September 20–26

The Sun goes south

Those of you living in the northern climes know it already. The evenings are calm but edged with a decided chill, the smell of burning leaves permeates the air, and the Sun is setting earlier and earlier. Summer is definitely on the wane and autumn is at the door, where, if you can't yet feel it in the air, you can sense it in the changing sky.

The autumnal equinox, which marks the first day of autumn, occurs the moment the Sun crosses the celestial equator as it moves to the south. This usually occurs on September 23, though it can also occur a few hours earlier on September 22. A similar situation occurs around March 21, when the Sun heads north across the equator marking the beginning of spring. We call that event the vernal equinox (March 15–21).

Throughout the year, the Sun goes no farther south (or north) of the celestial equator than 23.3°, because that's how much Earth's axis is tilted with respect to the plane of the solar system. If you were standing at Earth's equator at noon on September 23, the Sun would appear directly overhead. But if you were located somewhere further north of the equator, the Sun would appear south of your zenith at noon on that day by an amount equal to your latitude. Hence, the further north you are, the further south the Sun will appear.