equinox

The term equinox indicates any point-in-time within the year when day and night are of equal length (12 hours). There are two such equinoxes each year. The term equinox is also used for two directions, specifically, the two points on the celestial sphere in the directions along the line through Sun and Earth at the precise time of the year's equinoxes (as above). These equinox-directions slowly shift (precession of the equinoxes), those of each successive year pointing to slightly different locations within the celestial sphere. The term equinox also has a specific technical usage in astronomy: a selected such point-in-time, whose associated direction has been standardized for use defining the axes of various celestial coordinate systems.

Twice during the year, the geometry of the Earth's orbit and axial tilt are such that day and night are of equal length throughout the Earth. These are points in time when the Sun is exactly overhead at solar noon on some point on the Earth's equator. In a simplified geometry (assuming Earth and Sun are spherical), it occurs when a line through the centers of the Sun and Earth pass through the Earth's equator. Due to the tilt of the Earth's rotation compared to its orbit, and the fact that the Earth's poles (axis of rotation) continue to point to (roughly) the same spot in space, this happens twice a year. The two equinoxes are termed:

• vernal equinox - a point in time (and the associated direction) when the Sun moves northward across the celestial equator, by our calendar always on or near March 20. Other names are the March equinox, the vernal point, and the first point of Aries,
• autumnal equinox - similarly, on or near September 22, when the Sun analogously moves southward across the celestial equator. It is also known as the September equinox.

The precession of the equinoxes constitutes a precession of the Earth's axial tilt, a shifting of the direction that the Earth's poles point and of the equatorial plane (and the celestial equator), shifting the equinoxes roughly 50 arcseconds per year, too small to be obvious in a casual view of the sky, but substantial to astrometry. The shift is (roughly) constant but slow and at any given time, a current direction of the equinox can be calculated.

Some celestial coordinate systems use these directions as axes for designating points in the celestial sphere, one being the vernal equinox, and to specify a location in the sky, you have to specify not only the coordinate system and the coordinates, but which equinox you are tying them to. This is done by citing a point in time (date, hour, minute, and second). It does not have to be one of the two annual equinoxes because at any point in time, there is a calculated equinox-direction based upon the direction of Earth's poles at that moment. The phrase "coordinates using the calculated equinoxes as of January 1 2000", is generally abbreviated to "coordinates equinox J2000.0", aka J2000.0 equinox. (J2000.0 refers to the time January 1, 2000, 11:58:55:816 UTC). Both the ecliptic coordinate system and the equatorial coordinate system depend upon the time of equinox, specifically the vernal point, which is the direction the Sun is seen at the time of the vernal equinox, which is used as the basis for ecliptic longitude in the ecliptic coordinate system of right ascension in the equatorial coordinate system.