Is today first day of fall? Autumnal equinox arrives tonight

The autumnal equinox arrives tonight, September 22, at 8:05 p.m. ET. This marks the official start of astronomical fall in the Northern Hemisphere.

At that precise moment, the sun will sit directly above Earth’s equator, creating roughly equal day and night across most regions of the world.

The word equinox comes from Latin, meaning “equal night.” In the Southern Hemisphere, the same event marks the start of spring.

Two definitions of fall

Astronomers say summer ends September 22, making tonight the start of astronomical autumn. But meteorologists and climatologists consider fall to have begun September 1, since they group the year into three-month seasons based on temperature cycles, with June, July, and August being the hottest months.

El Nino effect

The start of fall doesn’t mean colder days and nights. The combination of El Niño and climate change makes warmer-than-normal temperatures likely this fall.

Forecasters expect this El Niño to peak later this fall as one of the strongest on record.

This summer already sets a record, with August being the warmest month worldwide for combined land and ocean temperatures, and the contiguous U.S. has seen its warmest year-to-date on record. Ten states, including Texas, California, and Arizona, had their warmest start to a calendar year.

Shorter days ahead

From now until the winter solstice, most U.S. states will lose 3 to 3.5 hours of daylight. Earth’s 23.5 degrees tilt means the Northern Hemisphere is gradually turning away from the sun, bringing darker nights.