Equinox (celestial coordinates): Difference between revisions

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{{About|the celestial coordinate system|the moment when the Sun is positioned directly over Earth's equator|Equinox}}
{{Use mdy dates|date=AprilMarch 20142019}}
In [[astronomy]], '''equinox''' is either of two places on the [[celestial sphere]] at which the [[ecliptic]] intersects the [[celestial equator]].<ref name='aa2019'>{{cite book |last= |first= |authorlink= |title=Astronomical Almanac for the Year 2019 |url= |accessdate= |year=2018 |publisher=United States Naval Observatory |location=Washington, DC |isbn=978-0-7077-41925 |page=M6 }}</ref><ref>{{cite book |last=Barbieri |first=Cesare |authorlink= |title=Fundamentals of Astronomy |url= |accessdate= |year=2007 |publisher=Taylor and Francis Group |location=New York |isbn=978-0-7503-0886-1 |page=31 }}</ref><ref>{{cite web |url=https://syrte.obspm.fr/iauWGnfa/NFA_Glossary.html |title=IAU Nomenclature for Fundamental Astronomy |last1= |first1= |last2= |first2= |date=2007 |website=Paris Observatory |publisher= |accessdate=December 23, 2018 }}</ref> Although there are two intersections of the ecliptic with the celestial equator, by convention the equinox associated with the sun's [[ascending node]] is used as the origin of celestial coordinate systems and referred to simply as ''the'' equinox. In contrast to the common usage of [[spring equinox|spring]] and [[autumn equinox|fall]], or vernal and autumnal, equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time.
The equinox moves because of perturbing forces, therefore in order to define a coordinate system it is necessary to specify the date for which the equinox is chosen. This date should not be confused with the [[epoch (astronomy)|epoch]]. Astronomical objects show real movements such as [[orbit|orbital]]al and [[proper motion]]s, and the epoch defines the date for which the position of an object applies. Therefore, a complete specification of the coordinates for an astronomical objects requires both the date of the equinox and of the epoch.<ref>{{cite book |editor-last=Seidelmann |editor-first=P. Kenneh |authorlink= |title=Explanatory Supplement to the Astronomical Almanac |url= |accessdate= |year=1998 |publisher=University Science Books |location=Mill Valley, CA |isbn=978-0-935702-68-2 |page=12 }}</ref>
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[[File:Equinox path.png|thumb|right|upright=1.4|The precession of the equinox]]
The equinox moves, in the sense that as time progresses it is in a different location with respect to the distant stars. Consequently, star catalogs over the years, even over the course of a few decades, will list different [[ephemerides]]. <ref>{{cite book |last=Chartrand |first=Mark R. |authorlink= |title=The Audubon Society Field Guide to the Night Sky |url= |accessdate= |year= 1991|publisher=Alfred A. Knopf |location=New York |isbn=978-0-679-40852-9 |page=53 |bibcode=1991asfg.book.....C }}</ref> This is due to precession and nutation, both of which can be modeled, as well as other minor perturbing forces which can only be determined by observation and are thus tabulated in astronomical almanacs.
=== Precession ===
Precession of the equinox was first noted by [[Hipparchus]] in 129 BC, when noting the location of [[Spica]] with respect to the equinox and comparing it to the location observed by [[Timocharis]] in 273 BC.<ref>{{cite book |last=Barbieri |first=Cesare |authorlink= |title=Fundamentals of Astronomy |url= |accessdate= |year=2007 |publisher=Taylor and Francis Group |location=New York |isbn=978-0-7503-0886-1 |page=71 }}</ref> It is a long term motion with a period of 25,800 years.
=== Nutation ===
Nutation is the oscillation of the ecliptic plane. It was first observed by [[James Bradley]] as a variation in the declination of stars. Because he did not have an accurate enough clock, Bradley was unaware of the effect of nutation on the motion of the equinox along the celestial equator, although that is in the present day the more significant aspect of nutation.<ref>{{cite book |last=Barbieri |first=Cesare |authorlink= |title=Fundamentals of Astronomy |url= |accessdate= |year=2007 |publisher=Taylor and Francis Group |location=New York |isbn=978-0-7503-0886-1 |page=72 }}</ref> The period of oscillation of the nutation is 18.6 years.
==Equinoxes and epochs==
==Sidereal time and the equation of the equinoxes==
[[Sidereal time]] is the [[hour angle]] of the equinox. However, there are two types: if the mean equinox is used (that which only includes precession), it is called mean sidereal time; if the true equinox is used (the actual location of the equinox at a given instant), it is called apparent sidereal time. The difference between these two is known as the equation of the equinoxes, and is tabulated in Astronomical Almanacs.<ref>{{cite book |last= |first= |authorlink= |title=Astronomical Almanac for the Year 2019 |url= |accessdate= |year=2018 |publisher=United States Naval Observatory |location=Washington, DC |isbn=978-0-7077-41925 |page=B21–B24,M16 }}</ref>
A related concept is known as the equation of the origins, which is the arc length between the [[Celestial Intermediate Origin]] and the equinox. Alternatively, the equation of the origins is the difference between the [[Earth Rotation Angle]] and the apparent sidereal time at Greenwich.
==Diminishing role of the equinox in astronomy==
In modern astronomy the ecliptic and the equinox are diminishing in importance as required, or even convenient, reference concepts. (The equinox remains important in ordinary civil use, in defining the seasons, however.) This is for several reasons. One important reason is that it is difficult to be precise what the ecliptic is, and there is even some confusion in the literature about it.<ref>{{cite book |last=Barbieri |first=Cesare |authorlink= |title=Fundamentals of Astronomy |url= |accessdate= |year=2007 |publisher=Taylor and Francis Group |location=New York |isbn=978-0-7503-0886-1 |page=74 }}</ref> Should it be centered on the Earth's center of mass, or on the Earth-Moon barycenter?
Also with the introduction of the International Celestial Reference Frame, all objects near and far are put fundamentally in relationship to a large frame based on very distant fixed radio sources, and the choice of the origin is arbitrary and defined for the convenience of the problem at hand, There are no significant problems in astronomy where the ecliptic and the equinox need to be defined.<ref>{{cite book |last1=Capitaine |first1=N. |last2=Soffel |first2=M. |chapter=On the definition and use of the ecliptic in modern astronomy |authorlink= |title=Proceedings of the Journées 2014 "Systèmes de référence spatio-temporels": Recent developments and prospects in ground-based and space astrometry |year=2015 |publisher= |location= |isbn=978-5-9651-0873-2 |pagepages=61–64 |arxiv=1501.05534 }}</ref>
==External links==
*[https://web.archive.org/web/20090712234011/http://csep10.phys.utk.edu/astr161/lect/time/coordinates.html Celestial Coordinate System, UTK]
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