Asidereal year (/saɪˈdɪəri.əl/,US also/sɪ-/; from Latin sidus 'asterism, star'), also called asidereal orbital period, is the time thatEarth or anotherplanetary body takes to orbit theSun once with respect to thefixed stars.
Hence, for Earth, it is also the time taken for the Sun to return to the same position relative to Earth with respect to the fixed stars after apparently travelling once around theecliptic.
In 2025, the sidereal year equals365.256363ephemeris days (365 days, 6 hours, 9 minutes and 9.8 seconds).[1]
The sidereal year differs from thetropical year, "the period of time required for the ecliptic longitude of the Sun to increase 360 degrees",[2] due to theprecession of the equinoxes. The sidereal year is 20 min 24.7 s longer than the mean tropical year(365.242189 ephemeris days), or 365 days, 5 hours, 48 minutes, 45.1 seconds.[1]
Ancient Egypt was aware their year and the sidereal year differed. This led to their development of theSothic cycle in the second millennium BC that completed on theheliacal rising of the starSirius on the new year, which offers a pseudo-sidereal year of just over 365 days and 6 hours.[3]
Before the discovery of the precession of the equinoxes byHipparchus in theHellenistic period, the difference between sidereal and tropical year was unknown to the Greeks.[4] For naked-eye observation, the shift of the constellations relative to the equinoxes only becomes apparent over centuries or "ages", so pre-modern calendars, such asHesiod'sWorks and Days, would give the times of the year for sowing, harvest, and so on by reference to the first visibility of stars, effectively using the sidereal year.[citation needed]
Historic estimates have been offered by the likes of:[5][6]
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