The days are getting shorter. Here in South Georgia, most of us welcome the Sun’s shortening reign of heat. On September 22 at 9:31 a.m., the Sun will be directly over the equator on its journey further south. It is the Fall or Autumnal Equinox for us, which marks the official beginning of Autumn in the Northern Hemisphere and the beginning of Spring in the Southern Hemisphere.
The term Equinox is derived from the Latin aequus (equal) and nox (night). At the equinoxes, days and nights are of equal duration. We experience two equinoxes, the Autumnal Equinox in September and the Vernal Equinox in March when the Sun crosses above the Equator again on its northern travel. Antithetical to the equinoxes are the solstices. The Summer Solstice marks the longest day and the Winter Solstice marks the longest night.
This travel of the Sun northward or southward is of course an illusion, just as we imagine the Sun rises and sets when actually the Earth turns daily under the Sun. As the Earth turns, its axis is tilted 23.5 degrees with respect to the plane of our annual orbit around the Sun. That tilt remains pointed in the same direction in space as we orbit the Sun. In Summer, the tilt is toward the Sun in the Northern Hemisphere and our days are longer. Six months later, we're tilted away from the Sun and our days are shorter. This causes the seasons.
Besides equal duration days and nights, during the equinoxes, the Sun rises due East and sets due West. This is true everywhere on Earth and is easily recognized, so many ancient cultures marked East and West with stone or adobe structures. The Sun rising due East over these marks on the Vernal Equinox signaled the beginning of the new year in many cultures. Many cultures today still recognize the Vernal Equinox as the beginning of a new year, rather than January 1.
For us, the Autumnal Equinox officially marks the seasonal promise of milder days. But later as we tire of the winter months and long for warmth, we will welcome the Vernal Equinox.
A couple of nights after the equinox, on September 24, the Moon, Jupiter, and Saturn will be close together in a line in the south after dark. The following night the Moon will have moved east, and the triple will form a triangle. You can’t “see” the equinox, but the trio will be a sight.