After I spent a few weeks of my life making a calendar application and thus suddenly understanding some of the complexities regarding the measurement of time, I have always been a bit curious about the different aspects of this. Consider this, the earth rotates to make a day, but at the same time it orbits the sun. So in order for the sun to reach what we call "high noon" again it rotates slightly more than 360 degrees. Yes you read that correctly. It rotates more than 360 degrees for each of our days. So what is then a year? Phil Plait explains it with pictures. Enjoy!