On Monday, observers of the skies will once again have the opportunity to experience a rare "black moon." There was already a black moon on December 1, which makes this celestial phenomenon extremely rare.

A black moon also refers to the new moon phase itself when the moon is positioned between Earth and the sun, making it nearly invisible from our view. The black moon is a new moon in Capricorn, an Earth sign associated with ambition, professional goals, societal structures and authority, according to Thomas.

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.