Fall equinox arrives as day and night balance for a moment
Autumnal equinox signals the start of fall in the Northern Hemisphere and spring in the Southern, with a partial lunar eclipse visible in some regions.
Day and night will be roughly equal on Monday as the autumnal equinox arrives, marking the start of fall in the Northern Hemisphere and spring in the Southern Hemisphere.
During an equinox, the tilt of Earth’s axis is oriented so that both hemispheres receive nearly equal amounts of sunlight. The sun will be directly overhead at noon along the equator on the equinox, a moment that occurs twice a year as the Earth continues its orbit around the Sun. The fall equinox usually falls between Sept. 21 and Sept. 24.
Equinoxes are named for what the Sun’s relationship to day and night produces: equality of length is a handy shorthand, since day and night are not perfectly equal everywhere due to atmospheric refraction and the Sun’s apparent size. On this day, the north and south poles are both illuminated by sunshine, a rare alignment that marks a change of seasons in many cultures.
In the Northern Hemisphere, daylight begins to shrink after the equinox as days grow shorter toward the winter solstice on Dec. 21, while in the Southern Hemisphere the opposite trend unfolds.
Across cultures, equinoxes have long carried festival significance. Harvest traditions abound, from Dozynki, the Polish harvest festival connected to the equinox, to other autumnal rituals that trace back to ancient harvest celebrations. At the Mayan site Chichen Itza in Mexico, visitors have gathered on the equinox to watch a shadow pattern that some describe as a serpent descending El Castillo.
This year, observers in the southern reaches of the globe may glimpse a partial lunar eclipse on Monday. The Moon’s shadow will be visible from Antarctica, New Zealand and a sliver of Australia, weather permitting.
Beyond the calendar date, the equinox reflects how the year is carved up by the Earth’s orbit and tilt. Astronomical seasons depend on the Sun’s position relative to the Earth; meteorologists define seasons by three-month temperature cycles. The two systems offer different, but complementary, ways of understanding the year’s rhythm.