When this occurs, day and night are equal in length. These points occur near March 21 and September 21 when the ecliptic and the celestial equator intersect as shown in the figure above.

Traditionally these points have been known as the "vernal equinox" (spring equinox), which heralds the beginning of spring in the Northern Hemisphere in March as the sun crosses the equator from south to north. The "autumnal equinox" coincides with the beginning of autumn in September in the Northern Hemisphere as the sun crosses the equator from north to south. Because of the historical origin of these names, the terms "March equinox" and "September equinox" are a way of avoiding the hemispheric bias in the names.

For a similar reason the preferred names for the points traditionally known as the "winter solstice" and "summer solstice" are "December solstice" and "June solstice", respectively. At these points the sun is at its farthest north, about June 22 and farthest south, about December 22.

The equinoxes and solstices are important for us because they mark the beginning of the four seasons.