An equinox is an astronomical event in which the plane of Earth's equator passes the center of the Sun, making night and day of approximately equal length all over the planet. The equinoxes are the only times when the solar terminator (the "edge" between night and day) is perpendicular to the equator. As a result, the northern and southern hemispheres are equally illuminated.

The March equinox or Northward equinox is the equinox on the earth when the Sun appears to cross the celestial equator, heading northward. The March equinox is the vernal equinox in the northern hemisphere and the autumnal equinox in the southern hemisphere.