The September equinox

The September equinox is the instant the Sun's center crosses the celestial equator heading south, apparent ecliptic longitude 180 degrees. The subsolar point stands on the equator, the terminator runs pole to pole, and the two hemispheres split the sunlight evenly. From here the Sun's daily arc sinks for the Northern Hemisphere and climbs for the Southern.

Exact equinox instants, 2026 to 2032

Computed with the same open-source ephemeris that runs every live tool on this site; instants are stated to the minute and reliable to about a minute. Your time zone can shift a calendar date by a day either way.

Times shown in your local time zone.

The equal-day myth and the equilux

Despite the Latin ("equal night"), day and night are not equal on the equinox: sunrise and sunset are timed from the Sun's upper edge, and refraction lifts the low Sun about half a degree, so the equinox day runs several minutes past 12 hours at most latitudes. The standard almanac convention places the Sun's center 50 arcminutes below the horizon at rise and set, 34 arcminutes of refraction plus 16 of solar radius. The true near-equal date, the equilux, falls days after this equinox in the Northern Hemisphere (the mirror of March, where it comes before), near September 26 at latitude 40 north and weeks later toward the tropics; at the equator no day of the year dips to 12 hours. The panel above computes yours.

The Sun rises due east today

On the equinox, and only around the equinox, the Sun rises due east and sets due west from every point on Earth outside the poles: the Sun stands on the celestial equator, and the celestial equator cuts every horizon exactly at east and west. It is the year's free compass calibration. Note the rise point against your skyline and you have marked true east until the buildings change.

The Harvest Moon connection

The full moon nearest this equinox is the Harvest Moon, and it earns its name from real geometry. On most nights the Moon rises about 50 minutes later than the night before; near the September equinox the evening ecliptic meets the eastern horizon at its shallowest angle of the year, so successive moonrises come only 20 to 30 minutes apart at mid-northern latitudes, and under 20 minutes farther north. For several evenings running, a bright Moon rises near sunset, which genuinely mattered to pre-electric harvest crews. The nearest-full-moon rule means the Harvest Moon can land anywhere from about September 8 to October 7, some years an October moon; our full moon calendar computes each year's and flags it. One deeper wrinkle for the dedicated: the strength of the shallow-angle effect is modulated by the 18.6-year regression of the lunar nodes, and a minor lunar standstill, the next around 2034, minimizes it; that cycle lives in our lunar nodes lesson.

Why the date drifts between September 21 and 24

The 365.2422-day tropical year pushes each equinox about 5 hours 49 minutes later every common year, and each leap day pulls it back about 18 hours. In Universal Time the September equinox ranges over September 21 to 24, and your time zone can move the local date another day. The same arithmetic once broke a calendar: the Julian year ran 11 minutes long, sliding the equinoxes a day earlier every 128 years until the Gregorian reform of 1582 reset the drift. Our tropical year profile has the full accounting.

The fastest-changing days of the year

Day length is changing at its yearly maximum right now, up to about three minutes per day at mid-latitudes, because the Sun's declination curve is steepest where it crosses the equator. Three minutes a day is an hour and a half in a month, which is why autumn darkness (or southern spring light) seems to arrive so abruptly compared with the slow-motion stall of the solstices.

Aurora season

The equinox weeks are statistically the best of the year for auroras, a twice-yearly rhythm in the geomagnetic record since the 1850s. The leading explanation, the Russell-McPherron effect (1973), is geometric: near the equinoxes the tilt of Earth's magnetic field relative to the interplanetary field makes the southward component that opens the door to solar wind energy statistically more likely. Odds, not promises: it takes solar activity to light the sky, and our sunspot cycle lesson covers that half of the equation.

Harvest tables and honest history

In Japan the day is a national holiday, Shubun no Hi, the center of the autumn Higan, a week of Buddhist observance for honoring ancestors; "higan" means "the other shore." At Chichen Itza the equinox sunset walks a chain of triangular shadows down El Castillo's balustrade to the stone serpent's head, an effect that is real, spectacular, and looser than the postcards suggest: photographic surveys show it running for weeks around the equinox, and whether it was deliberately engineered is still debated. Honesty is also owed to "Mabon": the name for the autumn equinox observance was coined in 1974 by Aidan Kelly, borrowing a figure from medieval Welsh literature with no historical tie to the equinox. The modern observance is real and meaningful; the "ancient Celtic festival of Mabon" is not history.

Frequently asked questions

Is day equal to night on the September equinox?

Not quite. Because sunrise and sunset are timed from the Sun's upper edge and refraction lifts the low Sun's image, the equinox day runs several minutes longer than 12 hours at most latitudes. The nearly equal date, the equilux, comes a few days after this equinox in the Northern Hemisphere; at the equator daylight exceeds 12 hours every day of the year.

What is the Harvest Moon and why is it special?

The Harvest Moon is the full moon nearest the September equinox, which can fall from about September 8 to October 7. Around it, successive moonrises come only 20 to 30 minutes apart at mid-northern latitudes instead of the usual 50, because the evening ecliptic meets the horizon at its shallowest angle of the year, so bright moonlit evenings run several nights in a row.

Does the Sun rise due east on the September equinox?

Yes, everywhere on Earth outside the poles, on the days around the equinox and at no other time of year. The Sun sits on the celestial equator, which meets every horizon exactly at the east and west points, making the equinox sunrise a natural compass calibration.

Is the September equinox always on September 22 or 23?

Usually one of those two, in Universal Time, but the full range is September 21 to 24, and your time zone can shift the local date by another day. The drift is the 365.2422-day tropical year working against the leap-year cycle.

Why does darkness seem to arrive so fast in autumn?

Because day length changes fastest at the equinoxes, up to about three minutes per day at mid-latitudes. The Sun's declination curve is steepest as it crosses the equator, and three minutes a day compounds to more than an hour and a half over a month.

Keep exploring: why the tilt makes the seasons, the tropical year behind the drifting dates, sunrise and sunset for any date, and The Sun with every equinox and solstice for two years.