The date at which the sun shines directly on the equator and the length of day and night is nearly equal. (The word equinox derives from the Latin for “equal night”). There are two equinox dates per year. In the northern hemisphere, the one in March is designated the spring or vernal equinox (which marks the transition from winter to spring), and the one in September is designated the the fall or autumnal equinox (which marks the transition from summer to fall).

Seasons, Solstice, and Equinox