How Supermoons Work


Moonageddon or Myth? Supermoon Misconceptions
Two men play in front of the supermoon in Turkey on Aug. 31, 2015. Ozkan Bilgin/Anadolu Agency/Getty Images
Two men play in front of the supermoon in Turkey on Aug. 31, 2015. Ozkan Bilgin/Anadolu Agency/Getty Images

Since the idea of supermoons was dreamed up by an astrologer, it's not surprising that phenomenon is seen by some as a sign of future events, and perhaps even as a cause of them — a "Moonageddon," as one Australian news website put it.

If you poke around the internet, you'll find seers linking supermoons to earthquakes, volcanic eruptions, monster storms and even wildfires, and noting that various high-profile natural disasters occurred right around the time that a supermoon loomed over the nighttime landscape.

The makers of those claims often stretch the concept of proximity to extremes — 2005's Hurricane Katrina, for example, has been linked in some accounts to supermoon effects, even though it actually occurred more than seven months after a supermoon [source: Portman].

And even when a supermoon does occur close in time to a natural disaster — such as the 9.1 earthquake and tsunami that hit Japan eight days before a supermoon in March 2011 -- that isn't necessarily evidence of a connection between two events.

As James Garvin, chief scientist at NASA's Goddard Space Flight Center, explained in a 2011 interview, the earth stores a tremendous amount of energy in its crust, and the relatively small amount of force exerted by lunar gravity isn't enough to alter the processes that cause earthquakes [source: NASA].

Supermoons don't have that much more effect on the earth than the moon usually does. The only exception is that when the moon is closer to the earth than usual, it does cause tides to be slightly higher — but only by slightly more than an inch, at most [sources: Sessions, ScienceDaily].

The belief that supermoons can affect human behavior is based upon the assumption that a supermoon, because it looks bigger than normal, exerts some sort of unusual force upon Earth's inhabitants. But there's no evidence that is so.

When it comes to gravity, for example, the effect of a full moon at perigee is only about three one-thousandths of an ounce (110 milligrams) greater than the moon exerts at apogee — 1/9th of the mass of a paper clip. That effect actually is smaller than you'll encounter when you're near a mountain or even a large building [source: Sessions].