

May 2, 2012: The full Moon has a reputation for trouble. It raises high tides, it makes dogs howl, it wakes you up in the middle of the night with beams of moonlight stealing through drapes. If a moonbeam wakes you up on the night of May 5th, 2012, you might want to get out of bed and take a look. This May’s full Moon is a "super Moon,” as much as 14% bigger and 30% brighter than other full Moons of 2012.

The scientific term for the phenomenon is "perigee moon." 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 50,000 km closer than the other ("apogee"). Full Moons that occur on the perigee side of the Moon's orbit seem extra big and bright.

Such is the case on May 5th at 11:34 pm Eastern Daylight Time1 when the Moon reaches perigee. Only one minute later, the Moon will line up with Earth and the sun to become brilliantly full. The timing is almost perfect.

A ScienceCast video explains the facts and fiction of "super-moons." Play it

Okay, the Moon is 14% bigger than usual, but can you really tell the difference? It's tricky. 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 can seem much like any other.



The best time to look is 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. On May 5th, this Moon illusion will amplify a full Moon that's extra-big to begin with. The swollen orb rising in the east at sunset should seem super indeed.

Diagrams: #1 #2 Perigee is the point in the Moon's elliptical orbit closest to Earth.

Folklore holds that all kinds of wacky things happen under the light of a full Moon. Supposedly, hospital admissions increase, the crime rate ticks upward, and people behave strangely. The idea that the full Moon causes mental disorders was widespread in the Middle Ages. Even the word "lunacy," meaning "insanity," comes from the Latin word for "Moon."

The majority of modern studies, however, show no correlation between the phase of the Moon and the incidence of crime, sickness, or human behavior. The truth is, the Moon is less influential than folklore would have us believe.

It's true that a perigee full Moon brings with it extra-high "perigean tides," but according to the National Oceanic and Atmospheric Administration this is nothing to worry about. In most places, lunar gravity at perigee pulls tide waters only a few centimeters (an inch or so) higher than usual. Local geography can amplify the effect to about 15 centimeters (six inches)--not exactly a great flood.

Super perigee Moons are actually fairly common. The Moon becomes full within a few hours of its closest approach to Earth about once a year on average. The last such coincidence occurred on March 19th, 2011, producing a full Moon that was almost 400 km closer than this one. As usual, no trouble was reported--unless you count a midnight awakening as trouble.

If so, close the drapes on May 5th. Otherwise, enjoy the super-moonlight.



Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit: Science@NASA