Starguide - A Practical Guide to Astro-Photography

An equinox (in astronomy) is one of two intersections of the ecliptic with the celestial equator where 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.

The equinox moves because of perturbing forces, and 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. 

Astronomical objects show real movements such as orbital and proper motions, and the epoch defines the date for which the position of an object applies. Therefore, a complete specification of the coordinates for an astronomical object requires both the date of the equinox and of the epoch.

The currently used standard equinox and epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch.

Reference:

[1] Equinox article in Wikipedia

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