You’ve probably heard the phrase “once in a blue moon.” Maybe you’ve even used it to describe an occasional occurrence in your life — the frequency with which you buy a new cellphone, for example.
But this weekend will bear witness to a true blue moon: the second full moon to appear in the sky in a single month.
The celestial event comes around seven times every 19 years or roughly every two to three years, according to EarthSky.
What is a blue moon?
The moon cycle has eight lunar phases, and it lasts 29.5 days — just shy of a typical month’s length. As a result of this mismatch, the occasional month will host two full moons. The second of these moons is called a blue moon — a calendrical blue moon, to be precise.
