On Sept. 22, 2025, at 2:19 p.m. EDT, the autumnal equinox officially marks the start of fall in the Northern Hemisphere. It’s the precise moment when the sun sits directly above the equator, creating nearly equal amounts of day and night across the globe.
While it’s often called the “first day of fall,” the equinox is more than a seasonal marker—it’s a natural event that shapes agriculture, wildlife, and even our internal body clocks.
What exactly is the autumnal equinox?
The term “equinox” comes from the Latin aequalis (equal) and nox (night). On this day, day and night are nearly equal in length worldwide. That balance happens because Earth’s axis isn’t tilted toward or away from the sun.
- In the Northern Hemisphere, it signals fall’s arrival.
- In the Southern Hemisphere, it’s the vernal equinox, or the beginning of spring.
Though we talk about “equal day and night,” they aren’t perfectly balanced. Thanks to Earth’s atmosphere bending sunlight, daylight actually lingers a few minutes longer. Astronomers note that the true day of exact balance usually comes a few days later.
Why does the equinox happen?
Earth doesn’t orbit upright. Its 23.5-degree tilt is the reason we have seasons. Twice a year—in March and September—the tilt aligns so that the sun shines directly over the equator.
- March equinox: Starts spring in the north, fall in the south.
- September equinox: Starts fall in the north, spring in the south.
Compare this with solstices:
- June solstice: Longest day of the year in the north.
- December solstice: Shortest day of the year in the north.
When does fall really start: astronomically or meteorologically?
Here’s where definitions diverge:
- Astronomers say fall begins with the September equinox.
- Meteorologists and climatologists mark the start of fall on Sept. 1, since they group seasons by the calendar’s warmest and coldest months.
That’s why, technically, summer “ended” Aug. 31 for meteorologists—even though astronomically, it lasted until Sept. 22.
What happens on the equinox itself?
On Sept. 22, most of the world will see the sun rise nearly due east and set almost due west. This only happens twice a year—during the March and September equinoxes.
For those living at the poles, equinoxes are dramatic. In the Arctic, it marks the end of months of sunlight; in Antarctica, it’s the end of months of darkness.
Why does the equinox matter for us on Earth?
The autumnal equinox isn’t just a calendar marker—it influences ecosystems, human health, and traditions.
Human health
Our circadian rhythm depends heavily on daylight. Shorter days and longer nights can affect sleep and mood. For some, this shift triggers seasonal affective disorder (SAD), a type of seasonal depression.
Animal behavior
Wildlife reacts to the change in light, not temperature. Birds begin long migrations, squirrels and bears prepare food stores, and nocturnal animals adjust their activity cycles.
Agriculture and harvest
Historically, the fall equinox signaled the start of harvest season. Farmers timed crop gathering to maximize storage before winter. Today, many harvest festivals around the world—from China’s Mid-Autumn Festival to North America’s Thanksgiving—have roots in equinox traditions.
Interesting facts about the autumnal equinox
- Day and night are “nearly” equal, but daylight wins by a few extra minutes.
- No government body or political organization “declares” fall—this is purely an astronomical event.
- At the equator, daylight length barely changes year-round, so equinoxes are less noticeable.
- Ancient monuments like Stonehenge and Mexico’s Chichén Itzá align with equinox sunrises and sunsets, showing just how long humans have tracked this event.
Looking ahead: darker days into winter
From Sept. 22 onward, the Northern Hemisphere tilts further away from the sun. Days will grow shorter, nights longer, and temperatures cooler until the winter solstice on Dec. 21, 2025, the year’s shortest day.
For many, that means cozy sweaters and pumpkin spice. But scientifically, it’s a reminder that our lives remain deeply connected to the tilt of our planet.