Harvest moon, Springfield, Illinois. (Credit: Seth Perlman/AP/REX/Shutterstock)
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Introduction

The fall equinox—also called the autumn equinox—takes place every year at some time between September 21 and September 24. It marks the first day of fall in the Northern Hemisphere. (The reverse is true in the Southern Hemisphere, where the September equinox signals the first day of spring.) People have celebrated the fall equinox for centuries. In the Northern Hemisphere, the September equinox coincides with the fall harvest, and many ancient harvest celebrations take place on or around the fall equinox.

Equinox comes from the Latin words “aequi,” which means equal, and “nox,” or night. On the equinox, day and night are of nearly equal length across the planet.

As the Earth orbits the sun, it is tilted at a fixed angle. For half the year, the North Pole is tilted slightly toward the sun, bringing longer days to the Northern Hemisphere, while the South Pole is tilted slightly away from the sun, bringing fewer hours of sunlight to the Southern Hemisphere.

Then, as the Earth continues to move around the sun at its fixed angle, the North Pole is tilted slightly away from the sun. The equinox marks the point of the year where this transition occurs, and on the equinox the part of Earth closest to the sun is the equator, rather than places north or south.

In the Northern Hemisphere, the September equinox marks the first day of fall. The reverse is true in the Southern Hemisphere where the September equinox signals the first day of spring.

Ancient cultures didn’t have clocks to calculate minutes of daytime and nighttime, but they could measure the sun’s position geometrically.

People observed that the sun’s rising and setting points moved slightly each day of the year. The summer solstice would occur when the sun reached its northernmost point. The sun’s southernmost point marked the winter solstice. The two days of the year when the sun rose exactly due east and set exactly due west marked the equinoxes.

Archaeologists believe a number of prehistoric sites were used by ancient peoples to track the position of the sun and predict equinoxes and solstices. Some of these sites include Stonehenge and Newgrange in the UK and the Majorville Medicine Wheel in Alberta, Canada.

Greek Mythology: To the ancient Greeks, the September equinox marks the return of the goddess Persephone to the darkness of the underworld, where she is reunited with her husband Hades.

Chinese Harvest Moon Festival: The full moon that falls closest to the autumnal equinox is sometimes called the Harvest Moon. The Chinese began celebrating the fall harvest at the Harvest Moon centuries ago, during the Shang dynasty. Ancient Chinese celebrated the successful harvest of rice and wheat and made offerings to the moon.

Ethnic Chinese and Vietnamese people still celebrate the Harvest Moon or Mid-Autumn Festival. During the Mid-Autumn Festival, lanterns adorn streets and family and friends gather to give thanks, share food, and watch the moon. Round pastries, called mooncakes, are often enjoyed at this time.

Japanese Higan: Higan is a holiday celebrated by some Japanese Buddhists. It takes place twice a year, during the fall and spring equinoxes.

During Higan, Japanese Buddhists will return to their hometowns to pay respects to their ancestors. Higan means “from the other shore of the Sanzu River.” In Buddhist tradition, crossing the mythical Sanzu River meant passing into the afterlife.

Harvest Festivals In Great Britain: The people of the British Isles have given thanks at fall harvest festivals since pagan times. Harvest festivals traditionally were held on the Sunday nearest the Harvest Moon.

Early English settlers took the harvest festival tradition with them to America. These tradition festivals, once celebrated around the equinox, formed the basis of American Thanksgiving, which we now celebrate in November.

French Republican Calendar: During the French Revolution, the French government designed and implemented a new yearly calendar.

Each new year would start midnight on the day of the autumnal equinox. In the revolutionary attempt to rid the calendar of religious or royalist influence, each month was named after a natural element.

The French followed this calendar from 1793 until Napoleon Bonaparte abolished it in 1806.

Modern Paganism: Modern pagans celebrate a feast called Mabon on the autumnal equinox. This harvest festival is a time to celebrate the gifts of the Earth.

In the far north, the autumnal equinox signals peak viewing of the aurora borealis or northern lights.

The celestial display of brilliantly colored lights happens when charged particles from the sun strike atoms in Earth’s atmosphere, causing them to light up. These light displays peak around the fall and spring equinox. That’s because disturbances in Earth’s atmosphere—known as geomagnetic storms—are strongest at these times.

Ancient Observatories—Timeless Knowledge. Stanford Solar Center.
Who, What, Why: What is an equinox? BBC.
Fall equinox ups chances of seeing Northern Lights. Space.com.