VOICES Voices Icon Ideas and Insight From Explorers


Watch Final Summer Supermoon Tonight


A "super moon" rises near the Lincoln Memorial
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.


  1. Patrick Fueta
    September 8, 2014, 11:29 am

    Thanks for I am now exposed to words, like perigee, apogee, perihelion and aphelion all about orbing of the earth. However, anything that gets people looking up and wondering is now appealing to me.

  2. Jason Turner
    Orem, UT
    September 8, 2014, 10:33 am

    Here’s a link to a little super moon time-lapse.

  3. Kurosh Munshi
    September 8, 2014, 9:13 am

    The moon looks bigger at the horizon because the mind believes its further away than when directly up in the sky. In reality this difference is negligible. Since our mind expects to see a smaller moon further away at the horizon, the constant angular width makes the moon appear larger.

  4. Xiaoying Xu
    September 6, 2014, 11:49 pm

    Chinese’s lunar year is so important for the astronemer!

  5. Barbara Rupert
    Oklahoma City
    September 6, 2014, 10:46 pm

    It’s a bit incredible that, this being National Geographic, the author does not mention that the full phase of the moon accomplished 22 hours after the supermoon will be the Harvest Moon.

    The Harvest Moon is the full moon closest to the autumnal equinox, after which the days get progressively shorter in relation to the length of the night. This year the Harvest Moon happens to come early, about two weeks before the autumnal equinox.

    What makes the Harvest Moon so special as a practical matter is that the moon rises as little as 23 minutes after sunset for several nights in a row.

    Fred Schaaf of the Farmers Almanac provides a summary of the science behind the Harvest Moon effect as well as an explanation of its significance to the people who live in the European latitudes, which, of course, extend around the entire northern hemisphere:

    “It must have seemed a boon that just when days were getting rapidly shorter and the Sun seemed to go down all too soon, the Harvest Moon arrived to extend the hours that the harvesting could be done.”

    For a modern frame of reference for the final harvest of the year, Reuters reported on August 28th that the French winter wheat harvest was nearly over with some fields still to be cut in the northern regions along the sea.

    Of course, work didn’t end with the harvest. It also had stored while prepared for use and stored again. For example, threshing wheat was a laborious process that would have been rather dangerous by firelight.
    Think about the significance of this to human civilization. Doesn’t it make you wonder why we do not hear much about the Harvest Moon in the Southern Hemisphere?

    Well, the average latitude of Europe is 50 degrees north. Run your finger along 50 degrees south on a globe or a map and what do you find?

    I highly recommend Deborah Byrd’s article about the Harvest Moon at earthsky.org and Fred Schaaf’s article at the Farmer’s Almanac.

  6. Rishi
    September 6, 2014, 9:56 pm

    Most beutiful picture…..rlly m imprsd.

  7. Namia I. Abana
    September 6, 2014, 7:02 pm

    I saw a Supermoon in the Philippines, it was not Red and huge, it was White.