Overview

• The astronomical start of the season falls at the autumnal equinox, which occurs on Sept. 22 this year.
• At the equinox the Sun is directly above the equator, giving both hemispheres nearly equal daylight.
• Seasons arise from Earth’s roughly 23.5-degree axial tilt, and the 365.2422-day solar year shifts equinox timing slightly each year.
• After the September equinox, days shorten and temperatures trend downward in the Northern Hemisphere as the Southern Hemisphere enters spring.
• Day and night are not exactly equal because atmospheric refraction extends sunrise and sunset, and some cultures observe the date with traditions such as Japan’s Shūbun no Hi.