Equinox (astronomy)
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From the Glossary of Astronomical Terms by the Harvard-Smithsonian Center for Astrophysics:
Either of the two points (vernal, autumnal) on the celestial sphere where the ecliptic (which is the apparent path of the sun on the sky) intersects the celestial equator. Due to precession, this point moves over time, so positions of stars in catalogues and on atlases are usually referred to a "mean equator and equinox" of a specified standard epoch.
Note that epoch is often confused with equinox when applied to astronomical positions. In nearly all cases where epoch is used, equinox is what is really meant.
In the standard equatorial coordinate system used to specify positions on the celestial sphere, the right ascension is measured eastward from the vernal equinox. The location of the equinox changes slowly with time because of precession of the equinoxes, making a full revolution in approximately 25000 years. This is a motion of the earth, rather than the distant stars, so the equatorial coordinates of a distant star (that lies in a fixed direction in space) will change with time. Thus astronomers refer to a set of coordinates as being measured with respect to the "equator and equinox" of a particular date. In modern work, coordinates are almost always referred to a standard moment in time at the beginning of 2000. This instant is called J2000, which is Julian date 2451545.0. The phrase "equator and equinox" is often shortened to "equinox". In this context it is meant to refer to the date on which the coordinates used actually lined up with the equator and equinox of the earth, leading to idioms such as "equinox J2000".
In contrast, epoch simply means the date the observation was made. The distinction becomes clearer when considering the case of a star with a large proper motion. The position of such a star with respect to other stars will change significantly with time. An observation of a fast-moving star taken in 2007 would reveal its position at epoch 2007, but could be referred to equinox J2000 -- the J2000 refers only to the coordinate system used to describe the location, not the actual location the star.
[edit] External Sources
- Badastronomy.com Epoch v. Equinox Discussion
- Equinox from Eric Weisstein's World of Astronomy
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