YEAR

time required for the Earth to travel once around the Sun, about 365 1/4 days. This fractional number makes necessary the periodic intercalation of days in any calendar that is to be kept in step with the seasons. In the Gregorian calendar a common year contains 365 days, and every fourth year (with a few exceptions) is a leap year of 366 days. In astronomy, several kinds of year are distinguished, having slightly different lengths. The solar year (365 days 5 hours 48 minutes 46 seconds), also called tropical year, or year of the seasons, is the time between two successive occurrences of the vernal equinox (the moment when the Sun apparently crosses the celestial equator moving north). Because of the precession of the equinoxes (an effect of a slow wobble in the Earth's rotation), the solar year is shorter than the sidereal year (365 days 6 hours 9 minutes 10 seconds), which is the time taken by the Sun to return to the same place in its annual apparent journey against the background of the stars. The anomalistic year (365 days 6 hours 13 minutes 53 seconds) is the time between two passages of the Earth through perihelion, the point in its orbit nearest the Sun. A lunar year (used in some calendars) of 12 synodic months (12 cycles of lunar phases) is about 354 days long. A cosmic year is the time (about 225 million years) needed for the solar system to revolve once around the centre of the Milky Way Galaxy.