September equinox at 2:19 p.m. EDT marks start of autumn

On Monday at 2:19 p.m. Eastern Daylight Time, the sun will cross the celestial equator heading south, marking the September equinox and the official start of astronomical autumn in the Northern Hemisphere and spring in the Southern Hemisphere. The moment, which occurs simultaneously around the globe, signals the transition from the long days of summer to steadily shortening daylight in the north and the reverse in the south.

Astronomers say the equinox is the point in Earths orbit when the planet's tilted axis is sideways to the sun, so both hemispheres receive nearly the same amount of sunlight. That near balance explains the Latin root of the word equinox, equal night. But specialists caution that day and night are not exactly equal on the calendar date commonly called the equinox. Because the sun appears as a disk rather than a single point of light and because Earth's atmosphere bends sunlight near the horizon, most locations experience a few extra minutes of daylight. Almanacs and astronomers note that the true date when day and night are equal at a given latitude, sometimes called the equilux, can fall several days before or after the equinox.