Autumn officially begins in the Northern Hemisphere on Monday, 22 September, as the autumnal equinox ushers in the first day of fall.
At this celestial milestone, the sun crosses directly over the equator, creating almost equal hours of daylight and darkness across the world.
While the north prepares for crisp mornings and golden leaves, the same moment signals the vernal equinox in the Southern Hemisphere, the first day of spring.
What is the First Day of Fall?
The autumnal equinox is not a whole day but a precise moment in time, as reported by USA Today. It happens when Earth's axis tilts neither toward nor away from the sun. According to the National Oceanic and Atmospheric Administration, at this point the planet seems to have no tilt with respect to the sun. This occurs only twice a year, in March and September.
NASA's Earth Observatory notes that Earth's 23.5-degree tilt drives seasonal changes. The very word equinox comes from Latin, meaning 'equal night'. The balance of daylight and darkness has fascinated people for centuries.
What to Expect During the Autumnal Equinox
In both the US and the UK, people can expect around 12 hours of daylight and 12 hours of night. However, the split is not perfectly equal. Time and Date explained that this is because of atmospheric refraction, daylight lasts a little longer.
On this day, the sun rises almost due east and sets nearly due west. From then until December's winter solstice, days in the Northern Hemisphere gradually shorten. Sunrises come later, sunsets arrive earlier.
The event has biological and cultural impacts. Humans adjust to shorter daylight hours, with some experiencing Seasonal Affective Disorder. Animals begin migrating or changing feeding patterns to prepare for winter. Historically, farmers connected this shift with the start of the harvest season.
8 Fun Facts About the Autumnal Equinox
The equinox holds many surprises beyond its link to seasonal change. Here are eight autumnal equinox fun facts that are not widely known.
It's Just a Moment, Not a Day
The equinox occurs at a single instant. This year, it will happen at 2:19 p.m. EDT.
Day and Night Aren't Exactly Equal
Atmospheric refraction gives slightly longer daylight. Depending on location, daylight lasts 12 hours and 6 to 16 minutes.
The Date Can Vary
While most fall on 22 or 23 September, equinoxes can rarely occur on 21 or 24 September. The next 21 September equinox will be in 2092.
It Sparks the Northern Lights
NASA notes geomagnetic activity is twice as likely around the equinox. This makes it prime time for aurora displays.
The Harvest Moon Effect
Around the equinox, the Harvest Moon rises earlier. The time between moonrises shrinks, helping farmers work later into the night.
Cultural Celebrations Worldwide
Druids in London hold ceremonies at Primrose Hill. In China and across Asia, the Moon Festival is tied to the Harvest Moon.
Mayan Equinox Serpent at Chichen Itza
At Mexico's El Castillo pyramid, sunlight forms a serpent shape on the steps during the equinox.
Equilux Happens Later
True equal day and night, known as the equilux, happens a few days after the equinox, not on the same date.
The first day of fall may feel familiar, yet the science and traditions surrounding it show just how remarkable this seasonal shift remains.
