Calendopaedia - Astronomical Calendar
Definition of a Year
The most common definition in the western world of the year is based on the
revolution of the Earth around the Sun and is therefore called a `Solar Year'.
However, there are several possibilities to define beginning and end of one
revolution and thus also several kinds of solar years:
- A tropical year is the interval between two successive
passages of the centre of the Sun through the mean vernal equinox
and lasts 365.242199 days UT. The name refers to the
changes of seasons (greek `tropai', the turning points)
which are fixed in this kind of year. It is for this
reason that the tropical year is of great importance
in the construction of calendars. The length of the tropical
year is a matter for debate and is discussed on this page.
- A sidereal year is the time required for the earth to complete an
orbit of the sun relative to the stars. It lasts 365.256366 days UT.
- An anomalistic year is the interval between two
successive passages of the Earth through the perihelion
(the point closest to the Sun) of its orbit and it lasts
365.259636 days UT.
The years so defined differ in length because of the precession of Earth's
rotation and the tumbling of the Earth orbit.
The Julian year (365.25 days UT) and the
Gregorian year (365.2425 days UT) as defined
in the calendars of the respective name are solar years as well.
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