Midway between the solstices, however, the sun is aligned vertically above the equator of the earth. This occurrence is known as an equinox, which means that day and night are of equal length everywhere on earth. About March 20 and September 21, the sun rises exactly in the east, follows the equator for 12 hours, and sets exactly in the west. At midday on an equinox, the sun is at its zenith over the equator and no objects will cast a shadow there.

Equinox

March 19, 20, or 21

Equinox

September 21, 22, 23, or 24