The Washington Post
On Sunday, we’ll see a special cosmological treat. The year’s biggest and brightest full moon will rise.
This year has three supermoons. We had one July 12, and have yet another one Sept. 9. But the supermoon this weekend is the most “super” of the bunch.
The term refers to the time when the moon, which orbits Earth in a slightly elliptical trajectory, is at the absolute closest it can get while also being full. Other full moons have come close. But as a supermoon, Sunday’s will be ever so slightly more magnificent.
“The size difference between even the dimmest and brightest full moon is only a bit more than 10 percent,” said Shawn Domagal-Goldman, research space scientist at NASA’ Goddard Space Flight Center in Greenbelt, Maryland. “So the difference between other ‘supermoons’ and this one isn’t huge.” But he said the event is a great excuse to take a moment to enjoy the beauty of the moon, something so often ignored in hectic daily life.
Lunar events should be especially exciting to urbanites.
“Normally, when cool stuff is happening in the night sky, we miss it because of the light pollution,” Domagal-Goldman said. “But there’s no such thing as too much light pollution to see the moon. All you need is nighttime and a clear sky. If you live in a city and want to share in the awe of the cosmos, this is the astronomical event for you.”
To get those clear skies, Domagal-Goldman recommends going out right at moonrise when there is less likely to be cloud cover. And some optical tricks will help your viewing experience at that time, too.
“It’s going to look biggest and brightest to us when it’s right next to the horizon,” Domagal-Goldman said. When the moon is next to things that we are familiar with, such as trees, it looks much bigger than when surrounded by other astronomical bodies, such as stars.
Known as the Sturgeon Moon (because of the abundance of sturgeon in the Great Lakes and Lake Champlain at this time of year), it becomes full officially this Sunday at 2:09 p.m., just 26 minutes after coming closest to Earth.
While the moon officially reaches full phase and cozies closet to Earth on Sunday afternoon, it will appear more or less full Friday, Saturday, and Sunday evening for night skywatchers (at 96 to 99 percent illumination).
Once it is high in the sky, observers should not expect to see much of a difference between this full moon and others.
“A 30 percent difference in brightness can easily be masked by clouds and haze,” according to NASA. “Also, there are no rulers floating in the sky to measure lunar diameters. Hanging high overhead with no reference points to provide a sense of scale, one full Moon looks about the same size as any other.”
Geoff Chester, an astronomer at the U.S. Naval Observatory, writes on his blog that it is difficult to tell one moon apart from another. Rather, the “moon illusion” – in which the moon appears much bigger than it actually is when it sits on the horizon – is what people remember most.
But Chester pointed to a possible benefit from this supermoon craze: “If it gets people out and looking at the night sky and maybe hooks them into astronomy, then it’s a good thing.”
The moon will rise at 7:55 p.m. Sunday and set at 7:20 a.m. Monday.