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.

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 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 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, 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.^[3]

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.^[4] 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.^{[citation needed]}

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.^{[citation needed]} 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 ^{[citation needed]}

Historic estimates

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

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