You might want to step outside tomorrow night (July 12), when a bulging “supermoon” will rise in the evening sky.
Tomorrow’s supermoon, which will be bigger and brighter than most other full moons this year, will be the first of three such moons in consecutive months. The next supermoons will occur on Aug. 10 and Sept. 9, according to NASA.
The moon follows an oval or elliptical orbit, and a supermoon occurs when the moon is in the part of its orbit closest to Earth. At this point, known as “perigee,” the moon is about 30,000 miles (50,000 kilometers) closer to the planet than at its farthest point, or “apogee.” [In Photos: Glitzy Images of a Supermoon]
Tomorrow and Sept. 9, the moon will be full on the same day as perigee, and on Aug. 10, it will be full during the same hour as perigee, making it especially bright.
Supermoons gained attention last year, when a June 2013 full moon was 14 percent bigger and 30 percent brighter than other full moons, according to NASA. But these monstrous moons aren’t all that uncommon, as it turns out.
“Generally speaking, full moonsoccur near perigee every 13 months and 18 days, so it’s not all that unusual,” Geoff Chester, an astronomer at the U.S. Naval Observatory in Washington, D.C., said in a statement. “In fact, just last year, there were three perigee moons in a row, but only one was widely reported.”
To the casual observer, it’s not easy to tell the difference between a normal full moon and a supermoon. Clouds and haze can obscure the supermoon’s brightness. “Also, there are no rulers floating in the sky to measure lunar diameters,” NASA officials said in a statement.
However, many people looking at the supermoons this summer may be tricked into thinking they’re bigger because of an illusion that, for reasons astronomers and psychologists don’t fully understand, makes moons look much larger near trees, buildings or other objects near the horizon. Consequently, people watching the moon rise tomorrow may confuse its size and think the supermoon is actually much larger than it is.