Equinox (celestial coordinates)

Therefore, a complete specification of the coordinates for an astronomical object requires both the date of the equinox and of the epoch.

[5] 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.

[6] 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.

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 the right ascension and declination of stars are constantly changing due to precession, astronomers always specify these with reference to a particular equinox.

Alternatively, the equation of the origins is the difference between the Earth Rotation Angle and the apparent sidereal time at Greenwich.

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.)

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.

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.