Between December 30 and 31, skywatchers will witness a rare celestial event—the Black Moon, the final astronomical phenomenon of 2024.

But have you heard of a "black moon?" The label describes a rare second new moon in a month. Per the U.S. Naval Observatory, a second new moon of December will occur tonight, rising at 4:27 p.m. CT.

That's because a black moon is the term for the second new moon to fall in a single calendar month, and new moons appear invisible to the naked eye from Earth. While new moons happen throughout the year, two new moons only fall in a single calendar month about once every 29 months, according to the Old Farmer's Almanac.