The International Space Station can also sometimes be seen at morning or evening twilight; and many other satellites can be seen at much fainter brightnesses throughout the night.