Crisp autumn air will soon be rolling in and the leaves on trees will be trading their vivid green hues for shades of orange, red and purple. Fall begins on the autumnal equinox. Although people may see it as the official kickoff to bonfires and flannels, that’s not the only interesting part of this remarkable day.

It’s a moment and not a day. Despite what many people think, the autumnal equinox is not an entire day. In fact, the equinox is a precise astronomical event. It occurs at the exact moment the sun crosses the celestial equator, an imaginary line in the sky directly above the Earth’s equator.

It’s not “equal night.” Although the word equinox comes from the Latin word “aequus” (equal) and “nox” (night), we do not get exactly 12 hours of daylight and 12 hours of darkness on the equinox. This is because of how the Earth’s atmosphere refracts sunlight. The sun appears above the horizon even after it has technically set, according to the National Weather Service.

Northern lights can be seen. Those hoping to catch a glimpse of the aurora borealis will find the weeks surrounding the autumnal equinox are the prime window to do so, according to NASA. Geomagnetic storms that trigger these stunning light displays occur twice as frequently around the fall and spring equinoxes compared to the summer and winter solstices.

It’s not always on the same day. The autumnal equinox does not fall on the same day each year. It usually lands on September 22 or 23, but it can occasionally occur on September 21 or 24 (in the northern hemisphere), indicates the U.S. Naval Observatory.