The autumnal equinox is the day in September when summer officially ends and fall begins. It happens when the sun crosses directly over Earth's equator, making day and night almost exactly equal in length everywhere on the planet.

This astronomical event occurs around September 22 or 23 each year in the Northern Hemisphere. After the autumnal equinox, nights grow longer than days as winter approaches, and the sun's path across the sky gets lower. In the Southern Hemisphere, the same date marks the vernal equinox, or the start of spring.

Ancient peoples built monuments like Stonehenge partly to track the equinoxes. These moments mattered enormously for knowing when to harvest crops or prepare for winter. Many cultures still celebrate harvest festivals near the autumnal equinox.

You can observe the equinox yourself: on this day, the sun rises almost exactly due east and sets almost exactly due west. After the equinox, you'll notice the sun rising farther south each morning and setting earlier each evening. The autumnal equinox reminds us that Earth is always moving through space, tilting on its axis as it orbits the sun, creating the rhythm of seasons that shapes life on our planet.