Year

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Year

The period of the Earth’s revolution around the Sun, or

of the apparent motion of the Sun on the ecliptic. It may

be defined in a number of ways, each of which leads to a

slightly different value:

Sidereal year. The time interval during which the Sun

apparently completes one revolution of the celestial sphere

relative to the stars (which, for this purpose, are regarded

as being fixed in space). This is equal to the revolution

period of the Earth around the Sun as measured relative

to the stars, and is equivalent to 365.2564 mean solar days.

Tropical year. The time interval between two

successive passages of the Sun through the vernal equinox.

Its length is 365.2422 mean solar days, about 20 minutes

shorter than the sidereal year. The difference arises

because of the effects of precession. As this definition of

the year is related to the recurrence of the seasons, the term

‘year’, if unqualified, is generally taken to mean ‘tropical

year’.

Anomalistic year. The interval between two

successive passages of the Earth through the perihelion

of its orbit which, because of a slow change in the position

of perihelion, is not quite the same as the sidereal year. Its

length is 365.2596 mean solar days.

Gregorian calendar year. This is the value of the year

adopted for calendar purposes, and is equal to 365.2425

mean solar days. For practical purposes it can be taken as

equal to the tropical year (the difference amounts to 0.0003

mean solar days).