The Science of Sustainability

SuperMoon—or Paul Bunyan Moon?

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People have a propensity for attraction to extremes. We are fascinated by the biggest, the smallest, the farthest, the nearest, the most wildly colorful—you name it. We're Extreme Adjectives Junkies!

This point was brought to my awareness again leading up to last Saturday's "SuperMoon" event: when the Full Moon phase on March 19 coincided with the Moon being at perigee–the closest point to Earth in its elliptical orbit–and therefore at its largest apparent size, its brightest, and its most influential, tidally speaking. By the numbers, this periodic event hadn't happened for 18 years–though it should be noted that the Moon gets just as close to the Earth every month, but it's usually not a Full Moon when it does.

It seems every news media source—on air, on web, on print—was running the story, and there was a great deal of public interest…which was, of course, mostly fueled by the media coverage! I got calls from several news agents wanting the facts and my thoughts about the SuperMoon—and of course what they wanted to hear was that the Moon's closeness and size and brightness and gravitational attraction were all magnitudes greater than normal, that the moment had measureable effects on tides, earthquakes, and volcanic eruptions, and of course that there would be a bumper crop of werewolves and lunatics running around the night….

I felt bad when I had to deliver the sober, perhaps tepid, facts: according to the gossip, the Full Moon would be 14% larger in the sky and about 30% brighter than it is at the also-rare "Apogee Full Moon" when it is farthest from us. I suppose 30% brighter is nothing to sneeze at, though since our night vision sensitivity adjusts to lighting levels, we might not experience that great a difference. But 14% larger, while a detectable difference to our human perception, probably isn't enough to make us take notice if we were not otherwise alerted to the situation; I feel the mere suggestion that the Moon appears larger is a greater influence than the physical fact.

Put another way, if you were standing in the middle of a vast, flat, treeless plain, and you saw someone standing 700 feet away from you, with no visual cues for comparison, could you tell the difference if that person was 6 feet tall instead of 5-foot-1? That's analogous to the difference between seeing the Moon when it's nearest to us (perigee, 217,862 miles) and farthest away (apogee, 243,418 miles).

By the way, when I run the numbers, I calculate the Moon appearing less than 12% larger at perigee than at apogee…so either I've done my math wrong, or the "story" has grown larger with the telling somewhere along the line…. The quoted nearness of the perigee Full Moon may also have grown in stature from the fact; I was hearing numbers like 50,000 miles closer than at apogee, but doing the numbers I get a bit over 25,000. I'm tempted to call this the Paul Bunyan Full Moon instead….

But the perigee Full Moon is factually the closest, largest, and brightest of all Full Moons, and quite rare to boot, and that is more than enough to excite us Extreme Adjectives Junkies.

Your best bet for seeing a truly huge, awesome Full Moon is to catch it when it's rising, or setting, because our brains play a perceptual trick when the Moon is close to the horizon, making the Moon seem much larger than when it's high in the sky. You can dispel this "Moon Illusion" easily, either by holding up your pinky finger at arm's length, measuring the Moon against your finger's width, and comparing the result to any other time you measure it—or by looking at the bloated, near-horizon Moon with your head upside down (try it if you don't believe me!).

As for the physical effects on Earth by the SuperMoon (other than the lycanthropic overtones), the ocean tide levels were expected to be about an inch higher than usual…nothing to run for high ground over.

Bottom line: The Full Moon is splendid to watch from any distance. Just tell yourself it's really big and beautiful, and let your brain do the rest….

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Category: Astronomy, Partners

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Ben Burress

About the Author ()

Benjamin Burress has been a staff astronomer at Chabot Space & Science Center since July 1999. He graduated from Sonoma State University in 1985 with a bachelor’s degree in physics (and minor in astronomy), after which he signed on for a two-year stint in the Peace Corps, where he taught physics and mathematics in the African nation of Cameroon. From 1989-96 he served on the crew of NASA’s Kuiper Airborne Observatory at Ames Research Center in Mountain View, CA. From 1996-99, he was Head Observer at the Naval Prototype Optical Interferometer program at Lowell Observatory in Flagstaff, AZ. Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.
  • Marilyn

    You are right..I have a very SPECIAL thing with the moon..I always want to mark my calendar and see Full Moon,
    all through the years of my life, UNREAL but true…and the MOON on that night was special and now I have learnt
    the name PERIGEE or supermoon, "I am in awe at the splendour of our NIGHT sky and the luminosity of our only MOON.