[1] The time it takes for a body in orbit around a star to complete a revolution around the star (this movement is called translation). The definition of what is a complete revolution creates different definitions of a year:
  • The Sidereal Year is the time it takes to return to the same direction in space.
  • If the orbit is elliptical, we can define an Anomalistic Year: the time between two passes at the perihelion. If the planet rotates around itself, and the rotational axis is inclined relative to the plane of the orbit, then the planet has seasons.
  • If a planet has seasons, a Tropic Year is the time between the beginning of one season and the next occurrence of same season (for example, the time from the beginning of one Winter and the start of the next Winter).
These three kinds of years are usually very close to one another. Also note that the duration of the year in days changes with time, due to the tidal friction. Due to the large size of Earth's moon, rotational deceleration has been particularly pronounced on Earth.

[2] A Standard year, the orbital period of Old Earth as used in the widespread Tranquility Calendar.
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    The distance light travels in one standard or metric (Earth) year, viz. 9.46x1012 km. Standard unit of measurement. It is equal to 6.323x104 astronomical units (AU) or 0.307 parsecs.
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Development Notes
Text by Trent Shipley
Initially published on 15 December 2001.