equinox

Although not a purely meteorological term, it is habitually referred to in Meteorology and Climatology texts. It has its etymological origin in the Latin word aequinoctium, arising from the fusion of the Latin words aequus (equal) and noctem, nox (night), from which it follows that it corresponds to the day of the year on which the length of day and night are equal. There are two equinoxes each year, and on each of those days the sun rises exactly in the east and sets in the west. In the northern hemisphere, the spring or vernal equinox usually falls on March 20 or 21, and the autumn equinox on September 22 or 23. This correspondence is reversed in the southern hemisphere. Both equinoxes and solstices mark the changes of the four astronomical seasons into which the year is divided.