Have you ever heard of the phrase, “Once in a blue moon”? If you have, did you ever think about where that saying could have came from?
If you were outside gazing at the stars last Friday, August 31st, you would have caught a glimpse of a large full moon out in the night sky. What you probably didn’t realize was that this full moon was one of the two full moons in August. You might be thinking, “But we’re only supposed to have one full moon a month!” — and if you are, you would be right.
The moon goes through a “lunar cycle,” meaning that it goes through phases (such as full, quarter, half, and new moon) — essentially, we can view the moon from Earth as a constantly changing fraction that can be figured out like a math equation! This lunar cycle takes about 29.5 days to complete, from new moon to full moon. If you divide that by the number of days in a year (365 days), you end up with about 12 lunar cycles (one for each month!). That means for each season of the year — spring, summer, fall and winter — there are 3 full moons, and each of these moons has a specific seasonal name (such as Moon After Yule and Grain Moon).
Here’s the catch: there are a couple more days in the calendar year than there are days in the 12 lunar cycles. What does this mean? It means that every few years (about 2.7, to be exact) the days of the year catch up with an extra lunar cycle, and one of the seasons gets a fourth full moon — one extra full moon in addition to the three full moons in a season. When this happens, the third moon is called the “Blue Moon.”
It would make sense to think that the fourth moon would be the Blue Moon, since it’s the extra one added on, right? Actually, the reason why the third moon is called the Blue Moon is because all of the other seasonal full moons have established and set names — we can’t change those because they go along with the times and seasons of the year, so the third moon gets the special title!
The next Blue Moon is supposed to appear in July of 2015. It’s going to be a while before we can see another one, but in the meantime, all of us can brush up on our facts about the moon and solar system in time for the next awesome thing that happens in space!