BOSTON (Reuters) - Sky watchers and others who spend time outdoors at night in the United States are in for a treat on Monday night in the form of an apparently larger-than-average, shinier full moon, the last of this summer's three "supermoons."
A "supermoon" occurs when the moon is full and at the point of its orbit that brings it closest to the Earth.
On Monday night, the moon will rise in the western sky shortly after sunset and officially be considered a "supermoon" at 9:38 p.m. ET, scientists said.
"Because the moon is at perigee, or the closest point of its orbit, it's going to be about 13 or 14 percent bigger, optically, and ... about 30 percent brighter," said Philip Erikson, principal research scientist at the Massachusetts Institute of Technology's Haystack Observatory outside Boston.
Monday's full moon will also be a harvest moon, the term for the full moon closest to the autumn equinox, which falls on Sept. 23. The harvest moon comes at a time in the lunar cycle when moonrise does not occur significantly later night after night - a change welcome in an agrarian era when farmers worked late into the night trying to harvest their crops before frost set in.
Because of the moon's low angle of rise during the harvest phase, it will appear redder than usual at moonrise, scientists said.
"It's a good reason to look up, because it's a good, bright, full moon," said Amanda Thompson at the Boston Museum of Science's Hayden Planetarium. "You will be able to see the maria on the moon, which are the dark, smooth areas. Those are cooled-off lava flows."
(Reporting by Scott Malone; Editing by Jonathan Oatis)