Just as the day is a unit of time based on Earth's rotation, the year is based on the revolution of Earth around the sun. Long before people understood that Earth was in motion, however, they still observed that the sun travelled through the sky relative to the stars, and that it took one year to return to the place it started.

The time it takes Earth to orbit the sun is one year, but as you might have already guessed, with so many definitions for something as simple as the length of the day, defining the length of the year is just as complex and depends on which reference point you use.

If we measure the time it takes the sun to circle the sky once and return to the same position relative to the distant stars, it takes 365 days, 6 hours, 9 minutes, and 10 seconds. This is the sidereal year.

The sidereal year, however, is not the most useful measurement for observing the sky because it does not allow for a phenomenon called precession.