Question

what is an Equinox? Explain long.

Answer 1

An equinox is a day of the year when day and night are exactly the same length: 12 hours each. They happen each year on March 20, called the vernal equinox in the Northern Hemisphere, and September 22, called the autumnal equinox in the Northern Hemisphere.
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Answer 2

An equinox is commonly regarded as the moment when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun,[2] which occurs twice each year: around 20 March and 22–23 September. In other words, it is the point at which the center of the visible Sun is directly above the Equator.

However, because the Moon (and to a lesser extent the other planets) cause the true motion of the Earth to vary from a perfect ellipse, the equinox is now officially defined by the Sun's more regular ecliptic longituderather than latitude. The instants of the equinoxes are currently defined to be when the longitude of the Sun is 0° and 180°.[3]There are tiny (up to 1¼ arcsecond) variations in the Sun's latitude (discussed below), which means the Sun's center is rarely precisely over the equator under the official definition. The two understandings of the equinox can lead to discrepancies of up to 69 seconds.

On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the Sun and atmospheric refraction. The word is derived from the Latin aequinoctium, from aequus (equal) and nox (genitive noctis) (night).