The Sun turns south
Over the year, the Sun’s path across the sky drifts north and south because Earth’s axis is tilted about 23.4 degrees. The celestial equator, the projection of Earth’s equator onto the sky, marks the halfway line.
From the June solstice onward, the Sun has been creeping south. At the September equinox it crosses the celestial equator. For that instant, the Sun is overhead at noon on Earth’s equator, and both poles sit right on the edge between day and night.
From then until the March equinox, the Sun stays south of the celestial equator. In the Northern Hemisphere that means shorter days and lower noon Sun; in the Southern Hemisphere, longer days and a higher Sun.
Dates and the calendar
The September equinox normally falls on September 22 or 23 in Coordinated Universal Time. Counting local time zones, the full range is roughly September 21 to 24, though the outer dates are rare.
The reason is the mismatch between the 365-day calendar year and the tropical year of about 365.24 days. Without leap years, the equinox would drift through the calendar by almost a day every four years. The Gregorian calendar’s leap-year rules hold it within a narrow band. For the exact instant in your time zone, see the seasons page.
When are day and night really equal?
Even on the equinox, day is a little longer than night almost everywhere. Sunrise and sunset are counted from the moment the Sun’s upper edge touches the horizon, and the atmosphere bends sunlight so that the Sun appears a little higher than it is. Together these add several minutes of daylight.
The day with the most nearly equal day and night, often called the equilux, therefore comes a few days after the September equinox in the Northern Hemisphere and a few days before it in the Southern Hemisphere. Its exact date depends on latitude. The sunrise and sunset page shows day length for your location.
Shrinking days
Days shorten fastest around the September equinox. At about 50 degrees north, for example, daylight drops by nearly four minutes per day in late September. Closer to the equator the change is smaller, and at the equator itself day length hardly changes all year.
| Hemisphere | Season that starts | Direction of day length |
|---|---|---|
| Northern | Astronomical autumn | Days shortening quickly |
| Southern | Astronomical spring | Days lengthening quickly |
A turning point at the poles
At the North Pole, the Sun sets around the September equinox and does not rise again until around the March equinox. At the South Pole, the Sun rises and stays up for about six months. Refraction stretches the polar day slightly at both ends, so these events are spread over a few days rather than happening exactly at the equinox.
Moon and sky around the equinox
The full moon that falls closest to the September equinox is traditionally called the Harvest Moon in the Northern Hemisphere. For a few evenings around it, the Moon rises at a similar time each night, shortly after sunset, which historically gave farmers extra light for harvest work. You can follow the moon phases to see when it falls.
Because the September equinox starts spring in one hemisphere and autumn in the other, the hemisphere-specific names can be confusing. The guide to the autumnal equinox explains how the naming works, and the March equinox covers the opposite crossing.