Equinox (celestial coordinates): Difference between revisions

new section and some stubbish sub-sections -- more to be added later
(further clarification)
(new section and some stubbish sub-sections -- more to be added later)
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>
 
== Motion 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= |publisher=Alfred A. Knopf |location=New York |isbn=0-679-40852-5 |page=53 }}</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 oscialltaion of the nutation is 18.6 years.
 
==Besselian equinoxes and epochs==