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The truth behind the supermoon

The year's closest Full Moon arrives February 19, but it will not blind us, explode, or bring about the apocalypse.
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This image shows the supermoon of 2013 rising above Nossa Senhora do Castelo, a church in Sesimbra, Portugal. The photographer positioned himself 1.2 miles (2 kilometers) from the church and shot through a 3.2-inch refractor.
Miguel Claro

Our culture has given us lots of terms for outstanding items, events, or people. But many of them contain one prefix that suggests the biggest or the best: super. Super Glue. Supernova. Super Bowl. Superman. Even the word that groups these expressions together contains those five letters: superlatives.

This month, you’re sure to hear a more recent addition: Supermoon. Quite simply, it refers to the one Full Moon each year (out of the 12 or 13 that occur) that lies closest to Earth. In 2019, the date that happens is February 19. Some, like CNN, are calling it the “Super Snow Moon” citing Native American and European traditions spelled out in The Old Farmer’s Almanac.

Origin

Actually, “supermoon” isn’t new. The term first appeared in a 1979 article for Dell Horoscope magazine. (I can imagine many of you groaning at this revelation, but remember that astrologers also named the planets and many of the constellations.) The author, American astrologer Richard Nolle, described it as “a New or Full Moon which occurs with the Moon at or near (within 90 percent of) its closest approach to Earth in a given orbit.” Nolle gave no reason for his “within 90 percent of” delimiter. Like most astrological numbers, the range gives the predictor a huge amount of wiggle room.

It’s easy to understand why a term like supermoon would catch on. It describes in general terms how one particular Full Moon each year rates, and — at least for the general public — it sure beats the astronomical term: perigee-syzygy Moon.

Perigee means “closest to Earth,” and syzygy is a term astronomers use for a lineup of three celestial bodies, in this case the Moon, Earth, and the Sun, in that order. Note that a class of supermoons also occurs at New Moon (when the syzygy would be Earth-Moon-Sun), but because we can’t see the Moon at its New phase, I chose not to discuss it here.

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To capture the supermoon on June 23, 2013, this imager created a 26-part mosaic. Each part contains 160 video frames taken through a 7-inch Maksutov-Cassegrain reflector.
Hidenori Nohara
The numbers

To give you some perspective on lunar perigee, during the period from 1990 to 2020, the Moon’s closest approach to Earth occurred November 14, 2016. On that date, it stood 221,524 miles (356,509 kilometers) away, and it was, indeed, a Full Moon. And, just so you know, astronomers measure these distances from the centers of the two bodies. So, their surfaces lie approximately 5,000 miles (8,000km) closer than the numbers suggest.

On February 19, at 4 A.M. EDT, the Full Moon will reach perigee 221,681 miles (356,761km) from Earth. Will it appear bigger? Yes. Will it be brighter? Yes. Will you notice? No.

However, millions of people each year who gaze at the supermoon convince themselves it’s the brightest they’ve ever seen.

And they’re right, in the most general sense. They know it’s the brightest because that’s what they’ve been told. But even the finest visual observers can’t remember how large a previous Full Moon appeared to compare it to a so-called supermoon.

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This image of the supermoon that occurred May 5, 2012, is a composite. The photographers took one shot to record the trees and sky and another shot for the Moon. They then merged the two for the final result.
Craig and Tammy Temple

OK, so it’s bigger and brighter, but by how much? As an example, let’s compare the August 10, 2014, supermoon to the smallest Full Moon of 2014, which occurred January 15. On that date, our lone natural satellite was 252,607 miles (406,532km) from Earth. That means August’s Full Moon was 30,842 miles (49,636km), or 12.2 percent, closer than January’s, which makes it 26 percent brighter (without taking into account the opposition effect).

A 26 percent difference in brightness equals a magnitude difference of 0.25. That’s about the disparity between Betelgeuse (Alpha [α] Orionis), which shines at magnitude 0.58, and Aldebaran (Alpha Tauri), which glows at magnitude 0.85.

When shown these stars (they lie near each other in the Northern Hemisphere’s winter sky), most people with normal vision pick Betelgeuse as slightly brighter. Correct! But imagine showing the best observer you know one of these stars one night and the other seven months later and saying, “Pick the brighter one.” The accuracy would equal that of flipping a coin.

A suggested approach

As an amateur astronomer, you can field comments and questions from family, friends, co-workers, and others regarding the supermoon in one of two ways. You can choose to be an unapproachable elitist snob who poo-poos the whole notion of calling any Moon “super.” Of course, when you think about it, that approach isn’t going to win you any friends, and you’ll probably solidify a few people’s notions of “science types” as unfriendly, unbending, and unable to communicate.

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Apogee and perigee describe the two points in the Moon’s orbit when it lies farthest from and nearest to Earth, respectively. Our lone natural satellite can show up to a 14 percent change in size between the two extremes. The photographer used the same telescope and camera to capture the Moon at apogee (left) February 13, 2006, and perigee July 21, 2005.
Anthony Ayiomamitis

Your second choice is to use this event as a teaching opportunity. Explain — in simple language — what’s causing the supermoon. Encourage people to go out and look at it. Do they have a camera? Give them a few tips about photographing the Moon (use different exposures, include some landscape, etc.), and offer to look at their pictures afterward.

You might even go so far as to set up a telescope and offer “Free Looks at the Supermoon!” I know the Full Moon is the worst time to observe old Luna, but you’d be surprised at how many people never have seen it through a telescope. To them, even a peek during Full Moon, when details are lowest, will be “super” cool.

Regarding observing the supermoon (or any Full Moon), I suggest you use a neutral density filter that screws into your eyepieces. It will dim the Moon, and you won’t be “snow blind” after looking at it. Alternatively, you can make an aperture mask. Just cut a piece of stiff cardboard to fit your telescope’s aperture. Then cut a small hole in the cardboard. If you have a refractor, cut the hole in the middle of the inset; for reflectors or Schmidt-Cassegrain scopes, just offset the hole from the center.


Have fun!

The media have a field day with the supermoon. So, why shouldn’t we? Let’s spread the word that astronomy’s not all about math, light-years, and dark energy. It can be fun, too.

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