Science

Who You Calling a Blood Moon?

What do all of these "rare" moon terms really mean?

Supermoon? Blue moon? Blood moon? Yeah, let’s go ahead and pump the brakes on those names, because the first was created by an astrologer, the second is highly subjective, and the third only became popular recently (thanks, this-must-be-prophecy types). Let’s clarify what these terms really mean.

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    A total lunar eclipse is, of course, when the moon passes through the shadow of the Earth.
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    But the Earth doesn’t actually cast a single, super-delineated shadow. There are two components: the penumbra and umbra.
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    If you glimpse the moon when the light is sneaking through in the penumbra, it still won’t have the reddish or orangish or brownish hue it takes on during the so-called blood moon.
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    “I think the term more recently, really in the last decade or so, has become popular[ized] by these religious zealots that keep proposing that it's the end of time,” says Fred Espenak, scientist emeritus of NASA's Goddard Space Flight Center.
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    A supermoon is, we're sorry to say, really not all that super. Because the moon’s orbit around our planet isn't perfectly circular, its distance from Earth varies over time, slightly changing the way you perceive its size.
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    When the moon is at apogee, compared to when it’s at perigee—and *where* it is at both points—you're talking about a maximum perceived difference in diameter of about 14 percent. This is not something you would notice with the human eye.
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    Now, the third and somewhat more innocuous bit of the celestial trifecta: the blue moon, a term given to the second full moon to occur in a month. Then again, it all depends on your vantage point from Planet Earth—one skygazer's blue moon is another skygazer's first full moon of the *following* month. It's all just lunacy.
Matt Simon is a senior staff writer covering biology, robotics, and the environment. He’s the author, most recently, of A Poison Like No Other: How Microplastics Corrupted Our Planet and Our Bodies.
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