The time taken for Earth to go once around the Sun. In astronomy, different kinds of year are distinguished by the reference point used to measure the period of revolution. A sidereal year is the time taken by Earth to make one complete circuit round the celestial sphere as seen from the Sun. (Or, equivalently, the time for the Sun to make one complete trip against the background stars as seen from the center of Earth.) A tropical year (or solar year) is the interval between successive vernal equinoxes. Because the equinoxes have an annual retrograde motion (due to precession), of 50.26" relative to the stars, the tropical year is about 20 minutes shorter than the sidereal year. An anomalistic year is the interval between successive passages of Earth through perihelion or aphelion. An eclipse year is the time between successive returns of the Sun to the same node of the Moon's orbit. This period is keyed to the regular recurrence of both solar and lunar eclipses, which can only take place when the Sun and Moon are close to the node. Nineteen eclipse years are 6585.78 days which is almost exactly the same as the ancient Saros cycle of 6585.32 days – the period that separates eclipses in a given series. A lunar year is made of 12 lunations or synodic months (354.3672 days). A civil year has an exact number of days, which is determined by the calendar being used.
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