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Watch Final Summer Supermoon Tonight

  A last act in a cosmic play, the third and final full supermoon of 2014 graces the night sky this weekend. And on Monday evening, a Canary Island telescope will webcast its arrival. (Related: “#supermoon.”) Although the full phase of the moon officially occurs at 9:38 p.m. EDT on Monday, September 8, it will be at its...


A “supermoon” rises near the Lincoln Memorial on March 19, 2011, in Washington, D.C. Image Credit: NASA/Bill Ingalls

A last act in a cosmic play, the third and final full supermoon of 2014 graces the night sky this weekend. And on Monday evening, a Canary Island telescope will webcast its arrival. (Related: “#supermoon.”)

Although the full phase of the moon officially occurs at 9:38 p.m. EDT on Monday, September 8, it will be at its closest point to Earth 22 hours earlier.

So sky-watchers will get to see the lunar disk at its largest on Sunday, September 7 at 11:38 p.m. EDT, when the silvery orb will be just 222,698 miles (358,398 kilometers) from Earth.

Astronomers say, though, that only the most keen-eyed observers will notice that the moon will appear 15 percent brighter and 7 percent larger than the run-of -the-mill full moon.

The super moon that occurred on August 10 was the closest and brightest of the lunar triad this year, when it approached the Earth at only 221,765 miles (356,896 kilometers).

Celestial Mechanics

In terms of celestial mechanics, what is happening during a full moon? The moon orbits the Earth on an egg-shaped orbit, with our planet sitting a bit off center. This means that once a month in its orbit, the moon reaches its closest point to Earth, known as its perigee. This is when the moon looks the largest in diameter. 

At the same time, the moon is also at the point in its 28-day-long orbit around the Earth that it passes opposite the Sun. When viewed from the Earth, the moon will be fully illuminated, or “full.” 

But because the Earth moves around the Sun, the exact position in the moon’s orbit where it reaches its full phase changes. So what this means for sky-watchers is that every once in a while, perigee and a full moon coincide. We like to call it a supermoon, but astronomers prefer to call the event by a less catchy name, a perigee full moon.

It’s the marriage of the two occurrences when we get a brighter and larger-than-normal full moon,” said Geza Gyuk, astronomer at the Adler Planetarium in Chicago.

 “While this is nothing special from a science perspective, it is no doubt very poetical and very romantic.”

See for Yourself

When is the best time to catch the event?

A full moon is visible, weather permitting, all night. The exact moment of perigee and the exact moment of fullness don’t matter too much, says Gyuk. 

“Just find a time that is convenient and where you can spend a few minutes just looking and appreciating,” he said.

“Try and look for the moon when it is near the horizon, that’s when it gives an extra thrill, as it appears larger and more colorful than when it is overhead.”

The moon will appear to rise above the local eastern horizon just after local sunset and will set at sunrise in the west.  Starting at 9:30 pm EDT, the Slooh observatory on the Canary Islands will webcast the full moon.

Webcast courtesy of Slooh.

These rising and setting times are also when photo hounds can get the best lunar portraits because the moon is perched just above foreground objects, like houses, trees, and bodies of water. 

“The setup isn’t too important, but I’d recommend something with not too large a field of view or the moon will simply seem too tiny, said Gyuk. 

“Slightly after sunset, when the moon is low in the sky and the sky is darkening, is very dramatic for viewing and photography.”

Illusion Confusion

What causes the moon to look bigger at the horizon?  

This is really still a mystery of sorts to scientists. It is clearly an optical illusion, because cameras show the moon as precisely the same size, regardless of where it is in the sky. However, it is a convincing illusion.

According to Gyuk, some research has suggested it’s because, at the horizon, we can compare it to objects we are familiar with, while others have claimed it’s because, as a species, we are tuned to pay more attention to things on the horizon that could pose more of a threat compared with those above. “No flying lions on the savanna,” he added.

While most professional astronomers may tire of hearing of the supermoon phenomenon, which has really gone viral in the last few years, some experts like Gyuk actually welcome the interest.

“I don’t think astronomers necessarily scoff at the publicity. They may be a little bemused, but it is wonderful that people take an interest in what is going on in the heavens,” he explained.

“Anything that gets people looking up and wondering is great in my book!”

Follow Andrew Fazekas, the Night Sky Guy, on Twitter and Facebook.

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Meet the Author

Andrew Fazekas
Andrew Fazekas, aka The Night Sky Guy, is a science writer, broadcaster, and lecturer who loves to share his passion for the wonders of the universe through all media. He is a regular contributor to National Geographic News and is the national cosmic correspondent for Canada’s Weather Network TV channel, space columnist for CBC Radio network, and a consultant for the Canadian Space Agency. As a member of the Royal Astronomical Society of Canada, Andrew has been observing the heavens from Montreal for over a quarter century and has never met a clear night sky he didn’t like.