A super moon rises behind the Washington Monument on June 23, 2013, in Washington, D.C. During this perigee, the moon was about 221,824 miles away, compared to the 252,581 miles away that it is at its furthest distance from the Earth (apogee). (Photo: NASA/Bill Ingalls)
ALBANY — If the full moon this evening looks bigger than usual — and maybe redder — it’s not your imagination, though some of it may be because of a bit of an illusion.
The full moon will rise around 8:17 p.m. and will be about 30 percent brighter than usual, the second of three consecutive so-called “super moons.” The real term is perigee moon. The moon looks bigger and brighter for a simple reason — it’s closer to the Earth.
At perigee, the moon is at the closest part of its elliptical orbit, just over 34,000 miles closer to Earth than when it is at its apogee. Officials with NASA say today’s moon is the second of three consecutive ones to become full at the same day they are closest to the Earth, though today’s could be argued to be a “more-super” moon because when it becomes full at 2:10 p.m., it will be the same hour it reaches perigee.
If this seems to be unusual, it really isn’t, according to Geoff Chester of the U.S. Naval Observatory in a report on NASA’s science news website.
“Generally speaking, full moons occur near perigee every 13 months and 18 days, so it’s not all that unusual,” Chester said. “In fact, just last year there were three perigee moons in a row, but only one was widely reported.”
Depending on sky conditions, it sometimes is difficult to tell whether the moon is appreciably brighter. And while it will be 14 percent larger in appearance than an apogee full moon, there are no reference points in the sky to help with that measurement. If you didn’t expect it to look bigger, you likely wouldn’t notice any difference.
The most common nickname for the August full moon is the sturgeon moon, owing to Midwest Native Americans who found it coincided with the best fishing time for sturgeon, but one of its nicknames also is the Red Moon. That’s because weather conditions tend to be conducive toward the August full moon having a more red or orange tint than normal when it rises.
When the moon is low on the horizon, there is more atmosphere between it and the observer. The longer wavelength red light travels better through the atmosphere, while shorter wavelength blue light tends to scatter. With more red light reflecting from the moon and reaching the eye, the moon appears redder. Dust and moisture in the air also can scatter blue light more.
That low point at moon rise or moon set (7:57 a.m. Monday) also creates an optical illusion that the moon is “bigger” when it rises or sets than when it is overhead. It may be that near the horizon where it can be compared to things, trees or buildings give the observer a perspective for comparison that is simply lost in the wide expanse of the night sky when the moon is high overhead. Chester told NASA that astronomers and psychologists aren’t sure why the moon seems to look so much bigger near the horizon, but he thinks that phenomenon will contribute to people’s reactions to it.
“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,” he said in the NASA report.