year

Unit of time measurement, based on the orbital period of the Earth around the Sun. The tropical year (also called equinoctial and solar year), from one spring equinox to the next, lasts 365.2422 days (nearly 3651/4 days). It governs the occurrence of the seasons, and is the period on which the calendar year is based. Every four years is a leap year, when the four quarters of a day are added as one extra day. A year on Mercury is only 88 days; a year on Mars is 23 months.

The sidereal year is the time taken for the Earth to complete one orbit relative to the fixed stars, and lasts 365.26 days (about 20 minutes longer than a tropical year). The difference is due to the effect of precession, which slowly moves the position of the equinoxes. The anomalistic year is the time taken by any planet in making one complete revolution from perihelion to perihelion; for the Earth this period is about five minutes longer than the sidereal year due to the gravitational pull of the other planets. The calendar year consists of 365 days, with an extra day added at the end of February each leap year. Leap years occur in every year that is divisible by four, except that a century year is not a leap year unless it is divisible by 400. Hence 1900 was not a leap year, but 2000 was.