An equinox is the actual point in time at which the sun crosses the celestial equator. The celestial equator is simply the projection of the earth’s equator into space. An equinox is a particular point of time, not a full day as is commonly thought.
The word equinox is a translation of the Latin words aequi, meaning “equal” and nox, meaning “night.” The equinox is commonly considered to be the day on which the day and night are equal in length on all parts of the earth, save for the poles. The poles will have either 24 hours of light or 24 hours of darkness. The word equinox, however, is a misnomer. Day and night are not equal on the day of an equinox, with daylight exceeding darkness by as many as 16 minutes. The days on which equal day and night occur are called equiluxes and usually occur either a few days before or after the date of the actual equinoxes.
Twice yearly, the sun crosses the celestial equator, the vernal and autumnal equinoxes. The vernal equinox, also called the spring equinox or the first point of Aries, occurs on or about March 20. The autumnal equinox, also called the fall equinox or the first point of Libra, occurs on or about September 22. The vernal equinox passes the equator from south to north, while the autumnal equinox indicates the sun is crossing the equator from north to south. The dates of each equinox are not fixed, however, because it takes the earth 365.25 days to orbit the sun. Although this is remedied, for the most part, by the addition of 1 day every 4 years, during the leap year, it does not allow a fixed date for the equinoxes to occur. As for the equinox, the sun crosses the equator at a particular point in time, which will occur approximately 6 hours later the following year, or 1 full day every 4 years.
Despite the occurrence of the equinoxes across the earth, both equinoxes are named in relation to the northern hemisphere. So, whereas the vernal equinox in the northern hemisphere indicates the beginning of longer days and the beginning of spring, it indicates shorter days and the start of fall in the southern hemisphere.
The equinoxes hold great meaning across cultures. Many celebrations and holidays are marked by each. Perhaps the best known is the calculation of the Christian holiday, Easter. Easter occurs on the first Sunday after the first full moon on or after the vernal equinox. Whereas Easter’s date of occurrence varies greatly from year to year, the date of March 20 or March 21 is used as the date on which the spring equinox occurs, whether true or not.
Many calendars, including the Iranian calendar, utilize the vernal equinox as the start of the year. In Japan, both the vernal and autumnal equinoxes are national holidays—days to visit family graves and hold reunions. The autumnal equinox indicates the beginning of the harvest in many cultures and is celebrated as such in the United Kingdom.
Whether celebrated on a grand scale or simply observed in passing, equinoxes remain a natural, consistent indicator of seasons and an accurate measure of the passing of time.
Amy L. Strauss
See also Earth, Revolution of; Eclipses; Leap Years; Seasons, Change of; Solstice
Dickinson, T. (1987). Exploring the night sky: The equinox astronomy guide for beginners. Buffalo, NY: Firefly Books.
Staal, J. (1988). The new patterns in the sky: Myths and legends of the stars. Granville, OH: McDonald & Woodward.