Year, a period of time well known within and near the temperate zones of the earth as that in which the four seasons run through their course, and indicated upon all parts of the earth's surface by the apparent return of the sun at midday to the same position in the heavens, as from its place at our summer or winter solstice forth and back to the same place again, the length of which period corresponds nearly to the time of 365¼ diurnal revolutions of the earth (i. e., days). For the astronomical principles that determine or explain many of the points in relation to the year, see Astronomy, Sun, Moon, Precession of the Equinoxes, and Nutation. - The year, as just denned, or that in which the sun, from having its place over either tropic, moves to the other and returns, or (what is the same thing) starting from the equator at the vernal equinox of our hemisphere performs its complete circuit to the vernal equinox again, is termed the tropical year; but it will be convenient, and is more instructive, to define the year in general terms as the period in which the earth completes the circuit of her orbit around the sun. Like all such intervals, the year varies in length according to the way in which it is measured.

If we consider the earth's path without reference either to its shape or to the earth's own figure, we must refer her motions to the sun as centre and to the surrounding star sphere. Supposing a line drawn always to the earth's centre from the sun and prolonged to the star sphere, this line would travel round like the hand of a mighty dial; and the time in which it would complete one circuit is called the sidereal year. This period may therefore be defined as the interval between the successive returns of the earth to the same heliocentric position among the fixed stars, or the period in which the earth viewed from the sun's centre would appear to complete the circuit of the ecliptic. The sidereal year is not absolutely constant, because the earth is exposed to the perturbing influence of the other planets. Its mean value is 365d. 6h. 9m. 9.6s. Whether, apart from perturbations, the sidereal year is undergoing a secular change of length, is a question as yet undecided; certainly any such change must be exceedingly' minute. - But instead of referring the earth's motion to the star sphere, we may consider it with reference to the shape of the earth's orbit.

This orbit has two axes, for example, and either extremity of either axis might be considered as a starting point from which the year might be measured; so that we might measure a year as the interval between successive passages of the perihelion, or of the aphelion, or of mean distance following perihelion, or of mean distance following aphelion. Any one of these periods might be called the anomalistic year, because its beginning would be counted from the time when the anomaly either vanished or had its maximum value. In practice, however, the term is limited to the year measured from the perihelion. Thus the anomalistic year is the interval separating successive passages by the earth of the perihelion of her orbit. As the perihelion advances, the earth, after completing a circuit from perihelion to the same heliocentric longitude, has still to pass over the arc by which perihelion has advanced in the interval. Accordingly the anomalistic year exceeds the sidereal year; its mean length at present is 365d. 6h. 13m. 48.6s. It may perhaps appear strange to have the mean length at present spoken of, instead of the absolute mean; but the motion of the perihelion is so irregular, and passes through so many varying conditions in the oourse of long intervals of time, that we must be content to consider its present general rate of advance.

It may be added that the interval between successive passages of the perihelion is not, as might be supposed, equal in length to the interval between successive passages of the aphelion. This is easily shown. The perihelion is at present advancing at the rate of 11.24" per annum; so that, neglecting the minute gain while the earth is traversing this arc, the actual anomalistic year, estimated from the perihelion, exceeds the sidereal year by the time occupied by the earth in traversing an arc of 11.24" with her perihelion motion of 1° 1'9.9' per diem. Now if the anomalistic year were estimated from aphelion, the same would be true, only the earth's daily aphelion motion of only 57' 115" must be substituted. Since, with the more rapid motion, the arc 11.24" would be traversed in less time, the anomalistic year estimated from the perihelion is slightly shorter than the anomalistic year estimated from aphelion. The length of the mean anomalistic year, however, is at present that above indicated, and therefore it follows that at present the actual interval between the earth's successive passages of perihelion is slightly shorter than the mean anomalistic year. - Thirdly, the year may be measured with reference to the earth's figure and position.

For example, the plane of the earth's equator cuts the plane of the ecliptic in a straight line, which is carried round with the earth, moving almost exactly parallel to itself. Twice in each circuit, therefore, this line passes through the sun's centre, and the year may be measured from one or other of these epochs. One corresponds to the vernal equinox, the other to the autumnal equinox. Astronomers have selected the former to measure from, and the year thus measured is called the tropical year, or year of seasons, and is defined as the interval between the earth's successive passages of her vernal equinox. Since this point (when the earth viewed from the sun is at = and the sun viewed from the earth is at ) retrogrades, the tropical year is less 'than the sidereal year. Its mean length is 365d. 5h. 48m. 48.6s. As in the case of the anomalistic year, the actual length of the tropical year depends on the point from which it is measured; for the earth's diurnal motion at the passage of her vernal equinox is not precisely equal to her motion at the passage. of her autumnal equinox. Thus the length of the tropical year is not absolutely constant.

Its length at present exceeds by nearly 13 sec. the length which it had in the time of Hipparchus. In like manner the length of the anomalistic year is slowly variable. The lengths of the four seasons, astronomically measured, are very nearly as follows: from the vernal equinox to the summer solstice, 92d. 22¾ h.; from the latter to the autumnal equinox, 93d. 13½h.; from this to the winter solstice, 89d. 16¾h.; from this to the vernal equinox, 89d. l½h. - For an account of the years and calendars of different nations, see Calendar, and Chronology.