Friday's autumn equinox marks the end of summer and the first day of fall for anybody living in the Northern Hemisphere, meaning colder weather and longer nights are on the way.
The 2017 autumn equinox, which signals the official change of season, occurs on Sept. 22 at 4:02 p.m. EDT, according to the U.S. Naval Observatory, which handles astronomical data and serves as the official source of time for the military and country.
On the day of the equinox — a Latin term that means equality between day and night — day and night are each 12 hours long, according to NASA. The U.S. space agency said equinoxes occur when the Earth is in alignment with the sun directly above the Earth’s equator.
Seasons change based on how the Earth, which spins on a tilted axis, moves around the sun each year, NASA said. For example, it’s summer when the North Pole is pointed toward the sun, which provides the Northern Hemisphere with direct sunlight. But it’s winter when the North Pole is pointed away from the sun.
Winter officially begins in the Northern Hemisphere this year on Dec. 21.