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From the Astronomical Dictionary

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The description of the word you requested from the astronomical dictionary is given below.

the equinox

aequinoctium = [Latin] equinox, from aequatio = [Latin] make equal, and noctium, nox = [Latin] night; plural equinoxes

The equinox is

  1. the moment when the Sun crosses the celestial equator. Close to an equinox, day and night have nearly the same length (12 hours) everywhere on Earth. The equinoxes signal the beginning of the seasons of spring and autumn (where those are relevant). The vernal or ascending equinox occurs in March (in the Gregorian calendar) and marks the beginning of spring in the northern hemisphere and of autumn in the southern hemisphere. The autumnal or descending equinox occurs in September and marks the beginning of spring in the southern hemisphere and of autumn in the northern hemisphere. The beginning of the other seasons is governed by the solstices.
  2. the place in the sky between the stars where the Sun is during the vernal equinox. This location is also called the vernal equinox. In the equatorial and ecliptic coordinate systems, the vernal equinox has longitude and latitude equal to zero.

    Because of the precession of the equinoxes, the equinox slowly moves between the stars, so when one quotes ecliptic or equatorial coordinates, one has to indicate relative to which equinox these coordinates are measured. Three equinoxes that are commonly used in stellar atlases and planetary calculations are those of 1950.0 (the beginning of the year 1950), 2000.0 (the beginning of the year 2000), and the equinox of the date (i.e., the equinox of the same date as the coordinates themselves).