The fall equinox and the first day of autumn arrives on Saturday, September 23, 2023, at 02:50 A.M. EDT in the Northern Hemisphere. The equinox occurs at the same moment worldwide.
The autumnal equinox is an astronomical event that marks the start of autumn (or “fall”). In the Northern Hemisphere, the autumnal equinox occurs in September; in the Southern Hemisphere, it occurs in March.
During an equinox, the Sun crosses what we call the “celestial equator”—an imaginary extension of Earth’s equator line into space. The equinox occurs precisely when the Sun’s center passes through this line.
For those in the Northern Hemisphere, when the Sun crosses the equator going from north to south, this marks the autumnal equinox; when it crosses from south to north, this marks the vernal equinox. In the Southern Hemisphere, it’s the reverse.
After the autumnal equinox, days become shorter than nights as the Sun continues to rise later and nightfall arrives earlier. This ends with the winter solstice, after which days start to grow longer once again.
The word “equinox” comes from Latin aequus, meaning “equal,” and nox, ”night.” On the equinox, day and night are roughly equal in length.