A sidereal year (, ; ), also called a sidereal orbital period, is the time that

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

or another

planetary body A planetary-mass object (PMO), planemo, or planetary body (sometimes referred to as a world) is, by geophysical definition of planet, geophysical definition of celestial objects, any celestial object massive enough to achieve hydrostatic equilib ...

takes to orbit the Sun once with respect to the fixed 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 the

ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...

. It equals for the

J2000.0 In astronomy, an epoch or reference epoch is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body, as they are subject to pe ...

epoch, or a little over 366 sidereal days. The sidereal year differs from the solar year, "the period of time required for the ecliptic longitude of the Sun to increase 360 degrees", due to the

precession of the equinoxes In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's Rotation around a fixed axis, rotational axis. In the absence of precession, the astronomical body's orbit would show ...

. The sidereal year is 20 min 24.5 s longer than the mean tropical year at J2000.0 . At present, the rate of

axial precession In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In parti ...

corresponds to a period of 25,772 years, so sidereal year is longer than

tropical year A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky – as viewed from the Earth or another celestial body of the Solar System – thus completing a full cycle of astronom ...

by 1,224.5 seconds (20 min 24.5 s, ~365.24219*86400/25772). Before the discovery of the precession of the equinoxes by

Hipparchus Hipparchus (; , ; BC) was a Ancient Greek astronomy, Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the equinoxes. Hippar ...

in the

Hellenistic period In classical antiquity, the Hellenistic period covers the time in Greek history after Classical Greece, between the death of Alexander the Great in 323 BC and the death of Cleopatra VII in 30 BC, which was followed by the ascendancy of the R ...

, the difference between sidereal and tropical year was unknown to the Greeks. For naked-eye observation, the shift of the constellations relative to the equinoxes only becomes apparent over centuries or " ages", and pre-modern calendars such as

Hesiod Hesiod ( or ; ''Hēsíodos''; ) was an ancient Greece, Greek poet generally thought to have been active between 750 and 650 BC, around the same time as Homer.M. L. West, ''Hesiod: Theogony'', Oxford University Press (1966), p. 40.Jasper Gr ...

's ''

Works and Days ''Works and Days'' ()The ''Works and Days'' is sometimes called by the Latin translation of the title, ''Opera et Dies''. Common abbreviations are ''WD'' and ''Op'' for ''Opera''. is a didactic poem written by ancient Greek poet Hesiod around ...

'' 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. The Indian national calendar, based on the works of Maga Brahmins, as are the calendars of neighbouring countries, is traditionally reckoned by the Sun's entry into the sign of Aries and is also supposed to align with the spring equinox and have relevance to the harvesting and planting season and thus the tropical year. However, as the entry into the constellation occurs 25 days later, according to the astronomical calculation of the sidereal year, this date marks the South and Southeast Asian solar New Year in other countries and cultures

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* * {{Portal bar, Physics, Astronomy, Stars, Spaceflight, Outer space, Solar System, Science Astronomical coordinate systems Types of year Hindu astrology Technical factors of astrology ru:Сидерический год

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