Equinox (celestial coordinates): Difference between revisions

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* [[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: