On September 27th, there will be a very rare event in the night sky

Coming soon for the first time in more than 30 years: you’ll be able to witness a supermoon in combination with a lunar eclipse.

Late on Sept. 27, 2015, in the U.S. and much of the world, a total lunar eclipse will mask the moon’s larger-than-life face for more than an hour. But what is this behemoth of the night sky? Not a bird, not a plane, it’s a supermoon! Although this incarnation of the moon comes around only once every year, it’s not as mysterious as you might think.

“Because the orbit of the moon is not a perfect circle, the moon is sometimes closer to the Earth than at other times during its orbit,” said Noah Petro, deputy project scientist for the Lunar Reconnaissance Orbiter at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “When the moon is farthest away it’s known as apogee, and when it’s closest it’s known as perigee. On Sept. 27, we’re going to have a perigee full moon—the closest full moon of the year.”

At perigee, the moon is about 31,000 miles closer to Earth than at apogee. That distance equates to more than once around the circumference of Earth. Its looming proximity makes the moon appear 14 percent larger and 30 percent brighter in the sky than an apogee full moon, which sparked the term “supermoon.”

“There’s no physical difference in the moon,” Petro said. “It just appears slightly bigger in the sky. It’s not dramatic, but it does look larger.” Source: NASA

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