Day and night share equal time as fall equinox arrives Monday

Day and night will share almost equal hours on Monday as much of the world enters the fall season, marking the autumnal equinox in the Northern Hemisphere and the start of spring in the Southern Hemisphere. At the equator, the sun will be directly overhead at noon, while both the north and south poles will be illuminated simultaneously — a phenomenon that occurs only during equinoxes.

In the Northern Hemisphere, daylight will gradually shorten each day until the winter solstice on December 21. Equinoxes have been observed and celebrated for centuries, often tied to harvest festivals. For instance, in Poland and parts of Eastern Europe, the fall harvest festival Dozynki traces its origins to the equinox. At Mexico’s Mayan site Chichen Itza, people gather during the equinox to witness the sun casting a shadow resembling a descending serpent on the El Castillo pyramid.

This year, residents of Antarctica, New Zealand, and parts of Australia may also witness a partial lunar eclipse on Monday, adding a rare astronomical bonus to the equinox.

Understanding the equinox

The equinox occurs as the Earth orbits the sun at a tilt. For most of the year, the planet’s axis leans toward or away from the sun, causing unequal sunlight across the hemispheres. During the equinox, however, the Earth’s tilt aligns with its orbit, giving both hemispheres nearly equal sunlight. The term equinox comes from Latin, meaning “equal night,” as day and night last almost the same length, though minor differences can occur depending on location. The Northern Hemisphere’s fall equinox generally falls between September 21 and 24, while the spring equinox occurs between March 19 and 21.

What is a solstice?

Solstices mark times when the Earth reaches its maximum tilt toward or away from the sun, resulting in the greatest difference between day and night. During the summer solstice, the Northern Hemisphere tilts toward the sun, creating the longest day and shortest night, typically between June 20 and 22. The winter solstice, between December 20 and 23, occurs when the hemisphere tilts away from the sun, giving the shortest day and longest night.

Meteorological vs. astronomical seasons

While astronomical seasons are based on the Earth’s position in its orbit, meteorological seasons follow the annual temperature cycle. According to meteorologists, spring begins on March 1, summer on June 1, fall on September 1, and winter on December 1.