J2000.0 equinox

J2000.0 equinox is a standard direction (or equivalently, an adopted specific point on the celestial sphere) that is currently used as a reference in astronomy, being the position of the vernal equinox (March equinox) as of epoch J2000.0 (the equinoxes undergo a slow, constant shift, i.e., the precession of the equinoxes). The current standard equatorial coordinate system (i.e., with coordinates right ascension and declination) are defined such that coordinates J000000+000000 represent this standard direction. Before J20000.0 equinox was adopted, earlier such standard equinoxes were used (similarly based upon earlier standard epochs), each equinox adopted for reports/catalogs occurring near in time to the standard epoch it was based upon. For example, prior to standardization of the J2000.0 equinox, the B1950.0 equinox (analogously based on epoch B1950.0) was used as the standard. The J2000.0 vernal equinox in the B1950.0 coordinates is approximately B235726.23391-001641.2260, a shift of roughly 40 arcminutes. Conversely, the B1950.0 vernal equinox in J2000.0 coordinates is approximately J000233.76588+001642.2244.