[AS.] The time which the earth takes to go round the sun. The tropical year is the interval between two successive passages of the sun through the first point of Aries. Its mean length is 365 days 5 hrs. 48 min. 49.7 sec. Owing to the precession of the equinoxes it is shorter than the sidereal year, which is the interval which elapses between the time of the earth's leaving a given point in its orbit and the time of its returning to it. It consists of 365 days 6 hrs. 9 min. 9.6 sec. The order of the seasons is determined by the tropical year. The year is accounted 365 days in length, except every fourth year, which is named leap year and has 366 days. As this would make the average year rather too long, the last year of the century is not leap year unless its unit number can be divided by four. Thus 1900 was not leap year, but 2000 will be.