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Sidereal year

A sidereal year (/saɪˈdɪəri.əl/, US also /sɪ-/; from Latin sidus 'asterism, star'), also called a sidereal orbital period, is the time that Earth or another planetary body takes to orbit the Sun once with respect to the fixed stars.

Thus, a sidereal year is the interval required for Earth to travel once around the ecliptic, completing one revolution about the Sun and returning to the same orbital position relative to the background stars.

In 2025, the sidereal year equals 365.256363 ephemeris days (365 days, 6 hours, 9 minutes and 9.8 seconds).[1]

The sidereal year differs from the tropical year, "the period of time required for the ecliptic longitude of the Sun to increase 360 degrees",[2] due to the precession of the equinoxes. The sidereal year is 20 minutes 24.7 seconds longer than the mean tropical year (365.242189 ephemeris days), or 365 days, 5 hours, 48 minutes, 45.1 seconds.[1]

Ancient Egyptian astronomer-priests recognized that their 365-day civil year did not remain aligned with the stellar year. This led to their development of the Sothic cycle in the second millennium BC that completed on the heliacal rising of the star Sirius 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 by Hipparchus in the Hellenistic period, the difference between the sidereal and tropical year was unknown to the Greeks.[4] Because the shift of the constellations relative to the equinoxes is perceptible only over centuries, pre-modern calendars, such as Hesiod's Works 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

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Historic estimates have been offered by the likes of:[5][6]

  • Egyptian Sothic (c. 1800 BC) - 365 days, 6 hours
  • Hipparchus (c. 190 – c. 120 BC) - 365 + ⁠1/4⁠ + ⁠1/144⁠ days (= 365.25694... days = 365 days 6 hours 10 min).
  • Paulisa Siddhanta (c. 300 AD) - 365 days, 6 hours, 12 minutes, 36 seconds.
  • Romaka Siddhanta (c. 300 AD) - 365 days, 5 hours, 55 minutes, 12 seconds (tropical year).
  • Surya Siddhanta (c. 500-800 AD) - 365 days, 6 hours, 12 minutes, 30.36.56 seconds.
  • Aryabhatiya (c. 510 AD) - 365 days, 6 hours, 12 minutes, 30 seconds.
  • Rājamṛgāṅka (c. 1050 AD) - 365 days, 6 hours, 12 minutes, 30.915 seconds.

See also

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Look up sidereal year in Wiktionary, the free dictionary.

Notes

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  1. ^ a b AA 2024, pp. C2, L9.
  2. ^ AA 2016, s.v. year, tropical.
  3. ^ "Why Sothic Dates Are Crucial to Egyptian Chronology (But Not as Traditionally Understood) – Synchronologies". March 3, 2025. Retrieved August 9, 2025.
  4. ^ Stern, David P. (October 10, 2016). "Precession". NASA's Polar, Wind and Geotail Site. Retrieved April 25, 2022.
  5. ^ Barnett, Lionel D. (April 30, 1999). Antiquities of India: An Account of the History and Culture of Ancient Hindustan. Atlantic Publishers & Dist. ISBN 978-81-7156-442-2.
  6. ^ "Hipparchus - Biography". Maths History. Retrieved August 4, 2025.

References

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Sources

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