Why Saturday’s full moon will be ‘blue’

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GRAND RAPIDS, Mich. (WOOD) — Our next full moon, visible Saturday, will be a blue moon — but perhaps not in the way that you would initially think.

Astronomy lovers are most likely familiar with this definition of a blue moon: the second full moon in a calendar month. However, there is another, less well-known definition for a blue moon. Every few years, it’s possible to see four full moons in a season, the third of which is named a blue moon.

Here’s how it works: Each year has four seasons and each season is roughly three months. The time between full moons is close to that of a calendar month, but it’s not exact. The offset can allow us to see two full moons in the same month or four full moons between an equinox and a solstice.

The vernal equinox fell on March 20 this year and we had a full moon March 20. The second full moon of the season was April 19. The full moon Saturday will be the third, or the blue moon. Our final full moon of the season will be on June 17, just a few days before the June 21 summer solstice.

Blue moons are not common, but not quite as rare as the phrase “once in a blue moon” might imply. There were two monthly blue moons in 2018, one in January and one in March. The seasonal blue moon Saturday will be the fourth in the past decade. The previous three were Nov. 21, 2010; Aug. 20-21, 2013; and May 21, 2016. Our next seasonal blue moon will come on Aug. 22, 2021.

It’s much less likely to only see two full moons in a season. The last time this happened was 1961-1962, when there was a full moon just before the winter solstice and a full moon just after the vernal equinox, leaving only two full moons in the entire winter season. The next time this phenomenon will happen will be the winter of 2314-2315.

In addition to the blue moon we’ll see this year, there are several other names for the full moon in May. It has been referred to as the flower moon, the milk moon or even the corn planting moon.

And despite what the name might lead you to believe, the moon will not take on any sort of blue coloring. It is possible for the moon to appear blue, but only when certain atmospheric conditions are present. Volcanic eruptions and wildfires can both release particles into the atmosphere that are the correct size to scatter out red light. Once the red light has been scattered, the blue light can filter through and give the moon a blue tint.  

The moon will officially be full at 5:11 p.m. Saturday.

***CORRECTION: An earlier version of this story incorrectly stated the full moon was March 21, not March 20. We regret this error, which has since been corrected.***

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