Autumnal Equinox Day

What the autumnal equinox tool does

The autumnal equinox is the astronomical first day of autumn. This tool gives you the date for any year from 1900 to 2050, for whichever hemisphere you pick. Choose the year and the hemisphere and it returns the day, drawn from accurate astronomical figures rather than a rough estimate.

How to use it

1. Year. Any year from 1900 to 2050.
2. Location. Northern or Southern hemisphere, since autumn arrives at opposite times in each.

Press Calculate and the date of the autumnal equinox appears.

The balance point of the year

An equinox is the moment the Earth's tilt leans neither toward the sun nor away from it, with the sun directly over the equator and day and night falling very nearly equal across the whole planet. There are two each year, and the autumnal one is the September crossing for the northern hemisphere, the point where the long days of summer have shortened back to even and keep shrinking from there. If the spring equinox is the year tipping toward light, the autumn equinox is it tipping back toward dark. The day and night are as balanced as they get, just on the way down rather than up.

September in the north, March in the south

Because the two hemispheres run on opposite seasons, the autumn equinox falls at different times depending on where you are, which is why the tool asks. In the northern hemisphere, autumn begins at the September equinox. In the southern hemisphere, that same September moment is the start of spring, and autumn instead arrives at the March equinox. So this tool gives you September for the north and March for the south. It is the exact mirror of the spring equinox, swapped between the halves of the world.

Example dates

For the northern hemisphere, the autumnal equinox falls on 22 September 2025 and 23 September 2026. For the southern hemisphere, autumn's equinox lands on 20 March 2025 and 20 March 2026. As with all of these, the equinox is one instant worldwide, timed in universal time, so a date close to midnight can shift by a day once you convert to your own time zone.

Harvest, and the tilt toward darker days

The autumn equinox has long been tied to the harvest, the point in the year when crops are gathered before the colder months close in, and that link runs through many of the festivals held around it, from Mabon in some traditions to the Mid-Autumn or Moon festivals celebrated across East Asia. Practically, it marks the moment when nights start to outlast days in your hemisphere, the slide toward winter that the shorter, cooler evenings make plain.

Questions people ask

What date is the autumnal equinox?

In the northern hemisphere it falls around 22 or 23 September, and in the southern hemisphere autumn's equinox is around 20 March. Pick the year and hemisphere for the exact day.

How is it different from the spring equinox?

Both have nearly equal day and night, but the autumn one comes as days are shortening toward winter, while the spring one comes as they are lengthening toward summer. They fall about six months apart.

Why does the hemisphere matter?

Autumn happens at opposite times north and south. The September equinox is autumn in the north but spring in the south, so the autumn equinox is September for the north and March for the south.

Is it always on the same day?

No, it shifts within a couple of days because a year is not a whole number of days long. In the north it is usually 22 or 23 September, which is why checking the specific year is worthwhile.

References

1. U.S. Naval Observatory, Astronomical Applications Department: Earth's Seasons, Equinoxes, Solstices, Perihelion, and Aphelion. https://aa.usno.navy.mil/data/Earth_Seasons