The Super Blue Blood Moon!

Kasia Krzyzanska

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Last Wednesday morning, shortly before dawn, we didn’t have just an ordinary full moon. This moon showcased the collision of three fairly rare lunar events, giving it the fabulous name of a super blue blood moon.

The three events that coincided were that the moon was a supermoon at its perigee, meaning that it was at the closest point in its orbit to the Earth; a blue moon, as it was the second full moon that month; and a blood moon, because it happened to also be a total lunar eclipse, making the moon look red.

On their own, these events aren’t entirely uncommon. The last total lunar eclipse was in 2015, the last blue moon on May 21, 2016, and the last supermoon on December 21, 2016. However, not only is it highly unlikely that all these events should occur at once, but it is also lucky that it was able to observe this from New York.

A supermoon only occurs because the moon’s orbit is elliptical, causing some points of it to be closer to Earth than others. When the moon’s closest perigee position in its orbit coincides with its full moon phase, it appears to be significantly bigger enough that we call it a “supermoon”. Though the moon is in perigee about once a month, supermoons occur only two or three times a year. Here’s another article from Kaleidoscope (shameless plug) describing this phenomenon if you’re interested.

Blue moons are less astronomically spectacular. They’re not even really blue- it’s just a what we call the second full moon that occurs in a single month. There’s nothing really sciencey about this, it just happens that lunar cycles don’t quite line up with human-designed months. Technically, there is a “lunar calendar” that contains the 12.37 lunar cycles that typically occur in a year. This was one of the first calendar systems used by civilization; however, as this calendar didn’t line up with the seasons, which are determined by the position of the Earth around the Sun, we eventually switched to a solar calendar. But I digress; the point is blue moons are mostly a random coincidence that happens one about every 2.7 years.

Blood moons, or total lunar eclipses, are much cooler. These actually are reddish in color (hence “blood”), and occur when the Earth’s shadow from the Sun covers the moon. There’s a handy chart explaining it below:

(Thanks, Diagram Site!)

As you might have noticed, the position necessary for a lunar eclipse is the same as the position for a full moon. The reason we don’t have a lunar eclipse EVERY full moon is because the moon’s orbit isn’t super flat all the time- it wobbles a bit. In fact, not every lunar eclipse is even a total eclipse. Sometimes Earth’s shadow covers the moon only enough that it looks like a part of it is missing in a weird place. Total lunar eclipses occur only once or twice a year, but the ones that are visible from New York are a little rarer than that. But why is it red (well, orange-y)? Basically, the Earth still doesn’t really block all of the Sun’s light reflecting off the moon. If it did, it would look entirely black. However, in this situation instead sunlight bends (or refracts) around Earth’s atmosphere, which is filled with many pesky molecules. These molecules scatter sunlight through a process called Rayleigh scattering. As short, blue light waves are scattered more than long, red ones, the waves reaching the moon are mostly this color. (As it happens, this is also what makes the sky blue).

These three events combined resulted in last week’s super blue blood moon. Aside from the general coolness of seeing it (assuming you didn’t happen to be surrounded by trees at the time, like some certain poor journalists 😞 ), this event poses the great opportunity to learn about not one but three astronomical events. Here’s until the next one in 2037!

Photograph of the super blue blood moon.

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