Skywatchers should find a clear eastern horizon to see the last of the three summer “supermoons” as it rises at 7:17 p.m. PDT Monday, Sept. 9. It’s also an early Harvest Moon, which in folklore is the full moon closest to the fall equinox — when night and day are equal.
A supermoon — a perigee full moon in astronomical terms — appears about 14 percent larger and about 30 percent brighter because the lunar orbit is elliptical and is sometimes closer to Earth, according to the website earthsky.org. “Apogee” is the term for when the moon is farthest from Earth.
Last month’s perigee full moon was the closest and largest-appearing of 2014. Even though three supermoons in a row might seem like an unusual event, it’s not that rare, said Geoff Chester of the U.S. Naval Observatory.
“Generally speaking, full moons occur near perigee every 13 months and 18 days, so it’s not all that unusual,” he told NASA Science News. “In fact, just last year there were three perigee moons in a row, but only one was widely reported.”
Watch a You Tube video on the subject by searching for “ScienceCasts: A Summer of Super Moons.”