A year on earth is measured by one complete trip around the sun. Seems simple enough but there is a problem. The earth doesnt travel in a path around the sun that returns it to its starting point. So how do we know when the year starts or ends?
[youtube=425,350]http://www.youtube.com/watch?v=IhqzW97_47w[/youtube]
I admit I feel asleep halfway through this, but you might enjoy it.