5 Surprising Facts About the Autumnal Equinox

Key Insights

September 22 marks the autumnal equinox, signaling the start of fall in the Northern Hemisphere. It’s the day when Earth is positioned just right in relation to the sun, causing day and night to be nearly equal in length. Almost, but not entirely. We’ll clear up this and four other intriguing details about the first day of fall.

1. It's Not a Full Day Event

Though we observe and even celebrate the autumnal equinox throughout the day on September 22, it’s actually just a brief moment in time — precisely when the sun crosses the celestial equator, an invisible line in the sky above Earth’s equator.

Typically, Earth orbits with a tilt of 2 degrees on its axis. But during this specific moment, its axis is aligned in such a way that it’s neither tilted toward nor away from the sun.

In 2024, this event will occur at 8:44 a.m. EDT. After this moment, the sun will begin to rise later and set earlier for those in the Northern Hemisphere.

Those in the Southern Hemisphere will experience the opposite. In fact, September 22 is known as the spring equinox in the Southern Hemisphere, marking the beginning of longer days ahead.

2. Day and Night Aren't Perfectly Equal

The term "equinox" derives from Latin, combining aequus (meaning "equal") and nox (meaning "night"). However, the duration of day and night on the equinox isn’t exactly 12 hours each.

Because of the refraction of sunlight (the bending of the sun's rays), the sun may appear above the horizon in some places even though it’s still technically below it.

Additionally, people living farther from the equator will experience slightly longer days, as the sun takes more time to rise and set from their location. Around the equinox, the sun might be visible for anywhere between 12 hours and 6 minutes to 12 hours and 16 minutes, depending on the latitude.

3. The Date of the Equinox Can Change

While the autumnal equinox typically occurs on September 22 or 23, it can sometimes fall on September 21 or 24. This happens because the Gregorian calendar used in the Western world defines a year as 365 days, which is the time it takes for Earth to complete its orbit around the sun.

In reality, Earth takes 365.25 days to orbit the sun. This means the September equinox will occur about six hours later each year. Leap years help adjust for this discrepancy, effectively resetting the date.

In 2092 and 2096, the autumnal equinox will occur on September 21. The last time this happened was in 1000 C.E.! You can find the exact dates for both autumnal and spring equinoxes, as well as the summer and winter solstices, up until 2025 at this link.

4. Equinoxes Trigger the Northern Lights

You’ve probably seen the mesmerizing aurora borealis, those dazzling light displays in the night sky. Well, the equinox marks the beginning of the season when these stunning lights are visible, typically lasting all the way through the spring equinox in March.

Auroras occur when solar winds interact with Earth's magnetic field. Solar winds are streams of plasma particles released from the sun and traveling through space. Due to Earth's axial tilt, the solar wind from the sun can more easily reach Earth’s atmosphere through the geomagnetic field. As a result, geomagnetic storms are more intense and frequent during spring and fall than in the summer or winter.

When these charged particles collide with Earth’s magnetic field, they strike atoms of oxygen, nitrogen, and other elements in the air. This causes the atoms to release photons of various wavelengths, producing the colorful auroras we see in the sky.

5. It's a Time for Celebration

The fall equinox is marked by several celebrations, including the neopagan festival of Mabon, a harvest celebration honoring the gathering of crops and the earth's abundance.

In China and other Asian nations, the moon festival is celebrated, typically in mid-September, around the time of the harvest moon. This full moon occurs closest to the autumnal equinox.