What are Eclipses, and how do they Work?

For those of you that don't know, here are the 2 main types of eclipse you can see:

Solar Eclipses

Solar eclipses, in case you don't know, are events where the moon travels between the earth and sun.

A diagram of a typical solar eclipse, taken from http://www.earthview.com

Although the sun and moon appear to be the same size in the sky, they are NOT!! The sun is 400 times bigger than the moon! "Then why are they the same size in the sky?" you ponder. The reason is that by some AMAZING coincidence, the sun is also 400 times further away from us than the moon is. Hey, get this! If the moon's diameter were just 140 miles smaller, the moon would be too small to cast a total eclipse... no one would see one! Ever! But it's NOT 140 miles smaller. It's just the right size and distance to allow us to see this marvellous occurrence.

Solar eclipses are quite rare, so when one does occur, it's a very special event (Maybe not as special as Christmas, but special nonetheless). The more alert ones among you will have realised that the moon takes 27-and-a-bit days to orbit the earth, so you're probably wondering why there isn't an eclipse every 27-and-a-bit days. You should use this model idea to help you understand: take a round object to represent the sun (a tennis ball or orange) and put it on a flat table. Place a smaller object about 10 cm away to represent the earth (a cherry or grape) and roll it round the 'sun'. This is how earth behaves in space. It goes around the sun as if on a an invisible (not to mention very large) table. You now need an even smaller object (a pea for instance) to represent the moon. Put it about 2-3 cm away from the 'earth' and roll the moon round it. Still don't get it? OK, the fact is that the moon doesn't roll around on our large invisible table. The moon's orbit is tilted by about 5 degrees. This slant will send the moon below the table at some points, and above at others. In fact, there are only 2 places in it's orbit that put it 'on the table' as it were. These points are called NODES, and only when the moon is at a node can an eclipse have a chance of taking place. If these nodes stayed in the same place above earth all the time, we as a whole race would virtually never see an eclipse at all! But as the earth journeys round the sun and rotates on it's axis, the moon's orbit rotates with it. The nodes move about, which increases the chances of eclipses vastly. (please don't ask me the details- I'm only 15!)