Vernal Equinox

Vernal Equinox, one of two times of year when the Sun's position makes day and night of equal length in all parts of the Earth. The vernal equinox usually occurs on March 20 or 21 and marks the beginning of the spring season in the Northern Hemisphere.

For information on:

• astronomical aspect of the vernal equinox, see Ecliptic; Pisces (astronomy); El Nath
• vernal equinox as the basis for the mapping of space, see Celestial Sphere; Coordinate System (astronomy); Time: Sidereal Time
• early descriptions of the vernal equinox, see Hipparchus; Copernican System
• cultural events centered on the vernal equinox, see Easter: Pre-Christian Tradition; Fasting; Passover
• synchronizing the calendar with the vernal equinox, see Calendar: The Gregorian Calendar
• fixing the date for Easter, see Easter: Rulings of the Council of Nicaea on the Date of Easter
• prehistoric structure possibly used for predicting the vernal equinox, see Stonehenge: Purpose of Stonehenge