The periodic “supermoon,” as described in recent years, is larger and brighter than the average full moon and certainly worth a skyward glance as it rises like a giant spotlight above the horizon.
But on Monday, the Earth, moon and sun will conclude an orbital do-si-do that leaves them in almost perfect alignment, producing a supermoon unlike any other full moon in 68 years.
NASA says we’re about to witness “an extra-supermoon,” which last occurred in 1948 and won’t recur until Nov. 25, 2034.
There’s a crazy scientific name for it — a perigee-syzygy moon. It occurs when the moon is on the opposite side of Earth from the sun while also at perigee, which is its closest orbital point to Earth.
The perigee-syzygy moment officially will occur at 8:52 a.m. Monday. That’s during daylight, so the best view will be moon rise on Sunday or Monday evening, with similarly sized nearly full moons occurring this weekend and a few days after.
The weather forecast in the Pittsburgh area calls for clear skis Saturday through Tuesday, so NASA said we can expect a beautiful spectacle.
A supermoon occurs within 90 percent of perigee, which means it’s somewhere along the elliptical curve of its orbit closest to Earth. On Monday, there will be a mere two-hour difference between full moon and perigee, which is about as good as it gets when it comes to alignment.
It also will represent one of three supermoons occurring in the final three months of 2016, with a previous one on Oct. 16 and another to occur Dec. 14. The last one notably will block any view of meteors streaking across the sky from the Geminid meteor shower. But that would occur with any full moon.
Sandhya Rao, a University of Pittsburgh research professor in the department of physics and astronomy, said the difference in moon size, from our earthling perspective, will merit a look skyward and serve as a perfect teaching moment for lunar science.
For example, she said, most people think the moon has a circular orbit with Earth at the center.
The fact is, the moon has an elliptical orbit around Earth, which sits off-center inside that ellipse. The moon’s orbit also is ever shifting in relationship to the sun. If perigee defines the moon at its closest position to Earth at 225,622 miles away, its farthest point, known as “apogee,” is 252,088 miles away.
The 26,000-mile difference between perigee and apogee is slightly longer than Earth’s circumference at the equator.
Bottom line, a full moon at perigee vs. apogee is 14 percent larger, NASA said. That’s comparable to the difference between a baseball and tennis ball. The lunar disk also appears 30 percent brighter.
Such events, said Ms. Rao — who holds a Ph.D. in physics — actually are of little scientific interest, with most astronomers more focused on deep space, with her own research involving galaxy evolution and quasars.
But the most dramatic view of the moon, she said, will happen as it rises above the horizon, due to an illusion that makes it appear larger in context with trees and buildings, in comparison with the moon high in the sky.
“I personally do marvel at the magnificent sight of a supermoon rising,” she said. “The big variable here is the weather. Hopefully it will be clear.”
As a researcher also interested in “dim little objects way off in the universe,” Rowen Poole, associate director of the Nicholas E. Wagman Observatory in Deer Lakes Regional Park, Frazer, said he prefers when the moon isn’t visible in the sky, citing its impact on the upcoming meteor shower.
“Some people like looking at the moon and this will be a nice, full, bright moon,” he said. “But a lot of people don’t look up anymore and don’t see the sky because of light pollution. When talking to the public, you like to tell them something interesting about what they are looking at. Here, we can attach a factual story to it, and that’s kind of cool.”
And kind of bright. And kind of rare. And definitely worth a look.