At 8:05 p.m. Eastern time on Tuesday, the sun will be directly overhead at the equator, marking the autumnal equinox. This celestial event signals the start of fall in the Northern Hemisphere and the beginning of spring in the Southern Hemisphere.
During an equinox, the Earth's axis is not tilted toward or away from the sun, resulting in nearly equal lengths of day and night worldwide. The equator receives the sun’s rays at a perpendicular angle, which is why the sun is said to be directly overhead at that point.
Observers in the United States will notice a shift in the daylight hours as the equinox passes. The sun will rise and set at roughly the same times for the next few days, after which daylight will gradually shorten.
Scientists and astronomers use the equinox as a key reference point for calendars and for studying the Earth's orbital mechanics. The event has been recorded for centuries and continues to be a milestone in many cultures around the globe.
<small>Source: PBS NewsHour — read the original story there.</small>