# What is the difference between a sidereal year and a tropical year? How is each calculated?

Mar 7, 2016

Sidereal year is for revolution of the Earth referred to stars. Tropical year is the period between two successive (same) equinox instants.

#### Explanation:

We have an equinox instant, once in six months, nearly. The two are Vernal Equinox and Autumnal Equinox. Vernal Equinox year is March 21 to March 20, nearly. This is tropical year = 365.2421871 days. The little longer sidereal year = 365.2563630 days.

At the equinox instant, the Sun is right overhead, at noon, at some longitude of the equator.

Equinox instant changes every year, due to precession of the equinox, by 20 m 23 s, nearly. The angular precession rate is (360/25800) deg/year = 50"/year, nearly.

Equinox rotation corresponds to Earth's spin-axis' rotation, about a mean position, through 360 deg, in a Great year of nearly 280 centuries

The tropical year is relevant to the seasons.

We can refer to a solstice instead, for defining tropical year.
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In astronomical parlance quite many terms are used here
For more details about each, refer to wiki..
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