Equinox (celestial coordinates): Difference between revisions

equation of the equinoxes
(new section and some stubbish sub-sections -- more to be added later)
(equation of the equinoxes)
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 oscialltaion of the nutation is 18.6 years.
==Besselian equinoxesEquinoxes and epochs==
===Besselian equinoxes and epochs===
A Besselian epoch, named after 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 Since 1984, Besselian equinoxes and epochs have been superseded by [[#Julian epochs|Julian equinoxes and epochs]].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.
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.
The standard equinox and epoch currently in use are [[#J2000.0|J2000.0]], which corresponds to January 1, 2000 12:00 [[Terrestrial Time]].
The J2000.0 epoch is precisely [[Julian date]] 2451545.0 TT ([[Terrestrial Time]]), or January 1, 2000, noon TT. This is equivalent to January 1, 2000, 11:59:27.816 [[International Atomic Time|TAI]] or January 1, 2000, 11:58:55.816 [[UTC]].
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:
* [[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
==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.