A day in March that is the beginning of spring and a day in September that is the beginning of fall, are examples of the equinox.
- the time when the sun in its apparent annual movement along the ecliptic crosses the celestial equator, making night and day of equal length in all parts of the earth: in the Northern Hemisphere the vernal equinox occurs about March 21 and marks the beginning of spring, and the autumnal equinox occurs about September 22 and marks the beginning of autumn
- either of the two points on the celestial sphere where the sun's path crosses the celestial equatoralso called equinoctial point
Origin of equinoxMiddle English ; from Old French equinoxe ; from Medieval Latin aequinoxium ; from Classical Latin aequinoctium ; from aequus (see equal) + nox, night
- Either of two points on the celestial sphere at which the ecliptic intersects the celestial equator.
- Either of the two times during a year when the sun crosses the celestial equator and when the length of day and night are approximately equal; the vernal equinox or the autumnal equinox.
Origin of equinoxMiddle English, from Old French equinoxe, from Medieval Latin aequinoxium, from Latin aequinoctium : aequi-, equi- + nox, noct-, night; see nekw-t- in Indo-European roots.
celestial sphere showing the positions of the autumnal and vernal equinoxes
(plural equinoxes or equinoctes)