Sidereal year

A sidereal year (Template:IPAc-en, Template:IPAc-en; Template:Etymology), 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.

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.

In 2025 the sidereal year equals Template:Nowrap (365 days, 6 hours, 9 minutes and 9.8 seconds).Template:Sfn

The sidereal year differs from the tropical year, "the period of time required for the ecliptic longitude of the Sun to increase 360 degrees",Template:Sfn due to the precession of the equinoxes. The sidereal year is 20 min 24.7 s longer than the mean tropical year Template:Nowrap, or 365 days, 5 hours, 48 minutes, 45.1 seconds.Template:Sfn

Ancient Egypt was aware their year and the sidereal year differed, and developed the Sothic cycle in the second millennium BC, the cycle 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.<ref>Template:Cite web</ref>

Before the discovery of the precession of the equinoxes by Hipparchus in the Hellenistic period, the difference between sidereal and tropical year was unknown to the Greeks.<ref>Template:Cite web</ref> 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'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.Template:Citation needed

Historic estimates

Historic estimates have been offered by the likes of:<ref>Template:Cite book</ref><ref>Template:Cite web</ref>

• Egyptian Sothic (c. 1800 BC) - 365 days, 6 hours
• Hipparchus (c. 190 – c. 120 BC) - 365 + Template:Sfrac + Template:Sfrac 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

• Anomalistic year
• Gaussian year
• Julian year (astronomy)
• Orbital period
• Precession § Astronomy
• Sidereal time
• Solar calendar
• Tropical year
• Mars time

Works cited

• Template:Cite book
• Template:Cite book