What’s Wrong with a Super Blue Blood Moon?

(Courtesy of Michael McKeag)

It was cloudy here on the morning of Jan. 31st, so we didn’t get to see the lunar eclipse – or photograph it.  Fortunately, my friend Mike McKeag was in southern California and managed to get this great shot of the Moon with background stars.

I did get a lot of questions about this much-hyped event, so thought I’d summarize the answers here:

A “Super Moon” just means that the Moon is near the time in its orbit where it is closest to Earth (at perigee), so it appears bigger and brighter. Since no one pays much attention to it other than at or near the full moon, the term “super moon” is generally used when perigee is more or less coincident with a full moon. This happens quite frequently, so it is not really rare. The increase in size and brightness is often overstated. For example, I heard a reporter say that it is 14% larger than normal, which is incorrect. At perigee the Moon is perhaps 13% larger than it is when at the farthest point, not the average, or “normal” size. A super Moon is 6 to 7% larger than average. The change in brightness is somewhat more dramatic, with a roughly 30% change from dimmest to brightest. However, human perception of brightness works in a logarithmic scale, so 30% doesn’t look like a big difference. In fact, very few people could tell a “Super Moon” from an ordinary one unless they could see them side-by-side.

A “Blue Moon” is widely recognized as the second full moon within a calendar month. This is actually incorrect, at least by the classical definition. The original definition of a “blue moon” is the 3rd full moon in a season (3 month period of Spring, Summer, Fall, or Winter) that has 4 full moons. This definition and the term “blue moon” were created by The Farmer’s Almanac. Due to an error on the part of an astronomy magazine, which published the other definition, the “2nd full moon in a month” definition has now become the more commonly understood meaning. Our calendar months and seasons are somewhat tied to Earth’s orbit around the Sun, but the Moon’s orbital period is essentially unrelated. Furthermore, the end points of months and seasons, while consistent from year to year, are arbitrary points in Earth’s orbit, so the coincidence of 2 full moons in a month or 4 in a season is quite arbitrary and meaningless. The reason for “blue” in this phrase is more complicated and somewhat lost in history, but it certainly has nothing to do with the color appearance of the Moon.

A lunar eclipse happens when Earth comes between the Sun and the Moon. That is, when the Moon is in Earth’s shadow. This can happen either partially or totally, depending on whether the Moon is completely within Earth’s shadow or not. In a total lunar eclipse the Moon is much darker than normal, as it is normally in direct sunlight. However, a significant amount of light from the Sun is refracted by Earth’s atmosphere and then lights up the Moon’s surface. This light has a reddish hue because the different colors of light refract differently. This is the same reason that a cloudless sky appears to be blue. Because of this coloration, a total lunar eclipse is sometimes called a “Blood Moon”. Some lunar eclipses are redder than others – it depends on how deeply shadowed the Moon is. Note that a lunar eclipse can only happen when the Moon is full, so that part of it is not coincidental at all.

When you combine the cycles of these 3 events it is true that it happens very rarely. But since the “blue moon” part of it is essentially arbitrary and the “super moon” is not very significant, the whole thing is a “made for TV” event. On the other hand, amateur astronomers are generally happy when people pay any kind of attention to what’s happening in the night sky, so we don’t complain about the excessive hype.

So I hope that you got to see it, but if not, the next total lunar eclipse (for the U.S.) will be Jan. 21, 2019.  If you want to see a really good one, mark your calendar for June 26, 2029!


About Greg Marshall

I am a retired electronics engineer and after a few months of enjoying my leisure I began to miss doing product development. My astronomy hobby always needed new solutions to unique problems, so I decided that whenever I came up with a good solution I would try to make it available to others.

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