From Glossary of Meteorology
- The period during which the earth completes one revolution around the sun.
This has several interpretations, as follows. 1) Sidereal year: time of true revolution around the sun, that is, the time it takes the earth (as seen from the sun) to reappear at the same fixed star, equal to 365.2564 mean solar days or 365 days, 6 hours, 9 minutes, 10 seconds; 2) Tropical year (
also called mean solar year, ordinary year): the time measured from one vernal equinox to the next, that is, the apparent revolution of the sun through the zodiac, equal to 365.2422 mean solar days or 365 days, 5 hours, 48 minutes, 46 seconds (this is not constant but decreases by about 5 seconds in one thousand years.); and 3) Calendar year: fixed by the Gregorian calendar of 365 days in ordinary years and 366 days in leap years.