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physical phenomena > event > celestial event > periodic celestial event > equinox

equinox
subject			fact

equinox	has period 1 year
	has definition One of two points in the sky that represent where the Sun appears to cross the plane of the Earth's equator. From the Earth's viewpoint therefore, the Sun reaches one point at a quarter, the other at three quarters of the way through the sidereal year: the vernal (spring) equinox is thus on or around 21 March, the autumnal on or around 22 September. The actual points in the sky change slightly every year through a process called precession.
	has definition Either of the two points on the celestial sphere where the celestial equator intersects the ecliptic.
	has definition Either of the two points on the celestial sphere at which the ecliptic intersects the celestial equator; also the time at which the Sun passes through either of these intersection points; i.e., when the apparent longitude (see apparent place; celestial longitude) of the Sun is 0° or 180°. (See catalog equinox; dynamical equinox for precise usage.)
	is a kind of periodic celestial event
event	has duration
	has synonym world point
	has time of occurrence

Kinds of equinox :

• autumnal equinox (2 facts) - See equinox
• vernal equinox (6 facts) (First Point of Aries, spring equinox) - The ascending node of the ecliptic on the celestial equator; also the time at which the apparent longitude (see apparent place; longitude, celestial) of the Sun is 0°., The point of intersection between the ecliptic and the celestial equator, where the Sun crosses from south to north. It is sometimes called the First Point of Aries because several thousand years ago it was in Aries. Because of precession it has now slid west into Pisces and in 200-300 years it will edge into Aquarius. By definition, the vernal equinox is at α = 0°, δ = 0°.