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An equinox occurs twice a year (around 20 March and 22 September), when the plane of Earth's equator passes the center of the Sun. At this time the tilt of the Earth's axis is inclined neither away from nor towards the Sun. 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) andecliptic intersect. These points of intersection are called equinoctial points: classically, the vernal point (RA = 00h 00m 00sand 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 (the place on Earth's surface where the center of the Sun is exactly overhead) is on the Equator. The subsolar point crosses the Equator moving northward at the March equinox and moving southward at the September equinox. (Since the sun's ecliptic latitude is not exactly zero it is not 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 perpendicular to Earth's equator. Thus the Northern and Southern hemispheres are illuminated equally. (At the solstices, that angle reaches its minimum of 66.5°, corresponding to 90° minus Earth's axial tilt).
Another meaning of equinox is the date when day and night are the same length. The equinox is not exactly the same as the day when day and night are of equal length for two reasons. Firstly, because of the size of the sun, the top of the disk rises above the horizon (constituting 'sunrise' which is the start of 'daytime') when the center of the disk is still below the horizon. Secondly, Earth's atmosphere refracts sunlight which means that an observer can experience light (daytime) even before the first glimpse of the sun's disk has risen above the horizon. To avoid this ambiguity the term equilux is sometimes used in this sense.[note 1] Times of sunset and sunrise vary with an observer's location (longitude and latitude), so the dates when day and night are of exactly equal length likewise depend on location. For places near the equator the daytime is always longer than the night, so they would never experience an equinox by this definition.
Diagram of the Earth'sseasons as seen from the north. Far right: December solstice.