Depending on where you live, an equinox can mean the start of longer, gloomier nights or the promise of warmer days and more sunlight. Either way, humanity has been tracking these celestial events for millennia. Ancient observatories around the world kept an eye on equinoxes, along with the more obvious solstices. These days, you don't need a stone circle or a fancy observatory - just check your calendar, which marks the equinox twice a year.

The word "equinox" comes from Latin: equi- meaning equal, and nox meaning night. But what exactly is an equinox? It's the exact moment when the center of the Sun crosses the plane of Earth's equator, and it happens twice a year, in March and September. In 2026, the equinoxes will occur on March 20 at exactly 14:46 UTC (or 7:46 AM EDT), and again on September 23 at 00:05 UTC (which is September 22, 2026, at 5:05 PM PDT for those keeping score).

On the day of an equinox, observers at the equator will see the Sun directly overhead at noon. After the March equinox, the Sun's path in the sky continues moving north until the June solstice, then heads south. It crosses the equatorial plane again at the September equinox and keeps going south until the December solstice, when it turns around. This is why some call the March event the northward equinox and the September one the southward equinox.

During an equinox, the Sun shines equally on both the Northern and Southern Hemispheres, making these the only times when both poles are simultaneously lit by sunlight. But don't expect a perfect 12-hour day and 12-hour night. The actual equal-day-and-night split depends on your latitude and can happen a few days before or after the equinox. Why? Blame the Sun and our atmosphere. The Sun is a sphere, not a point light source, so its edge gets refracted by our atmosphere as it rises and sets, adding several minutes of light each day. The Sun doesn't just wink on and off like a light bulb, so there's no perfect split - but it's close.

Equinoxes are tied to the changing seasons. In March, Northern Hemisphere observers welcome the longer, warmer days of their vernal (spring) equinox, while Southern Hemisphere observers brace for shorter days and longer, cooler nights with their autumnal (fall) equinox. Come September, the roles reverse.

Originally posted by Dave Prosper: February 2022