Equinox (celestial coordinates): Difference between revisions

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In [[astronomy]], '''equinox''' is a moment when the [[vernal equinox|vernal]] point, [[celestial equator]], and other such elements are taken to be used in the definition of a [[celestial coordinate system]]. The position at other [[equinox]]es can be computed by taking into account [[precession]], [[nutation]], and [[aberration of light|aberration]], which directly affect e.g. [[right ascension]] and [[declination]].
 
Contrastingly, ''[[epoch (astronomy)|epoch]]'' is a moment for when a given position of an [[astronomical object]] is valid. The position at other epochs can be computed by taking into account [[proper motion]], [[parallax]], and/or [[orbital elements]].
 
In the case of orbital elements for objects within the [[Solar System]], only a few of the classical orbital elements are affected by a switch of equinox: the [[longitude of the ascending node]] and (to a much lesser extent) the [[orbital inclination|inclination]]. If another set of orbital elements are used, such as the position and velocity vectors for a particular epoch, all components can be affected by a switch of equinox.
 
Equinox is often confused with [[epoch (astronomy)|epoch]], with the difference between the two being that the equinox addresses changes in the coordinate system, while the epoch addresses changes in the position of the celestial body itself. The currently used standard equinox (and epoch) is [[#J2000.0|J2000.0]], which is January 1, 2000 at 12:00 [[Terrestrial Time|TT]]. The prefix "J" indicates that it is a [[#Julian epochs|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]] (ICRS) coordinate system (which is essentially{{clarify}} 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:
* [[NASA]]/[[North American Aerospace Defense Command|NORAD]]'s [[Two-line elements]] format with fractional day: 00001.50000000
 
==Besselian equinoxes/ and epochs==
A Besselian epoch, named after the German mathematician and astronomer [[Friedrich Bessel]] (1784–1846), is an epoch that is based on a ''Besselian year'' of 365.242198781 days, which is a [[tropical year]] measured at the point where the [[Sun]]'s longitude is exactly 280°. Since 1984, Besselian equinoxes and epochs have been superseded by [[#Julian epochs|Julian equinoxes and epochs]]. The current standard equinox and epoch is [[#J2000.0|J2000.0]], which is a Julian epoch.
 
Besselian epochs are calculated according to:
Since the [[right ascension]] and [[declination]] of stars are constantly changing due to [[precession]], astronomers always specify these with reference to a particular equinox. Historically used Besselian equinoxes include B1875.0, B1900.0, B1925.0 and B1950.0. The official constellation boundaries were defined in 1930 using B1875.0.
 
==Julian equinoxes/ and epochs==
A Julian epoch is an epoch that is based on [[Julian year (astronomy)|Julian years]] of exactly 365.25 days. Since 1984, Julian epochs are used in preference to the earlier Besselian epochs.
 
Since the [[right ascension]] and [[declination]] of stars are constantly changing due to [[precession]], (and, for relatively nearby stars due to [[proper motion]]), astronomers always specify these with reference to a particular epoch. The earlier epoch that was in standard use was the [[#B1950.0|B1950.0]] epoch.
 
When the ''mean'' equator and equinox of J2000 are used to define a celestial reference frame, that frame may also be denoted J2000 coordinates or simply J2000. Technically, thisThis is different from, but similar to, the [[International Celestial Reference System]] (ICRS): the mean equator and equinox at J2000.0 are distinct from and of lower precision than ICRS, but agree with ICRS to the limited precision of the former. Use of the "mean" locations means that [[nutation]] is averaged out or omitted. NovicesThis are sometimes confused by findingmeans that the Earth's rotational North pole does not point quite at the J2000 celestial pole at the epoch J2000.0; the reason is that the true pole of epoch nutates away from the mean one. The same differences pertain to the equinox.<ref name="Equinoj2000">{{cite journal |title=Rotation matrix from the mean dynamical equator and equinox at J2000.0 to the ICRS|doi=10.1051/0004-6361:20031552|author1=Hilton, J. L. |author2=Hohenkerk, C. Y.| journal=Astronomy & Astrophysics | volume=413 | pages=765–770 |year=2004 | bibcode=2004A&A...413..765H}}</ref>
 
The "J" in the prefix indicates that it is a Julian equinox/ or epoch rather than a Besselian equinox/ or epoch.
 
==References==
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