A year is the length of time that it takes for a planet, satellite, or other celestial body to complete one orbit around the Sun (or in the case of extrasolar planets, around their star). For comparison, the orbital period of the Moon around the Earth is related to the month.
The exact length of a year depends on which reference point is used for the orbital motion. For the Earth, the following alternatives exist:
|Reference point||Type of year||Length of year|
|The fixed stars||Sidereal period||365.25636 mean solar days|
|The equinoxes||Tropical year||365.24219 days|
|The perihelion position||Anomalistic year||365.25964 days|
Note that none of these three years are exactly the same length – all are slightly longer than the calendar year of 365 days, and shorter than the leap year of 366 days.
For the other planets in the Solar System, the approximate length of a year (measured in Earth tropical years) is:
|Planet||Length of Year|