September 2016 Harvest Moon or Supermoon: Will Maryland Weather Cooperate?

Whether you call the big, orange moon on display Friday, Sept 16, a harvest moon or a supermoon, the bigger question for Maryland sky-watchers may be: Will we see it?

We'll be under cloudy skies and eventually see some thunderstorms this weekend, according to the latest forecast from the National Weather Service. Partly cloudy skies start rolling in Friday afternoon across much of the area, but wet weather isn't expected to arrive until Sunday. So go outside after 7 p.m. tonight and you should catch some glimpses of the supermoon, or harvest moon, whichever you prefer to call it.

The best chance for storms will be Sunday night and into Monday morning, but forecasters at the weather service note amounts will vary widely across the region.

The harvest moon will make an appearance at 7:05 p.m. Eastern Time on Sept. 16. The moon will reach the crest of its full phase at 3:05 p.m. ET on Friday in North America. Perigee comes around 5 p.m. Eastern Time on Sept. 18, giving us the whole weekend to enjoy.

“So, it’s not the biggest possible moon on our sky’s dome, but it’s slightly bigger than usual,” Earthsky.org explained on its website. “Still, in any year you might think the harvest moon looks bigger or brighter or more orange.”

A trifecta of supermoons follow in October, November and December.

The upcoming harvest moon, so named because it’s the closest full moon to the fall equinox on Sept. 22, will be especially bright because it’s near perigee. What’s that, you who slacked in science class, ask? “Supermoon” is the unofficial term for “perigee,” when a full moon coincides with the moon’s closest approach to Earth in its oval-shaped orbit, making it appear 14 percent bigger in the night sky and often taking on an orange hue.

Why is that?

“The orange color of a moon near the horizon is a true physical effect,” Earthsky.org went on. “It stems from the fact that — when you look toward the horizon — you are looking through a greater thickness of Earth’s atmosphere than when you gaze up and overhead. The atmosphere scatters blue light – that’s why the sky looks blue. The greater thickness of atmosphere in the direction of a horizon scatters blue light most effectively, but it lets red light pass through to your eyes. So a moon near the horizon takes on a yellow or orange or reddish hue.”

• See Also: 2016 Meteor Shower Guide

So, if the moon is bigger, why isn’t it a supermoon?

Actually, it’s not bigger. It’s what the French call trompe l'oeil, or “fool the eye,” and what astronomers call a “moon illusion.”

Astrologer Richard Nolle, who coined the term supermoon, says the Sept. 16 harvest moon doesn’t qualify under his definition of a full moon that occurs at or near — within 90 percent — of its closest approach to Earth in a given orbit.

However, astrophysicist Fred Espenak, a NASA scientist emeritus, lists the Sept 16, Oct. 16, Nov. 14 and Dec. 13 full moons as supermoons, and also includes the January 2017 full moon on the list. Their disagreement comes from ambiguity on how “90 percent of perigee” is measured.

Supermoons during the winter months tend to look larger than Supermoons that occur during the rest of the year in the Northern Hemisphere. At this time of the year, the Earth is closer to the Sun. Because of this, the Sun's gravity pulls the Moon closer to Earth, making any winter Super Full Moons look bigger than summer Perigee Moons.

»Photo of a harvest supermoon courtesy of NASA.