Equinox

An equinox occurs twice a year (around 20 March and 22 September), when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, night and day are about equal length.

At an equinox the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect. These points of intersection are called equinoctial points: classically, the vernal point (RA = 00h 00m 00s and longitude = 0º) and the autumnal point (RA = 12h 00m 00s and longitude = 180º). By extension, the term equinox may denote an equinoctial point.

The equinoxes are the only times when the subsolar point is on the Equator. This point (the place on the Earth's surface where the center of the Sun can be observed exactly overhead) crosses the Equator moving northward at the March equinox and crosses the Equator moving southward at the September equinox. (Since the sun's ecliptic latitude isn't exactly zero it isn't exactly above the equator at the moment of the equinox, but the two events usually occur less than 30 seconds apart.)

The equinoxes are the only times when the terminator is inclined 90° to the Earth's Equator (while at solstices, that inclination reaches its minimum of 66.5°, corresponding to 90° minus Earth's axial tilt).

Another meaning of equinox is the date at which day and night are of equal length. Because times of sunset and sunrise, unlike the phenomenon described in preceding paragraphs, vary with an observer's geographic location (longitude and latitude), these dates likewise depend on location and do not exist for locations sufficiently close to the Equator. To avoid this ambiguity, the term equilux is sometimes used in this sense.