When many people look for known objects in the sky at night, they can often find what they are looking for by first identifying patterns in the stars, and working out from there where they need to look. However, for more detailed work it is necessary to some universal system for saying where things are in the sky; something that can be used by everyone.
Astronomers use something called the Equatorial Co-ordinate System. You can see in the animation below that the Equatorial Co-ordinate System surrounds the Earth and that from the point of view of an observer on the Earth the stars and other celestial bodies can all have their positions identified by using the grid. This makes it very easy to say where an object is at any one time.
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(approx.2.6MB)
In the image below, you can see an asteroid (called Harmonia) labelled in the centre. Around this you can see a number of stars, and the Equatorial Co-ordinate System is shown as a grid of red lines. To find the position of the asteroid, astronomers would refer to a star catalogue that tells them the exact positions of the stars in the grid, and from that information they would know where to put the grid lines for the Equatorial Co-ordinate System. Once they know the locations of the stars, they are able to work out the position of the asteroid.
The units of measurement for the Equatorial Co-ordinate System are Right Ascension and Declination (often shortered to R.A. and Dec.), and these are the co-ordinates you will be using to report the positions of the asteroids you identify in this activity.
Stars can be used in this way because they appear to stay in relatively fixed positions over a long period of time, and provide a useful method for working out the positions of objects that move with relative to the grid. Most of these objects are much closer to the Earth and are within our Solar System.
