Sidereal year: Difference between revisions
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{{Short description|Time taken by the Earth to orbit the Sun once with respect to the fixed stars}} | {{Short description|Time taken by the Earth to orbit the Sun once with respect to the fixed stars}} | ||
{{Use mdy dates|date=October 2025|cs1-dates=ll}} | |||
A '''sidereal year''' ({{IPAc-en|s|aɪ|ˈ|d|ɪər|i|.|əl}}, {{IPAc-en|USalso|s|ɪ|-}}; {{etymology|la|{{wikt-lang|la|sidus}}|asterism, star}}), also called a '''sidereal orbital period''', is the time that [[Earth]] or another [[planetary-mass object|planetary body]] takes to orbit the [[Sun]] once with respect to the [[fixed stars]]. | A '''sidereal year''' ({{IPAc-en|s|aɪ|ˈ|d|ɪər|i|.|əl}}, {{IPAc-en|USalso|s|ɪ|-}}; {{etymology|la|{{wikt-lang|la|sidus}}|asterism, star}}), also called a '''sidereal orbital period''', is the time that [[Earth]] or another [[planetary-mass object|planetary body]] takes to orbit the [[Sun]] once with respect to the [[fixed stars]]. | ||
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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]]. | 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 {{Nowrap|{{val|365.256363}} [[ephemeris day]]s}} (365 days, 6 hours, 9 minutes and 9.8 seconds).{{Sfn|AA|2024|pp=C2, L9}} | |||
The sidereal year differs from the [[tropical year]], "the period of time required for the ecliptic longitude of the Sun to increase 360 degrees",{{Sfn|AA|2016|loc=s.v. year, tropical}} due to the [[Axial precession|precession of the equinoxes]]. | |||
The sidereal year is 20 min 24.7 s longer than the mean tropical year {{Nowrap|({{val|365.242189}} ephemeris days)}}, or 365 days, 5 hours, 48 minutes, 45.1 seconds.{{Sfn|AA|2024|pp=C2, L9}} | |||
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>{{Cite web |date=March 3, 2025 |title=Why Sothic Dates Are Crucial to Egyptian Chronology (But Not as Traditionally Understood) – Synchronologies |url=https://synchronologies.com/2025/03/03/why-sothic-dates-are-crucial-to-egyptian-chronology-but-not-as-traditionally-understood/ |access-date=August 9, 2025 |language=en-US}}</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>{{Cite web| last = Stern| first = David P.| title = Precession| work = NASA's Polar, Wind and Geotail Site| accessdate = April 25, 2022| date = October 10, 2016| url = https://pwg.gsfc.nasa.gov/stargaze/Sprecess.htm}}</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>{{Cite web| last = Stern| first = David P.| title = Precession| work = NASA's Polar, Wind and Geotail Site| accessdate = April 25, 2022| date = October 10, 2016| url = https://pwg.gsfc.nasa.gov/stargaze/Sprecess.htm}}</ref> | ||
For naked-eye observation, the shift of the constellations relative to the equinoxes only becomes apparent over centuries or "[[Astrological age|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|date=August 2020}} | For naked-eye observation, the shift of the constellations relative to the equinoxes only becomes apparent over centuries or "[[Astrological age|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|date=August 2020}} | ||
==Historic estimates== | |||
Historic estimates have been offered by the likes of:<ref>{{Cite book |last=Barnett |first=Lionel D. |url=https://books.google.com/books?id=LnoREHdzxt8C&pg=PA193 |title=Antiquities of India: An Account of the History and Culture of Ancient Hindustan |date=April 30, 1999 |publisher=Atlantic Publishers & Dist |isbn=978-81-7156-442-2 |language=en}}</ref><ref>{{Cite web |title=Hipparchus - Biography |url=https://mathshistory.st-andrews.ac.uk/Biographies/Hipparchus/ |access-date=August 4, 2025 |website=Maths History |language=en}}</ref> | |||
* Egyptian Sothic (c. 1800 BC) - 365 days, 6 hours | |||
* [[Hipparchus]] (c. 190 – c. 120 BC) - 365 + {{sfrac|1|4}} + {{sfrac|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 (Ayurveda book) |Rājamṛgāṅka]] (c. 1050 AD) - 365 days, 6 hours, 12 minutes, 30.915 seconds. | |||
== See also == | == See also == | ||
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== Works cited == | == Works cited == | ||
*{{Cite book |title=Astronomical Almanac for the Year 2025 |date=2024 |publisher=[[United States Naval Observatory]], HM Nautical Almanac Office |location=Washington, D.C., and Taunton |ref={{SfnRef|AA|2024}} }} | |||
*{{Cite book |title=Astronomical Almanac for the Year 2017 |date=2016 |chapter=Glossary |page=M19 |publisher=[[United States Naval Observatory]], HM Nautical Almanac Office |location=Washington, D.C., and London |ref={{SfnRef|AA|2016}}}} | *{{Cite book |title=Astronomical Almanac for the Year 2017 |date=2016 |chapter=Glossary |page=M19 |publisher=[[United States Naval Observatory]], HM Nautical Almanac Office |location=Washington, D.C., and London |ref={{SfnRef|AA|2016}}}} | ||
{{Time topics}} | {{Time topics}} | ||
Latest revision as of 20:06, 11 November 2025
Template:Short description Template:Use mdy dates
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:Val ephemeris days (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:Val ephemeris days), 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.[1]
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.[2] 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.Script error: No such module "Unsubst".
Historic estimates
Historic estimates have been offered by the likes of:[3][4]
- 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
Notes
Works cited
- Script error: No such module "citation/CS1".
- Script error: No such module "citation/CS1".
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