Equinox (celestial coordinates): Difference between revisions

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Equinox is often confused with epoch,{{According to whom|date=December 2018}} with the difference between the two being that the equinox specifies a coordinate system, while the epoch is an arbitrary time reference agreed to by international standards.<ref name='aa2019' /> The currently used standard equinox and epoch is J2000.0, which is January 1, 2000 at 12:00 [[Terrestrial Time|TT]]. The prefix "J" indicates that it is a Julian epoch. The previous standard equinox and epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch. Before 1984 Besselian equinoxes and epochs were used. Since that time Julian equinoxes and epochs have been used.<ref name="Equinoccicc">{{cite book |url=https://books.google.com/books?id=WDjJIww337EC&pg=PA20&lpg=PA20&dq=julian+epoch+equinox&source=bl&ots=p8s-ilXgiV&sig=Y7PYY-JtJ0537ELO8BLJ7nNKjHk&hl=ca&ei=Cv1aSt3LH4_QjAex4bQb&sa=X&oi=book_result&ct=result&resnum=3 |title=Astronomy on the Personal Computer, p. 20 |accessdate=July 13, 2007 |publisher=[[Google books]]}}</ref>
 
Other equinoxes and epochs that have been used include:
* The [[Bonner Durchmusterung]] started by [[Friedrich Wilhelm August Argelander]] uses B1855.0
* The [[Henry Draper Catalog]] uses [[#B1900.0|B1900.0]]
* [[Constellation]] boundaries were defined in 1930 along lines of [[right ascension]] and [[declination]] for the B1875.0 epoch.
* Occasionally, non-standard equinoxes have been used, such as B1925.0 and B1970.0
* The [[Hipparcos Catalog]] uses the [[International Celestial Reference System]] (ICRS) coordinate system (which is essentially{{clarify|date=January 2018}} equinox J2000.0) but uses an epoch of J1991.25. For objects with a significant [[proper motion]], assuming that the epoch is J2000.0 leads to a large position error. Assuming that the equinox is J1991.25 leads to a large error for nearly all objects.<ref>{{cite journal | bibcode = 1997A&A...323L..49P | author=Perryman, M.A.C. | display-authors=etal | title=The Hipparcos Catalogue | journal= Astronomy & Astrophysics | volume=323| pages=L49-L52|year=1997}}</ref>
 
Epochs and equinoxes for orbital elements are usually given in [[Terrestrial Time]], in several different formats, including:
* [[Gregorian Calendar|Gregorian]] date with 24-hour time: 2000 January 1, 12:00 TT
* Gregorian date with fractional day: 2000 January 1.5 TT
* [[Julian day]] with fractional day: JDT 2451545.0
* [[NASA]]/[[North American Aerospace Defense Command|NORAD]]'s [[Two-line elements]] format with fractional day: 00001.50000000
 
==Besselian equinoxes and epochs==
 
The "J" in the prefix indicates that it is a Julian equinox or epoch rather than a Besselian equinox or epoch.
 
== Other equinoxes and their corresponding epochs ==
 
Other equinoxes and epochs that have been used include:
* The [[Bonner Durchmusterung]] started by [[Friedrich Wilhelm August Argelander]] uses B1855.0
* The [[Henry Draper Catalog]] uses [[#B1900.0|B1900.0]]
* [[Constellation]] boundaries were defined in 1930 along lines of [[right ascension]] and [[declination]] for the B1875.0 epoch.
* Occasionally, non-standard equinoxes have been used, such as B1925.0 and B1970.0
* The [[Hipparcos Catalog]] uses the [[International Celestial Reference System]] (ICRS) coordinate system (which is essentially{{clarify|date=January 2018}} equinox J2000.0) but uses an epoch of J1991.25. For objects with a significant [[proper motion]], assuming that the epoch is J2000.0 leads to a large position error. Assuming that the equinox is J1991.25 leads to a large error for nearly all objects.<ref>{{cite journal | bibcode = 1997A&A...323L..49P | author=Perryman, M.A.C. | display-authors=etal | title=The Hipparcos Catalogue | journal= Astronomy & Astrophysics | volume=323| pages=L49-L52|year=1997}}</ref>
 
Epochs and equinoxes for orbital elements are usually given in [[Terrestrial Time]], in several different formats, including:
* [[Gregorian Calendar|Gregorian]] date with 24-hour time: 2000 January 1, 12:00 TT
* Gregorian date with fractional day: 2000 January 1.5 TT
* [[Julian day]] with fractional day: JDT 2451545.0
* [[NASA]]/[[North American Aerospace Defense Command|NORAD]]'s [[Two-line elements]] format with fractional day: 00001.50000000
 
==References==