What is equinox?
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equinox, either of the two moments in the year when the Sun is exactly above the equator and day and night are of equal length; also, either of the two points in the sky where the ecliptic (the Sun’s annual pathway) and the celestial equator intersect. The vernal equinox, marking the beginning of spring in the Northern Hemisphere, occurs about March 21, when the Sun moves north across the celestial equator. The autumnal equinox falls about September 23, as the Sun crosses the celestial equator going south.
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'sequator. 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, the night and day have approximately 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, thevernal 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.
The equinoxes are also 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.
Anequinoxis a day when the sun is in itszenithover theEquator, and the Earth experiences days and nights of generally equal length. There are two equinoxes in a year: around March 21 and around Sept. 22.In theNorthern Hemisphere, March 21 is the spring orvernal equinox. September 22 is the autumnal equinox. Many people are starting to use the terms March equinox and September equinox, instead of vernal, which means spring, and autumnal, which means autumn or fall. March and September are spring and fall in the Northern Hemisphere, but it's the opposite in theSouthern Hemisphere. The Southern Hemispheres spring equinox takes place in autumn.Equinoxes are the only times of the year when thesubsolar pointis directly on the Equator. The subsolar point is an area where the sun's rays shineperpendicularto the Earth's surface—aright angle. Only during an equinox is the Earth's 23.5-degreeaxisnot tilting toward or away from the sun: the center of the sun is in the sameplaneas the Equator.Mostplanets experience equinoxes. On thegas giantSaturn, equinoxes are particularly dramatic. Saturn's spectacular ring system extends along the same plane as the planet's equator. Although the rings extend thousands of kilometers into space, they are actually very thin, only about a kilometer wide. During Saturn's equinoxes, the rings (and Saturn's equator) line up perfectly with the sun. Photos taken from the solar perspective reveal the rings as a razor-thin line.
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as the sun rays directly fall on the equator on 21 march and 23 sep , on these two days, the days and nights are equal all over the world . march 21 is known as vernal equinox or spring equinox in the n. hemisphere and autmn equinox in the s. hemisphere . sep 23 is autumn equinox in the n . hemisphere and spring equinox in the s. hemisphere