On 27 September there will be a very rare event in the night sky – a supermoon lunar eclipse, the first in more than 30 years.
It occurs when a total lunar eclipse masks the moon’s larger-than-life face for more than an hour.
“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 27 September we’re going to have a perigee full moon — the closest full moon of the year.”
At perigee, the moon is about 50,000 kilometres closer to Earth than at apogee.
“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.”
As for the supermoon and a lunar eclipse occurring simultaneously, Petro said, “It’s just planetary dynamics. The orbit of the moon around Earth is inclined to the axis of Earth and the orbital plane of all these things just falls into place every once in a while. When the rhythms line up, you might get three to four eclipses in a row or a supermoon and an eclipse happening.”
The total eclipse will last one hour and 12 minutes, and will be visible to North and South America, Europe, Africa, and parts of West Asia and the eastern Pacific.
Bill Condie is a science journalist based in Adelaide, Australia.
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