The Autumnal Equinox

Nature Watch
October 11, 2007

By Susan Benson
Director of Education
Cable Natural History Museum

The Autumnal Equinox occurred on September 22nd, officially marking, in astronomical terms, the beginning of autumn. Did you feel the difference? Probably not, but a huge change did occur.

The Autumnal Equinox marks the time when the sun moves south of the equator. We speak of this in terms of the sun, but the sun does not move; the change is actually brought about by the tilting of the earth. If you think of the earth as a person and North America as being on that person’s belly, the earth-person leans back during this time of year and points its belly into the darkness of space, away from the sun. The earth-person reclines further and further back throughout the winter, causing the sun’s daily peak height to be a little lower in the southern sky and the amount of daylight to progressively decrease. When the earth-person has tilted its belly as far as it can away from the sun, the sun is at its lowest point, or nadir. We mark this date with the Winter Solstice, which occurs around December 21st. This is the shortest day of the year and the first day of winter in the Northern Hemisphere.

The Winter Solstice also marks the point at which the earth-person begins tipping forward again. Eventually, the earth-person sits up straight, and its belt (the equator) is level with the sun. This happens around March 21st, the Vernal Equinox, or first day of spring (“vernal” means “spring”). Day and night are of equal length on this day. As the earth-person begins tipping forward and its belly comes closer to the sun, the sun moves north of the equator, the amount of daylight gradually increases, and the sun appears higher in the sky each day. It reaches its peak, or zenith, around June 21st, the Summer Solstice, which is the longest day of the year.

If that was a little tough to follow, maybe it will help to think of it this way. The word “equinox” means “equal night,” referring to the equal duration of light and darkness on the day of an equinox. More simply, equinox = equator. An equinox occurs when the sun crosses the equator, either headed south for the winter (the Autumnal Equinox) or north for the summer (the Vernal Equinox). On the other hand, “solstice” …… well, the word “solstice” does not lend itself to a clever memory trick; it just isn’t the equinox. It may help to know that the origin of the word “solstice” comes from the Latin for “sun stands still.” A solstice occurs when the sun reaches its highest or lowest point, its zenith or its nadir, and it marks the beginning of summer (at its zenith; the Summer Solstice) and the beginning of winter (at its nadir; the Winter Solstice).

Explanations for the cause and effects of the equinox and solstice are entirely scientific, but the occurrence of these changes holds great importance in many religious and cultural contexts. In general, the Autumnal Equinox marks the end of the harvest season and a time to reflect on the year as we light our home fires and approach the darkness of winter.

Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.