Twice a year, Earth tilts just right, giving nearly equal amounts of day and night across the globe. One of those moments arrives Monday: the fall equinox, officially ushering in the arrival of autumn.
While equinoxes don’t always fall on the same date, they occur in mid-September. This year’s fall equinox happens Monday, Sept. 22, at 12:19 p.m. MDT, according to NASA. It marks one of only two days each year when the sun shines directly over Earth’s equator, creating nearly equal lengths of day and night.
“Observing and predicting equinoxes is one of the earliest skills in humanity’s astronomical toolkit,” NASA notes. The word itself comes from Latin — “aequus,” meaning equal, and “nox,” meaning night.
The September equinox signals the start of shorter days and longer, cooler nights in the Northern Hemisphere. South of the equator, it’s the opposite — spring begins and daylight stretches longer.
The autumn equinox across the world
The fall equinox has long been a time of balance, harvest celebration and reflection around the world.
In Japan, Higan is observed during both the spring and autumn equinoxes, marking life’s transitions. During this holiday, families honor ancestors by visiting graves, cleaning them and offering flowers or traditional Japanese foods. Chinese and Vietnamese communities celebrate the harvest moon with family gatherings and mooncakes.
From Europe’s Michaelmas harvest feasts to the Mayans’ Pyramid of Kukulkan aligning with sunlight to form a serpent, each culture has its own unique way of celebrating the autumn equinox.
Equinoxes across the solar system
Equinoxes aren’t unique to Earth. “They occur twice a year on every planet of the solar system,” according to National Geographic. On Saturn, for example, an equinox happens every 14.7 years and will last for about seven years, BBC’s Sky at Night Magazine reports.
This equinox offers skywatchers a bonus. “The ringed planet (Saturn) will be incredibly bright in the sky throughout September in the eastern evening sky and western early morning sky. On September 21, Saturn will be at opposition, meaning Earth will find itself in between Saturn and the Sun, temporarily lined up. This also means that Saturn is at its closest and brightest all year,” NASA says.
For those back on Earth, Monday’s celestial moment is simpler: the official first day of fall.