Ask Ellen: How can a blue moon happen in the middle of a month?

GRAND RAPIDS, Mich. (WOOD) — The saying “once in a blue moon” is used to describe something as being rare, but the blue moon we saw Monday evening was especially so. It has to do with why a moon is given a descriptor of “blue” at all.

We only see a full moon once every 29.5 days. Most months have 30 or 31 days. This means typically, only one full moon is possible each month. However, since the monthly calendar doesn’t match up perfectly with the lunar cycle, every 33 months, two full moons land in a single month. The second of these full moons is given the title of “blue” moon.

For this to happen, the first full moon of the two must happen at the start of the month, like the first or second. The second full moon then arrives 29.5 days later at the end of the month. So, how was the full moon on Aug. 19 a blue moon?

SEASONAL BLUE MOONS

Apparently, NASA has a second qualifier for a moon to be blue. It has to do with how many full moons land in an astronomical season. Typically, during each of our four seasons, we see three full moons. But, if there are four full moons in one astronomical season, as experienced this summer, the third of the four moons is considered “blue.”

The next seasonal blue moon will not happen again until May 2027. The next seasonal blue super moon will not happen until January 2037.

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