How this calculator works
There are two common ways to date the start of autumn. Meteorological seasons follow the calendar: autumn begins on 1 September in the northern hemisphere and on 1 March in the southern hemisphere, and each season is three whole months. Astronomical seasons follow the Sun: autumn begins at the equinox, when the Sun crosses the celestial equator and day and night are close to equal. In the northern hemisphere that is the September equinox; in the southern hemisphere it is the March equinox. The astronomical date shifts by a day or so from year to year because the calendar year is not exactly the time the Earth takes to orbit the Sun. The times on this page are in UTC, so a local time zone may place the equinox on the previous or following calendar day.
Formula
days until autumn = the next autumn start instant minus the entered instant, in seconds, then split into whole days, hours, minutes and seconds. The astronomical start is the September equinox in the north or the March equinox in the south. The meteorological start is 1 September 00:00 UTC in the north or 1 March 00:00 UTC in the south.
Worked example
From 1 November 2026 at 00:00 UTC, the next northern autumn begins at the September equinox on 23 September 2027 at 06:02 UTC, which is 326 days, 6 hours and 2 minutes away. In the southern hemisphere the same starting instant is 139 days, 20 hours and 25 minutes from the March equinox on 20 March 2027 at 20:25 UTC. With the meteorological definition, the northern autumn begins on 1 September 2027, 304 days after 1 November 2026.
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Frequently asked questions
Why is the equinox on a different date each year?
The Earth takes about 365.242 days to orbit the Sun, while the calendar year is 365 or 366 days. The extra fraction accumulates and the leap day pulls it back, so the equinox drifts by about six hours a year and jumps back a day after a leap year.
Is the equinox the same date everywhere?
The instant is the same worldwide, but the local calendar date can differ. An equinox at 00:06 UTC on 23 September is still the evening of 22 September in the Americas. This page reports UTC dates, so check your time zone if you need the local day.
Why do some calendars start autumn on 1 September?
Meteorological seasons use whole calendar months so that records, forecasts and comparisons line up. Autumn is September, October and November in the north, and March, April and May in the south. The astronomical date is more precise for the Sun position but less convenient for monthly statistics.
Does the southern hemisphere autumn start in March?
Yes. Seasons are opposite across the equator, so the southern autumn begins at the March equinox and the northern autumn begins at the September equinox. The meteorological definitions are three months apart in the same way.