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.

A rare Black Moon will rise at the end of the year, and it will give both professional and amateur astronomers a great opportunity to view some far-away celestial bodies.