Astronomical autumn begins with the fall equinox, the moment when Earth’s tilt relative to the sun places neither hemisphere in favor. Day and night are nearly equal in length, and the sun crosses the equator heading south, a sign that summer is giving way to fall.
The fall equinox typically occurs on September 22 or 23, though the exact date and time vary slightly from year to year. At that precise moment, the sun’s direct rays shine over the equator. Afterward, the Northern Hemisphere begins tilting progressively away from the sun, leading to shorter days and longer nights.
The word “equinox” comes from the Latin aequus (“equal”) and nox (“night”) , reflecting the roughly balanced split between daylight and darkness. Most locations receive a few more minutes of daylight than darkness on the equinox because of atmospheric refraction and the way sunrise and sunset are defined. Still, it’s one of the two times each year when daylight and night are closest to equal.
A seasonal pivot
While the equinox marks the official start of astronomical fall, it doesn’t mean cool weather arrives on cue. September and October can still bring late-summer warmth. But daylight is rapidly decreasing, and the sun’s arc across the sky is noticeably lower and shorter than it was in June.
In DC, the fall equinox brings about 12 hours and 7 minutes of daylight, and daylight decreases by about 2½ minutes per day. Sunsets shift earlier at an accelerating pace, a change that becomes especially noticeable by October.
Plants respond to decreasing light levels by triggering autumn color and leaf drop . Migratory birds begin heading south. And temperatures, though often lagging behind the solar cycle, trend downward as the Northern Hemisphere receives less sunlight.
Why the date varies
Like the spring equinox, the fall equinox doesn’t land on the exact same calendar date every year because Earth’s orbit around the sun doesn’t take precisely 365 days. Leap years and orbital variations cause the timing to shift by several hours annually, occasionally nudging the date between September 22 and 23.
Astronomical vs. meteorological fall
There are two common ways to define the start of fall:
- Astronomical fall begins at the equinox – the precise moment when Earth’s tilt is perpendicular to the sun’s rays and the sun appears directly overhead along the equator. The sun rises due east and sets due west on the equinox, then gradually moves toward the southern horizon.
- Meteorological fall runs from September 1 through November 30, aligning with temperature patterns and simplifying climate statistics.
Meteorologists prefer the monthly definition for consistency in record-keeping, but the astronomical definition is based on Earth’s orbit around the sun.
What you’ll notice after the equinox
After the equinox, nights grow longer than days, and the sun continues to drop lower in the sky. This shift sets the stage for cooler air masses, stronger temperature swings and, eventually, the approach of winter.
Even though early fall weather can still feel summery, the amount of solar energy available to heat the atmosphere is steadily decreasing. This gradual loss of daylight is what drives the cooling trend that defines autumn.
