The Southward equinox (or September equinox) is the moment when the sun appears to cross the celestial equator, heading southward. Due to differences between the calendar year and the tropical year, the Southward equinox can occur at any time from the 22nd to the 24th day of September. At the equinox, the sun rises directly in the east and sets directly in the west. Before the Southward equinox, the sun rises and sets more and more to the north, and afterwards, it rises and sets more and more to the south.

In the Northern Hemisphere the Southward equinox is known as the autumnal equinox. In the Southern Hemisphere it is known as the vernal or spring equinox.