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 harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...

or another planetary body takes to orbit the Sun once with respect to the

fixed stars In astronomy, fixed stars ( la, stellae fixae) is a term to name the full set of glowing points, astronomical objects actually and mainly stars, that appear not to move relative to one another against the darkness of the night sky in the backgro ...

. 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 the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agai ...

. It equals for the J2000.0 epoch. The sidereal year differs from the

solar 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 of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time ...

, "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 rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In particu ...

. 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 particu ...

corresponds to a period of 25,772 years, so sidereal year is longer than tropical year 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 (; el, Ἵππαρχος, ''Hipparkhos''; BC) was a 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 equ ...

in the

Hellenistic period In Classical antiquity, the Hellenistic period covers the time in Mediterranean history after Classical Greece, between the death of Alexander the Great in 323 BC and the emergence of the Roman Empire, as signified by the Battle of Actium in ...

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

ages Ages may refer to: *Advanced glycation end-products, known as AGEs *Ages, Kentucky, census-designated place, United States * ''Ages'' (album) by German electronic musician Edgar Froese *The geologic time scale, a system of chronological measuremen ...

", and pre-modern calendars such as

Hesiod Hesiod (; grc-gre, Ἡσίοδος ''Hēsíodos'') was an ancient Greek poet generally thought to have been active between 750 and 650 BC, around the same time as Homer. He is generally regarded by western authors as 'the first written poet i ...

's ''

Works and Days ''Works and Days'' ( grc, Ἔργα καὶ Ἡμέραι, Érga kaì Hēmérai)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 ...

'' 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 The Indian national calendar, sometimes called the Saka calendar, is a solar calendar that is used alongside the Gregorian calendar by ''The Gazette of India'', in news broadcasts by All India Radio, and in calendars and official communications ...

, 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 The traditional New Year in many South and Southeast Asian cultures is based on the sun's entry into the constellation Aries. In modern times, it is usually reckoned around the 14th of April. Origins As most countries and cultures of South and So ...

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

See also

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