year

year The Earth's period of orbital revolution around the Sun, and hence by extension the orbital period of any planet. Astronomically the Earth's year can be defined in several ways. The actual orbital period with reference to the fixed stars is the sidereal year, 365.256 36 days. However, the positions of the stars are gradually changing because of precession. The interval between successive passages through the mean equinox is termed the tropical year, 365.242 19 days, which takes precession into account. This is the most commonly adopted definition of the year as it is the one that relates directly to seasonal changes. A further form of year is the anomalistic year, which is the average interval between successive passages of the Earth through the perihelion point of its elliptical orbit, 365.259 64 days. This differs slightly from the sidereal year due to small perturbations of the Earth's orbit by the gravitational influence of the other planets. Another definition of the year in astronomical use is the eclipse year of 346.620 03 days. This is the average interval between successive passages of the Sun through a node of the Moon's orbit. See also calendar year.