The Intihuatana Stone in Machu Picchu, Peru. (Photo provided — Mark Bender, Myshot, National Geographic Society)

This year, the autumnal equinox falls on the 22nd of September — today. It typically occurs on the 22nd or 23rd but, due to differences between the calendar year and the solar year (365 days versus 365 and 1/4 days), may take place anytime between Sept. 21 and Sept. 24.

The last time an autumnal equinox was on the 21st, however, was in 1931. And the next Sept. 21 equinox isn’t until 2076. The last time one occurred on the 24th was in 1907. That won’t happen again until 2303.

An equinox takes place when the Earth’s axis is turned neither away from nor towards the sun, which when seen from the equator rises due east and sets due west. Day and night are of approximately equal length everywhere in the world. The word equinox comes from the Medieval Latin word equinoxium, which means equality of day and night.

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The harvest moon

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In many regions of the world, the autumnal equinox represents the end of the growing season, the harvest. And the full moon nearest the equinox is known as the harvest moon. This year, the harvest moon peaked on Sept. 20, at 7:54 p.m, Eastern daylight time. It appeared full for the past three days.

Throughout the year, the moon rises about 50 minutes later with each passing night. But, as the harvest moon arrives, the difference from night to night is just 20 to 30 minutes; a phenomenon resulting from the angle of the moon’s orbit and the tilt of the Earth at the time of the equinox.

With many of our nation’s farmers and growers harvesting at this time, it’s important to remember that before electricity, the harvest moon, rising full in the evening, enabled agrarians to continue harvesting their crops long into the night.

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Harvest festivals

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The Bavarians, who lived where Germany sits today, were quite possibly the first to have an annual festival linked to the equinox.

Beginning near the end of September and continuing into early October, it was a celebration of the harvest associated with high community spirits and a lot of drinking. Although it’s widely believed that the annual German Octoberfest is directly tied to the 1810 wedding celebration for Bavarian Crown Prince Ludwig (King Ludwig 1), perhaps this is where the merriment associated with the Octoberfest celebration has its real origins.

Pagans celebrate the equinox with Mabon. Interestingly however, there is no historical evidence for that name among any ancient culture. The Druids called the celebration Mea’n Fo’mhair (translation: September).

Perhaps the Christian counterpart was Lammas (Loaf Mass); formerly observed in Britain as an Aug. 1 harvest festival during which, after a long summer of toiling under the heat of the sun, farmers offered their thanks for the blessings of the harvest by baking the communion bread for their churches from the newly-harvested early wheat crop.

A harvest supper was offered on Michaelmas day; the feast of Saint Michael, the Archangel. Dating back to the 3rd century, Michaelmas is still observed on Sept. 29 in some western Christian churches, although it is no longer a holy day of obligation.

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Ancient astronomers

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While ancient people had a limited understanding of astronomy, they were so in tune with the seasons and, as such, with equinoxes and solstices, easily observable phenomena that occur on a regular basis, that they built monuments and ancient observatories which, because of their orientation and physical features, record those astronomical events to this day.

Among them are:

¯ The Intihuatana at the Inca citadel of Machu Picchu, in Peru. There’s a huge stone carving at the top of this sacred mountain; a sundial of sorts. It’s situated at a northward angle of about 13 degrees, the latitude of Machu Picchu, with its corners facing due north, south, east, and west. It’s called Intihuatana, which translates from the native Quechua language as “place to tie up the sun.” The stone casts a shadow at all times throughout the year, except when the sun reaches its zenith (midday) during an equinox. Then and only then, for a very short time, it casts no shadow, whatsoever.

¯ The Temple of Kukulcan at Chichen Itza, Mexico. Built circa 1000 CE, the temple is an homage to the Mayan feathered serpent deity, Kukulcan. On equinox days, the intricately designed architecture of the pyramid manipulates sunlight and shadow to create a truly remarkable optical effect. At sunset, what appears to be a 120-foot-long shadow of a snake can be seen seemingly slithering down the side of the pyramid, until it joins the sculpted serpent’s head, at the foot of the steps. A time-lapse video of this, from the Smithsonian National Museum of the American Indian, can be found by searching for “The Descent of Kukulcan” on YouTube.

Among scores of other sites are Angkor Wat in Cambodia; the Karnak Temple in Egypt; the Mnajdra temples in Malta, Fajada Butte at Chaco Canyon, New Mexico; Hovenweep Castle, on the Colorado-Utah Border, Woodhenge in Cahokia, Illinois, several sights in Ireland, and (arguably) the most famous prehistoric astronomical monument of all, Stonehenge, in England.