Supermoons tumble out of the calendar like galactic marbles these days, with at least one measurement counting four close encounters of the lunar kind this year.
But on Monday, the supermoon of supermoons – a moon so plump and rare it hasn't been seen in nearly 50 years – will rise in the east-northeast heralding a fly-by that won't occur again until 2034.
The stellar display marks the closest a full moon has been to Earth since January 1948. That means the moon will appear as much as 14 percent bigger than the smallest moon and 30 percent brighter than the average full moon.
“As long as humans have been alive, they’ve had the moon, and now we have this modern understanding of what’s going on with our celestial neighbor,” said Noah Petro, a research scientist at NASA Goddard Space Flight Center. “We want people to recognize the moon is there and to go out and admire it, not just Monday but for the weeks and months and years to follow.”
Monday’s moon will actually reach fullness in the morning, with perigee – its closest approach to Earth in its orbit – happening shortly after.
The distance between Earth and its moon changes because the moon’s orbit is not a perfect circle, but more elliptical, so that one side is closer to Earth than the other. A moon becomes full when it is in alignment with the sun and Earth, creating a stronger gravitational pull that stretches the orbit to an even more elliptical shape.
That pull will also impact fall’s ballooning tides, with coastal flooding expected during high tides.
While the average full moon is about 236,790 miles from Earth, Monday’s moon will be 221,525 miles away – a difference of 15,265 miles.
“That’s a big number to you and me, but to the moon and space, it’s not that huge of a deal,” Petro said. “It’s an interesting quirk in orbital dynamics.”
If it sounds like Petro is trying to temper enthusiasm about the supermoon, he is.
Scientists struggle with stoking excitement while at the same time not wanting people to be disappointed if they aren’t overly impressed by an astral display, be it a meteor shower or swollen full moon.
Even the term “supermoon” has fueled debate. It was coined by astrologer Richard Nolle as a way to define a moon that is at 90 percent or more of its closest approach to Earth. But since then, others, including astronomers, have picked up on the catchy nickname and brought their own definitions to the table.
That’s why, depending on the source, there could be between four and six supermoons on average per year. One NASA blog counts three in 2016, including Monday’s, while Petro said he and his colleague’s consider only November’s full moon super this year.
“It’s not an astronomy term or a physics term. It’s kind of a new development,” said Bob Berman, an astronomer with the online astronomy website Slooh and a columnist for Astronomy magazine. “The concern about the hype is that many people will think they are going to see this gigantic moon, but it’s only about 8 percent closer than the average moon.”
As the moon breaks the horizon, it will appear enormous, which Berman said is an optical illusion that occurs because of how celestial objects are perceived on Earth. It also has to do with the ability to compare the moon as it breaks the horizon with objects in the foreground such as trees or homes.
“If you want to see a big moon, watch it rise. It’s a psychological effect,” Berman said.
Astronomers at Slooh are calling Monday’s full moon a “mega beaver moon” combining its super status with its Native American association. According to The Old Farmer’s Almanac, November was a time to set beaver traps before streams froze.
Florida Atlantic University astronomer Eric Vandernoot, who runs the school’s astronomical observatory, said hype or not, he hopes the supermoon piques the public’s interest in the moon and the mysteries it still holds.
NASA’s Lunar Reconnaissance Orbiter is revealing the moon to be more dynamic than anyone thought, including the discovery of new lunar “skylights,” which are voids in the moon where the surface has collapsed.
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