Glossary Yy


YEAR - Length of time required for a planet, satellite, or other celestial body to complete one orbit around the Sun (or in the case of extra-solar planets, around their star). The exact length of a year depends on which reference point is used for the orbital motion. For the Earth, the following alternatives exist: sidereal period (referenced to fixed stars) of 365.25636 mean solar days; tropical year (referenced to the equinoxes) of 365.24219 days; and anomalistic year (referenced to the perihelion position) of 365.25964 days.