What is a Supermoon?

A Full Moon That is Closest to Earth

Jul 27, 2018
Supermoon Silhouette

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What is a supermoon? We agree it’s catchy—and anything that encourages us to explore the night sky is positive—but also let’s get our facts right.

The term “supermoon” is not an astronomical term.  It did not even exist until someone concocted it recently.

  • First, it was applied to the closest Moon of the year.
  • Then, it started to mean the closest two or three Moons each year.
  • More recently, I saw an article quoting, ”4 to 6 supermoons each year.” It was accompanied by a photo of a giant red Moon; they were obviously confused between a totally eclipsed Moon and a merely close Moon. Additionally, they explained that a supermoon “looks 14% larger than normal.”

No, no, and no. Let’s put this to rest. This will also explain why we don’t use the term “supermoon” in our astronomy pages in the annual Old Farmer’s Almanac

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Supermoon Facts

The official astronomical name for a supermoon is “perigee full Moon” or “perigee syzygy.” Not quite as catchy, perhaps.

The word “perigee” refers to the Moon’s closest point to Earth in its monthly orbit, and a “syzygy” is when celestial bodies line up (as the Sun, Earth, and Moon do during a full Moon).

The Moon has an oval or elliptical orbit, not a round one, which means that every night it’s either a bit closer or farther from the Earth than it was the night before.

  • At its closest extreme, the Moon can be 14% closer and thus look 14% larger than at its farthest yearly extreme.
  • Put another way, it varies 7% from its average size

Here’s the key fact: Nobody can perceive a 7% difference in the Moon’s size.

perigee-apogee_full_width.jpg

And if you could somehow place the year’s biggest possible Moon next to the smallest one in the sky, you’d just barely tell the difference. And that’s with the absolute extreme Moons. 

Bottom line: It’s hard to truly perceive any difference at all between the Moon’s size from one month to the next, or one night to the next.

How to see a Huge Moon

But if you want to be guaranteed of seeing a huge-LOOKING Moon, it’s easy…

Simply watch the Moon when it’s rising or setting.

A Moon down near the horizon will always look enormous, thanks to the well-known Moon illusion, which makes our minds exaggerate the size of objects near the skyline.

Try it! If you want a Supermoon, you can have it—any night!

If you’re a Moon lover, see the Almanac’s free Full Moon Guides for each month here.

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s blog on stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe

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perceived moon size

Thank you for explaining... moon illusion... I often wondered why the size seemed to change so very much as the moon's distance from the horizon lengthened.

Best times to breed animals, particularly rabbits

According to the Farmers Almanach, are there better, or best times of the month, or days to breed for show quality animals? I raise pedigreed Silver Fox rabbits for show, and want to breed them for the best quality, without having to breed them more than 3 times per year. I don't want to breed them any more times than I have to. Thank you for your assistance. Barb

SUPER MOON BS

I am an astronomer and just wanna THANK YOU for your clarifying and shedding true Moon light on this endless absurd super moon BS !!!

Best articles

I just wanted to comment that I really look forward to Bob Berman's articles here on the Almanac site! They are so readable and entertaining, and I learn something new every time. He is SUPER! Ha!

How do I stop smoking using the dates on Farmers almanac ?

I have heard of using the dates to stop smoking what all do I need to do? Do I smoke that day? Do I clean out all ashtrays ? Please help I've smoked for 29 years and I want to stop and the pills do help me.

Best Days to Stop Smoking

Hi Stacy, You can find the best days to stop smoking here: http://www.almanac.com/calendar
Yes, if you wish to smoke smoking, get rid of the ashtrays and lighters and everything related to smoking, including any ashtrays or lighters in your car. Even clean your clothes and car from cigarette smell. Move your furniture around or get rid of any “smoking chairs” or spots. Get together a no-smoking kit with the pills, water, gum and toothpicks, hand toys. Stuff candy and toothpicks in your pocket for the day and in your car. Make a list of why you want to stop smoking and put it all over the house. Stay really busy your first days. Good luck!

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