### What Is a Year

I don't often just post a link to other people's articles, but this is one that's so interesting and relevant to today, that that's what I'm going to do.

Today is February 29th, a day that only occurs every four years. If you know a bit about how the solar system works, you probably think you understand the reason for a leap year - the Earth's orbit around the Sun is just a little over 365 days, so we need an extra day every so often to get the calendar back in sync with the Earth's orbit. But in fact, that's not the full story. The measurement we use for the length of a year is a bit more complicated than that, and doesn't actually result in the Earth being at quite the same position relative to the Sun year after year. To understand why, go read this article from Phil Plait at Bad Astronomy:

Another orbit? Why, you don't look a rotation older than 4.56 billion years!

And if you want to know a bit more about leap years, and why they occur every four years, but not on some years divisible by 100, but on years divisible by 400, then go read this article of his: