The full moons of summer 2014 will all be supermoons, NASA says. The first of the trio will occur Saturday, July 12.
The scientific term for the phenomenon is “perigee moon,” according to NASA. Perigee moons will also occur on Aug. 10 and Sept. 9.
Full Moons vary in size because of the oval shape of the Moon’s orbit. The moon follows an elliptical path around Earth with one side (“perigee”) about 31,000 miles closer than the other (“apogee”). Full Moons that occur on the perigee side of the Moon’s orbit seem extra big and bright.
This coincidence happens three times in 2014. On July 12 and Sept. 9, the moon becomes full on the same day as perigee. On Aug. 10, it becomes full during the same hour as perigee — arguably making it an “extra-super moon,” NASA says.
“Generally speaking, full moons occur near perigee every 13 months and 18 days, so it’s not all that unusual,” says Geoff Chester of the U.S. Naval Observatory. “In fact, just last year there were three perigee moons in a row, but only one was widely reported.”
In practice, it’s not always easy to tell the difference between a supermoon and an ordinary full moon. A 30 percent difference in brightness can easily be masked by clouds and haze. Also, there are no rulers floating in the sky to measure lunar diameters. Hanging high overhead with no reference points to provide a sense of scale, one full moon looks about the same size as any other.
“The ‘Moon Illusion’ is probably what will make people remember this coming set of full moons, more than the actual view of the moon itself,” he says.
The illusion occurs when the moon is near the horizon. For reasons not fully understood by astronomers or psychologists, low-hanging moons look unnaturally large when they beam through trees, buildings and other foreground objects. When the moon illusion amplifies a perigee Moon, the swollen orb rising in the east at sunset can seem super indeed.
“I guarantee that some folks will think it’s the biggest moon they’ve ever seen if they catch it rising over a distant horizon, because the media will have told them to pay attention to this particular one,” says Chester.