Sidereal year

A sidereal year is the time Earth takes to complete one orbit around the Sun relative to the distant stars, about 365.256 days.

Tropical year: The interval between successive passages of the Sun through the vernal equinox, about 365.242 days. Precession makes this seasonal year shorter than the sidereal year.
Earth's orbit: The path Earth follows around the Sun, shaped primarily by their mutual gravitational attraction. One completed revolution along this orbit supplies the sidereal year’s cycle.
Earth: The third planet from the Sun and the only known world with abundant surface liquid water and life. The sidereal year measures this planet’s revolution around the Sun.
Sidereal time: A timekeeping system based on Earth’s rotation relative to distant stars rather than the Sun. Its daily cycle uses the same stellar reference that defines the sidereal year.
Anomalistic year: The interval between successive perihelion passages of Earth, about 365.259 days. Earth’s orbit shifts its perihelion, giving this year a different length.
Fixed stars: Distant stars that appear nearly stationary relative to one another over short astronomical timescales. Their apparent background provides the reference against which Earth’s orbital return is defined.
Astronomical unit: A unit of length defined as exactly 149,597,870,700 meters, close to the mean Earth–Sun distance. It expresses the scale of the orbit whose completion defines the year.
Astrometry: The measurement of the positions and motions of celestial objects. Accurate stellar positions help establish the reference frame for orbital measurements.
Calendar year: A year as defined by a calendar system, commonly 365 or 366 days in the Gregorian calendar. Calendar years approximate seasonal cycles rather than measuring Earth’s orbit against the stars.
Precession: The slow change in the orientation of Earth’s rotation axis and orbital reference directions. Precession shifts equinoxes relative to the stars and separates the sidereal year from the tropical year.

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