This weekend’s Blue Moon isn’t the kind of Blue Moon you’re used to (and also isn’t blue) | Popular Science
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This weekend’s Blue Moon isn’t the kind of Blue Moon you’re used to (and also isn’t blue)

What is going on with my moon?

moon

The Full Moon

Regular-style.

NASA

This post has been updated, because you still care about the moons.

Blue moons, pink 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.

One recent example was the super worm moon on March 20. It was the third and last full supermoon of 2019, which didn't necessarily mean it was a moon particularly worth gazing upon. We get these super-duper-lunar events (remember the super blood wolf moon eclipse?) by smashing together all the qualifiers we've historically used to keep track of full moons throughout the year, and in the age of the internet, we can go a little overboard.

On Saturday, May 18 we're getting a “blue flower moon,” which you're probably wise enough to know isn't going to be blue in color. But confusingly, this blue moon isn't even the kind of not-blue blue moon we're used to. It's another kind of moon allegedly referred to as a blue moon, which is frankly infuriating.

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

In March of 2018, we had our second “blue moon” of that year, to much acclaim. 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 a nickname for when two full moons fall in the same calendar month, and we hadn't previously had two in one year since 1999. We won’t have it happen again until 2037. Astronomer David Chapman 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. In 2018 (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.

Getting two blue moons a year is rare, but we have individual blue moons every few years. The next one will occur on Halloween of 2020, so you can expect people to really lose their minds over that one. Also, fun fact: not actually blue. A moon can indeed take on a moody blue hue, but this only happens when particles of just the right size disperse through the sky—and it has nothing to do with the moon's status as "blue." Big clouds of ash from volcanic eruptions or fires can do the trick, but it doesn’t happen often, and the stars would certainly have to align for two such rare instances to occur at once.

Blue Moons that are not actually Blue Moons (see: May 18, 2019)

If you're good at counting, you may be asking yourself how it's possible that we're seeing a blue moon—the second full moon in a month—so early in May. The answer is that it's not possible! The moon is full every 29.5 days, so blue moons are only even possible on the last day or two of any given month. The full moon on May 18 is the first and only full moon for the month of May.

And yet.

Surprise! There's another kind of moon that some farmer's almanacs refer to as blue. Just as there's typically one full moon a month, there are generally three full moons a season. And just as there are sometimes two full moons in a month due to our calendar almost-but-not-quite following the lunar cycle (ugh) there are sometimes four full moons in a season. April's full moon landed right as spring began, leaving enough time for another three (the last will rise on June 17, less than a week before summer officially kicks off and long after we've all started acting like spring is dead and buried). Other outlets may breathlessly refer to this as a rare occurrence, but it happens every couple of years, which is not a lot of time.

Weirdly, the blue moon moniker is apparently applied not to the fourth full moon in a season (which is the one that only happens once-in-a-you-know-what) but to the third. Why? Who knows. What's the fourth full moon in a season called? A full moon. ¯_(ツ)_/¯

Flower Moons, Worm Moons, Pink 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? Or a super blood wolf moon? Lots of websites will tell you that "wolf moon" is the traditional name of the first full moon of the year in "Native American" cultures, which is kind of a weird thing to claim given that there are 573 registered Tribal Nations in the U.S. alone today, not to mention historically. The idea that hungry, howling wolves were such a universal constant in January that all of North America with its disparate cultures, geographies, and languages spontaneously came up with the same nickname is—well, it's dumb. It's a dumb idea.

The flower moon is one name (of many) historically given to the first or only full moon in May. The idea is that it comes around as plants are blooming. But even though the flower moniker probably really did exist somewhere, this is another great example of how bizarre it is to cite full moon names as coming from "Native American" cultures: even the Farmer's Almanac says that some tribes called it the mother's moon, because balmy temperatures were more baby-friendly, and the corn planting moon (self-explanatory).

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 (or a moon full of beavers in November, for that matter), though I wish it were so. Full moons can turn up with a slight tint, but that has nothing to do with what month it is.

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 cast. 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, 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. But anyone who crams both "blood" and "eclipse" into their moniker for a moon is just trying to win the search engine optimization game; a blood moon is just a lunar eclipse that's going through a goth phase. Ryan F. Mandelbaum at Gizmodo makes the case that we should really just stop throwing the phrase "blood moon" around and call them lunar eclipses, which is tough but fair, because they're lunar eclipses and not evidence of bloody battles between the sky gods.

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

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 average distance between the two bodies is about 238,855 miles, so while not insignificant, this shift is far from earth-shattering.

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. Maybe. If you're lucky. 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. Not all supermoons are created equal, by the way: March 2019's super worm moon was super because the date of the full moon lined up with the closest the moon would get to us during March, but the moon was actually at its closest for the year in February.

Micro Moon

See above; it’s the opposite of the super one. Size isn’t everything. We've got one coming up in September 2019, but you probably won't see tons of news outlets crowing over the micro full corn moon.

Paschal Moon

You may have heard that April's pink moon was also a Paschal moon. This is true! That just means it was used to determine the date of Easter Sunday. All of this is just calendar nonsense. In fact, the fall of a full moon just a few hours after the 2019 Vernal Equinox made this calendar nonsense way more confusing than usual. We refuse to go into this calendar nonsense further.

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