A year is a division of time defined basically by the period of revolution of the Earth about the Sun. Various kinds of astronomical
years and
calendar years have been defined. The astronomical year of chief importance is the tropical year, which is the time interval between successive occurrences of the spring equinox. Its length is now about 365.2422 mean solar, or civil, days, the number decreasing very slowly as a result of small, progressive
changes in the Earth's rotational speed and its orbit about the Sun. The seasons repeat, on the average, in this interval. The sidereal year, determined by the Earth's position with respect to the stars, is 365.25636 mean solar days. It is longer than the tropical year because it is not subject to the shortening effects of precession.The Gregorian solar calendar contains 365 days in a common year and 366 in a leap year; the average is 365.2425. Attempts to establish a better leap-year rule than the Gregorian are of little value because of uncertain changes in the Earth's speed of rotation.The number of days in a year in a religious calendar may vary widely. The Jewish lunar-solar calendar has from 353 to 355 in a common year and 383 to 385 in a leap year in a 19-year cycle. The Moslem lunar calendar, which does not have leap years, has either 354 or 355 days in a year. About 33.5 Moslem years equal 32.5 Gregorian years.