What is a blue moon? | Popular Science
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What is a blue moon?

What a strawberry moon? What is a worm moon? What is going on with my moon?


The Full Moon



Blue moons, strawberry moons, supermoons. For some reason your news aggregation algorithm of choice thinks you really really really want to know all about these moons. “Catch This Weekend’s AMAZING SUPERMOON,” one headline (or, like, 500 of them) will announce. “The Supermoon Isn’t Actually A Big Deal And You’re All Ruining Astronomy,” another will grouse.

On March 31—that’s Saturday, also known as tomorrow in some circles—we’ll have our second “blue moon” of the year. And while that’s not necessarily special in an oh-gosh-get-out-and-look-at-it kinda way, it’s certainly special: a blue moon is the nickname for when two full moons fall in the same calendar month, and we haven’t had two in one year since 1999. We won’t have it happen again until 2037. Astronomer David Chapman recently explained for EarthSky that this is merely a quirk of our calendar; once we stopped doing things based on the moon and started trying to follow the sun and the seasons, we stopped having one reliable full moon per month. The moon cycle is 29.53 days long on average, so on most months we still end up with a single new moon and a single full one. But every once in a while, things sync up so that one month steals a full moon from another. This year (and in 1999, and again in 2037) both January and March stacked full moons on the first and last nights of the month, leaving February in the dark.

There you have it, folks: this year’s blue moons are interesting, for sure, but the moon is still waxing and waning to the same rhythm it’s followed since before we were around to stare at it. Our calendar is just rigged to make that pattern occasionally seem like a big deal.

Consider this your go-to resource for all moon-gazing news. Here’s what you need to know about this week’s lunar event.

Regular Ol’ Full Moon

Look, it’s okay if you don’t know. There are probably loads of folks who walk around pretending they totally know why that thing in the sky seems to get bigger and smaller at regular intervals who totally do not.

The moon orbits Earth, and it’s tidally locked—that means it always shows us the same face, instead of twirling around like our planet does. That’s why you can always see the man on the moon (or the moon rabbit, depending on your cultural preferences) even as it spins around us. But while the moon is big and bright in the sky when it’s full, that’s only because it’s reflecting light from the sun. But the moon is always moving, so it’s getting hit with sunlight at different angles. It’s invisible to us during the “new moon,” because our satellite is parked right between us and the sun; the so-called dark side of the moon is lit up like a Vegas, but the side we can see is in shadow. A full moon happens when the earth is right between the sun and the moon, so sunlight hits the part we can see. And all the other phases are just the transition from one of those extremes to the other.

Blue Moon

See above. Getting two blue moons a year is rare, but we have individual blue moons every few years. Also, fun fact: not actually blue. A moon can in fact take on a moody blue hue, but this only happens when particles of just the right size disperse through the sky. Big clouds of ash from volcanic eruptions or fires can do the trick, but it doesn’t happen often.

Paschal Moon

You may have heard that the super special second blue moon of 2018 is also a Paschal moon. This is true! That just means it’s the first full moon of spring, which is often used to determine the date of Easter Sunday. All of this is just calendar nonsense and we refuse to go into it further.

Super Moon

The moon isn’t always exactly the same distance from Earth, because its orbit isn’t perfectly circular. We call the closest point perigee, and the most distant point is apogee. 2018’s closest perigee and most distant apogee both happened in January, and the difference was about 30,000 miles.

The reason you care about this middling change in distance is that it turns a moon super. When a full moon happens close to perigee, it’s going to look a smidge bigger. Honestly, the difference is not that profound, but if you’re in a position to photograph the supermoon next to something that shows the slight increase in scale, it can look pretty cool.

Super Moon

Super moon

This is a "super moon" rise from 2017. Using a 1200mm lens makes the moon appear larger in the scene.

Stan Horaczek

Micro Moon

See above; it’s the opposite of the super one. Size isn’t everything.

Blood Moon

Objectively the most metal moon, these only occur during total lunar eclipses (which can happen a few times a year in any given location). When the moon slips through our shadow, we give it a reddish tint. The moon can also look orange whenever it’s rising or setting, or if it hangs low in the horizon all night—the light bouncing off of it has to travel through thicker atmosphere there than when we view it sitting directly above us, which scatters more blue light away. But you’ll probably only see that deep, sinister red during an eclipse.

A lot of headlines about moons are just silly (you do not need to be particularly excited about a blue moon, it just looks like a regular ol’ moon), but you should definitely roll out of bed to look at a blood moon if one is going to be visible in your region.

Snow Moons, Worm Moons, Strawberry Moons…

Sometimes you’ll see a headline that promises a moon with so many qualifiers it makes your head spin. A superblueblood worm moon, mayhaps?

Many cultures have traditional names for the full moon in a given month or season, so there’s quite a list to draw from if you’re trying to really plump up a story on a slightly-bigger-than-average view of the moon. But these are all based on human calendars and activities and folklore; you will not go outside and see a pink moon in April, though I wish it were so.

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