- Constellations -
What is a constellation?
It is nothing more than an arbitrary grouping of stars that appears to make a picture.
Most of the ones we currently use come from the ancient Babylonians and Greeks and date from about 4,000 years ago. These people envisioned the figures of their mythical gods, heroes, animals and monsters onto the sky in the form of constellations. Some of the more recent (mainly southern hemisphere) ones date from only a few hundred years ago. They were made often in the celebration of then recent inventions, however a few were attempts by their inventors to immortalise themselves for all of eternity.
Of course every civilisation around the world had their own patterns in the sky. However in 1930 the International Astronomical Union decided that there should only be 88 constellations, all with well defined boundaries so that no part of the sky was outside of a constellation.
It is possible to "join the dots" and make a picture with the stars simply because the distances to them are so great that they all appear to be at the same distance from the earth. That is, we cannot visualise them in 3 dimensions. Consequently the stars all appear as if they are on the inside of a huge celestial sphere surrounding the earth.
The stars are moving, albeit slowly, so in a few thousand years the current patterns we see will be destroyed.
Why do we keep the constellations today, even though we no longer believe in Centaurs, Sea Monsters and the like? Constellations are handy to find your position in the sky in the same way that countries are handy to know where you are on the earth. If you say that you were looking at something in the constellation of Centaurus then immediately someone else knows approximately the area of sky to start looking.
As mentioned before a lot of the constellations represent the myths and heroes of the early civilisations in an attempt to de-mystify the heavens. Consequently there is usually a lot of lore that goes along with every constellation. In fact a lot of the pictures are quite intricately connected and it is often possible to get an entire epic story using a number of constellations. For example, the voyage of Jason and the Argonauts is up there in its entirety for everybody to see (read?). Other interesting stories relate to Orion and Scorpius.
How do we find the constellations?
We now know that the constellations allow us to identify a particular portion of the sky but how do we actually find where they are when we look up?
Fortunately there exist things known as star maps. These maps show the location of the brightest stars in the sky and usually also denote which stars belong to what constellation.
Using this map now allows us to find our way about the sky and to locate most of the constellations and brighter stars. In order to use the map all you have to do is orient yourself and the map so that the marked directions match up with the real directions. Once this is done then hopefully you will be able to match things in the sky to those on the map by noting the relative locations on both. Of course one way to make it easier when beginning is to recognise something in the sky, locate it on the map and then use that as a starting and reference point.
There is of course one major problem in using a star map of this simplicity, it is only good for a very specific time and date of the year. This is due to mainly two reasons. Firstly, everything in the sky appears to move throughout the night and secondly, from night to night they seem to rise earlier then the previous night. This gradually causes the sky to change each night until after a year they are back to rising at the same time on the same date again.
The first motion is due entirely to the fact that the earth is rotating. Although it may appear that the stars are moving and we are staying still it is really the other way around. But since we can't feel that we are moving, from our point of view it looks like the stars move. So, over the course of one day the stars will do one complete rotation about the sky. This means that the sky will appear different depending on the time of night you look.
The second motion has to do with the motion of the earth around the sun. We use the sun in order to keep time and measure the length of the day. The average time from noon to noon is 24 hours. But if we measured the length of the day by using the stars we would find that the day was only 23 hours and 56 minutes (approximately) long. This discrepancy comes about because as the earth rotates once about its axis, it has also moved a little in its orbit around the sun. This means that if we were to look at the sun overhead at noon on one day, then in order for the sun to again appear overhead the earth has to rotate once plus a little bit more. Since we tend to keep track of the time using our watches that are based on this noon to noon time (24 hours) what we find is that the stars, whose motion is based on the actual rotation of the earth once about its axis, seem to rise a little earlier (4 minutes) each night. Eventually, after the earth has done one complete lap around the sun the stars appear to come back to the same spot at the same time (as measured by our watches). So each night the stars shuffle a little bit further across the sky until in 365 days they're back to where they started. Incidentally 24 hours divided by 365 is 4 minutes. This means that we also need to take into account with our star maps what time of the year it is that we are looking.
	In order to get around this problem there are a number of things you could do. One would be to have an extremely large number of maps for each night of the year and a number of times throughout each night. Obviously not a very practical thing to do. The best solution is to use something known as a planisphere. This is a star map that can be altered to show the stars visible from a specific latitude on any date and at any time of the night.
	Both the simple star maps and the planisphere will show you roughly what can be seen in the sky but by necessity they are not very detailed. To the human eye it is possible to see thousands of individual stars. It would be completely impractical to put this number onto such a small piece of paper.
There exist however more detailed maps that are readily available. These come in various forms and complexity. Some are only marginally more detailed than a planisphere, others are extremely detailed and show only a small section of the sky on a page. Most of these are known as atlases although there are also a number of books around that treat each constellation by itself, providing a detailed map of each along with information about things to be found within its boundaries.
Once you have determined (by using the planisphere) what constellations are visible in the sky at the time you are observing and roughly what stars make up the constellation then is possible to go to the more detailed map in the atlas. This will allow you to work out exactly what stars you can see in the sky that make up the constellation picture.