National Geographic News
A partial lunar eclipse above Greek mountains.

A partial lunar eclipse stains the full moon red in an undated file picture.

Photograph by Detlev van Ravenswaay, Picture Press/Alamy

Ker Than

for National Geographic News

Published June 1, 2012

Earth's dark shadow will appear to take a bite out of the full moon Monday during a partial lunar eclipse.

At the same time, the moon will be at perigee—the lunar orb's closest approach during its egg-shaped orbit around our planet—creating an eclipsed "supermoon."

Lunar eclipses happen when the full moon, Earth, and the sun are lined up so that the moon crosses through Earth's shadow. (See lunar eclipse pictures.)

"It's a very cool kind of thing to see. It gives us an almost 3-D feeling for space that we don't normally get," said Mark Hammergren, an astronomer at the Adler Planetarium in Chicago.

Coined in 1979, the term "supermoon" is often used to describe a full or new moon that coincides with perigee—something that happens about once a year, on average. (Related: "Titanic Sunk by 'Supermoon' and Celestial Alignment?")

We don't see an eclipse with every full moon because our natural satellite follows a tilted orbit around Earth. This tilt also means that an eclipse can be full or partial, depending on how much of the lunar disk falls in our planet's shadow.

The last total lunar eclipse was in December 2011, and the next one doesn't happen until April 2014.

Monday's partial eclipse will be visible to sky-watchers in most of North and South America during the early hours before sunrise.

Meanwhile, observers in Australia, the eastern parts of Asia, and across the Pacific Ocean will see the partial lunar eclipse Monday evening just after sunset.

The eclipse will last for more than two hours and will reach its peak around 4 a.m. PT, when about a third of the moon's disk will be covered in shadow.

Lunar Eclipse Writ Large

In early May the moon was at perigee within minutes of the official full moon phase, creating a supermoon that appeared 16 percent bigger and 30 percent brighter than the year's other full moons.

Next week the moon will officially reach perigee the day before the full moon, swinging just 222,750 miles (358,482 kilometers) from Earth.

But the lunar orb is only 0.26 percent farther away on the days before and after its closest approach, Hammergren said.

That means the full moon will still look slightly larger than normal during Monday's partial eclipse.

"It won't be a very noticeable effect," Hammergren said. "The full moon will look only about 5 percent larger than it does on average, appearing perhaps just a little bigger than it usually does."

Still, he added in an email, a supermoon eclipse is a relatively rare sight.

"If you restrict the label of 'perigee' to within only one day of true perigee, then such eclipses will only occur every 10 to 12 years."

(Related: "Lunar Eclipse + Winter Solstice—First in 372 Years.")

Eclipse "Visible Proof" of Earth's Shape

While partial lunar eclipses are entertaining events for modern sky-watchers, in ancient times they served as some of the first evidence that Earth is not flat, Hammergren added.

"The Greeks understood that lunar eclipses happen when the shadow of the Earth is falling on the moon," Hammergren said.

(Related: "Ancient Eclipse Found in The Odyssey, Scientists Say.")

During a partial eclipse, Earth's shadow on the moon's face clearly traces a circle.

"No matter what your angle was when the eclipse occurred, the shadow was always circular. And the only thing that projects a circular shadow no matter its orientation is a sphere," he said.

If Earth was flat, by contrast, the shadow would sometimes appear as a thin line or a squashed oval.

An eclipse, therefore, is "visible proof that anyone can see."

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