Sky-gazers can enjoy a rare celestial event tonight as the second full moon of August shines overhead.

This "blue moon" refers not to color, but to rarity. Blue moons are defined as either the fourth full moon in a season, or, more recently, as the second full moon in a month. The first full moon this month was on Aug. 1.

Blue moons happen because our calendar months don't precisely sync with the moon's orbit. It takes the moon 29.5 days to wax and wane from full to new to full again. With the exception of February, months are longer than that, meaning that once in a while the timing works out so there are two full moons in one month, according to a story from LiveScience.