Once every blue moon. You’ve heard that phrase. But a black moon is just as rare. The next black moon will happen tonight, the last day of July and the first time since 2016.
Blue and black moons do not refer to the actual color of the moon, although the black moon is just that. The color refers to the overlapping that occurs in the lunar cycle during a single calendar month, which doesn’t happen often. This overlap occurs because lunar months are slightly shorter than calendar months. A lunar month (from new moon to new moon) takes roughly 29.5 days, while a calendar month is 30 or 31 days (except for February), which causes the moon phases to shift by about 1 day in each successive month.