- The equinox is defined as a day that occurs twice per year when the sun crosses the equator and the night and day are the same length.
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.
equinox definition by Webster's New World
- 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 occurs about March 21 and marks the beginning of spring, and the 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: Middle English ; from Old French equinoxe ; from Midieval Latin aequinoxium ; from Classical Latin aequinoctium ; from aequus (see equal) plush nox, night
equinox definition by American Heritage Dictionary
- 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: Middle 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
equinox - Cultural Definition
equinox - Science Definition
- Either of the two points on the celestial sphere where the ecliptic (the apparent path of the Sun) crosses the celestial equator . ♦ The point at which the Sun's path crosses the celestial equator moving from south to north is called the vernal equinox . The vernal equinox marks the zero point in both the equatorial and ecliptic coordinate systems; horizontal angular distances (right ascension in the equatorial system and celestial longitude in the ecliptic system) are measured eastward from this point. The vernal equinox is also known as the first point of Aries because when first devised some 2,000 years ago this point occurred at the beginning of Aries in the zodiac. Because of the westward precession of the equinoxes, the vernal equinox is now located at the beginning of Pisces. ♦ The point at which the Sun's path crosses the celestial equator moving from north to south is called the autumnal equinox .
- Either of the two corresponding moments of the year when the Sun is directly above the Earth's equator. The vernal equinox occurs on March 20 or 21 and the autumnal equinox on September 22 or 23, marking the beginning of spring and autumn, respectively, in the Northern Hemisphere (and the reverse in the Southern Hemisphere). The days on which an equinox falls have about equal periods of sunlight and darkness. Compare solstice.