One event, two months
The autumnal equinox is defined by season, not by month. It is the equinox after which nights are longer than days in your hemisphere:
- Northern Hemisphere: the September equinox, around September 22 or 23, is the autumnal equinox.
- Southern Hemisphere: the March equinox, around March 20, is the autumnal equinox.
So when Canada and Europe are entering autumn in late September, Australia, New Zealand, southern Africa and South America are entering spring. Six months later, the roles swap.
Because this can be confusing, many astronomers prefer the neutral terms September equinox and March equinox. The guide to the vernal equinox covers the spring side of the same story.
The astronomer’s definition
In astronomy, the autumnal equinox also names a fixed point on the sky: the place where the Sun crosses the celestial equator heading south. By convention this is always the September crossing, because the terminology was set by observers in the Northern Hemisphere.
That point was once called the first point of Libra, since it lay in Libra more than 2,000 years ago. The slow wobble of Earth’s axis, called precession, has since moved it into Virgo.
What changes after the autumnal equinox
After the autumnal equinox, the Sun’s noon position keeps getting lower in your sky and nights keep getting longer, until the winter solstice. The change is fastest right around the equinox: at mid-latitudes daylight shrinks by a few minutes each day. You can watch it happen on the sunrise and sunset page.
The weather tends to follow with a delay. Oceans and land release heat stored over the summer, so the coldest part of the year usually comes well after the winter solstice.
The Harvest Moon
The full moon closest to the September equinox is known in the Northern Hemisphere as the Harvest Moon. Normally the Moon rises about 50 minutes later each day. Around the September equinox, the angle at which the Moon’s path meets the evening horizon is shallow in the Northern Hemisphere, so the delay shrinks, at mid-northern latitudes to around half an hour or less. For several evenings in a row, a bright, nearly full Moon rises soon after sunset, which once helped farmers bring in crops after dark.
Whether a given year’s Harvest Moon falls in September or early October depends on the lunar cycle. Moon phase calendars show the upcoming dates.
Traditions around the autumnal equinox
People have marked the turning of the season in many ways:
- Japan: Autumnal Equinox Day (Shūbun no Hi) is a national holiday in September. Families often visit ancestral graves during the surrounding week of Higan.
- France: the French Republican calendar, introduced during the Revolution, began its years at the autumnal equinox. Its first year started on September 22, 1792.
- Harvest festivals: many farming communities in the Northern Hemisphere hold harvest celebrations around this time of year, although most are tied to local customs rather than the exact date of the equinox.
Equinox, not equal
The name suggests equal day and night, and that is roughly true everywhere on Earth on the equinox. But because sunrise is counted when the Sun’s upper edge appears, and the atmosphere lifts the Sun’s image slightly, the day still wins by several minutes. Nights only become longer than days a few days after the autumnal equinox.
For the exact date and time of each equinox in your time zone, see the seasons page.