The revolution of the earth around the sun is how we define the year. A year is the time it takes to make one revolution - a little over 365 days.
We all learn in grade school that the planets move at differing rates around the sun. While earth takes 365 days to make one circuit, the closest planet, Mercury, takes only 88 days. Poor, ponderous, and distant Pluto takes a whopping 248 years for one revolution.
This is an interesting little calculator that tells you your age on different planets in our solar system, I am only 2 years old on Jupiter, and 14 on Mars...oh, those were the days...
