Autumnal equinox
What is the autumnal equinox?
When is the autumnal equinox?
Autumnal equinox, two moments in the year when the Sun appears exactly above the Equator; also, either of the two points in the sky where the ecliptic (the Sun’s apparent annual pathway through the sky) and the celestial equator intersect. In the Northern Hemisphere the autumnal equinox falls about September 22 or 23, as the Sun crosses the celestial equator going south. In the Southern Hemisphere the equinox occurs on March 20 or 21, when the Sun moves north across the celestial equator.
The autumnal equinox is not a day when the day and night are equally long. Because daytime is defined from when the Sun breaks the horizon to when it completely sets, the day is longer by the time it takes the Sun to begin setting to when it completely sets. Atmospheric refraction also bends the Sun’s light, so there is light even when the Sun is actually below the horizon. However, there is a day called the equilux when day and night are equally long. In both hemispheres it occurs a few days before the vernal equinox and a few days after the autumnal equinox.
According to the astronomical definition of the seasons, the autumnal equinox also marks the beginning of autumn, which lasts until the winter solstice (December 21 or 22 in the Northern Hemisphere, June 20 or 21 in the Southern Hemisphere).
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