The Autumnal Equinox Marks Our Seasonal Transition

You’ve probably noticed that September just “feels” like a time of transition. For those of us in the Northern Hemisphere, it’s the time of the year when temperatures cool off, birds begin gathering for migration, and the trees start showing color.

All of this occurs around the autumnal equinox, between September 21 and September 24 (in 2023, the equinox is the 23^rd) marking the start of fall for people in the Northern Hemisphere.

Earth’s tilt and orbit make it all happen

Our planet is tilted about 23.5 degrees on its axis as it travels around the sun, and it stays tilted in the same direction as it orbits. This means the sun’s light hits the Earth at different angles. So, different parts of the Earth receive varying amounts of the sun’s light and warmth throughout the year — in other words, we have seasons.

However, twice during Earth’s orbit, on the autumnal (September) equinox and the vernal, or spring (March), equinox, the sun is directly over the planet’s equator, and everybody across the globe experiences a nearly equal amount of daylight and darkness.

A few interesting facts about the equinoxes and Earth’s orbit:

• The word equinox comes from two Latin words: aequus (equal) and nox (night).
• During the autumnal and vernal equinoxes, people across the globe can see the sun rise above the horizon due east and set due west. So, it’s a good time to find due east and west.
• Because of the Earth’s tilt on its axis, its northern and southern hemispheres trade places throughout the year receiving the sun’s light and warmth most directly.
• For people in the Southern Hemisphere, the equinoxes signal the opposite seasonal transitions: September marks the beginning of spring, and the March equinox marks the beginning of autumn.
• If you live in the Northern Hemisphere, starting on the autumnal equinox, you can observe a slight daily change in the sun’s arc across the sky — it’s shifting to the south.
• A number of prehistoric sites, like Stonehenge and Newgrange in the United Kingdom, were possibly used by ancient cultures to predict equinoxes and the summer and winter solstices — the longest and shortest days of sunlight during the year).

To learn more about the Earth's orbit, our solar system, the stars, and beyond, check out the fifth edition of Astronomy For Dummies.

Celebrations around the fall equinox

Over the centuries, before humans could scientifically explain the autumnal equinox, they observed the sun’s changing position in the sky and nature's transitions in the fall and spring, and they marked these times with rituals and celebrations.

Some of these ancient observations were incorporated into Greek mythology and other cultures' mythology, and into religious practices. Today, there are still many cultural and religious traditions practiced around the equinoxes. Here are summaries of just a few:

• Mabon — United Kingdom: Mabon is a fall equinox tradition created by the ancient Celtic people and celebrated by pagans today. It is one of the oldest harvest festivals in Europe. Acknowledging the autumnal equinox, the holiday is meant to give thanks for the warm, summer months, the fall harvest, and to get ready for the beginning of winter.

• The Snake of Sunlight — Mexico: The ancient Mayan and Aztec civilizations in Mexico celebrated the equinoxes at the site of Chichen-Itza, a city that existed about 1,500 years ago in what is now the state of Yucatan. When you visit the Chichen-Itza ruins today, you see a massive pyramid, a monument the Mayans built to honor the god Kulkulcan (Quetzalcoatl to the Aztecs). The deity was a feathered serpent, and the ancient Mayans believed it visited the temple twice a year — on the autumn and spring equinoxes.

  Chichen-Itza is a popular tourist attraction, and many come to witness a special effect that happens on the equinoxes. The pyramid’s steps are oriented so that in the afternoon of the two equinox days, the shadow on the pyramid looks like a snake slowly slithering down the stairs, with its tail at the top and its head at the bottom.

• Higan — Japan: Higan is a Buddhist tradition taking place around the equinoxes – three days before the equinox day and three days after. Higan means crossing over to the “other shore,” symbolizing the world of enlightenment, or spiritual awakening.

  For Buddhists, Higan is a time to reflect on one’s life and renew religious practices. During Higan in Japan, people visit their ancestors’ graves, where they tidy up the gravesites and place flowers and incense.