reader George Garcia sent in his photograph of the September 2012 harvest moon tackled Sept. 29, 2012, in Montebello, CA. (Image credit: George Garcia)
Earth"s moon is the brightest object in our night sky. It appears quite large, however that is only due to the fact that it is the closestly celestial body. The moon is a bit an ext than one-fourth (27 percent) the size of Earth, a much bigger ratio (1:4) than any type of other planets and their moons. Earth"s moon is the fifth largest moon in the solar system.

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The moon"s mean radius is 1,079.6 mile (1,737.5 kilometers). Double those figures to acquire its diameter: 2,159.2 miles (3,475 km), less than a 3rd the width of Earth. The moon"s equatorial one is 6,783.5 miles (10,917 km).

"If earth were the size of a nickel, the moon would be around as large as a coffee bean," according to NASA.

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The moon"s surface ar area is about 14.6 million square mile (38 million square kilometers), which is less than the complete surface area that the continent the Asia (17.2 million square mile or 44.5 million square km).

Mass, density and also gravity

The moon"s fixed is 7.35 x 1022 kg, about 1.2 percent the Earth"s mass. Put one more way, planet weighs 81 times more than the moon. The moon"s density is 3.34 grams per cubic centimeter (3.34 g/cm3). That is around 60 percent that Earth"s density. The moon is the second densest moon in the solar system; Jupiter"s moon Io is denser, v 3.53 g/cm3.

The moon"s gravitational pressure is only about 16.6 percent that Earth"s gravity. A 45-kilogram human being would sweet 100 lbs. Top top Earth but only 16.6 lbs. Top top the moon. A human who can jump up 10 feet on earth would be able to jump almost 60 feet top top the moon.

Like many of the solar mechanism worlds, the moon"s gravity varies based upon its surface ar features. In 2012, NASA"s heaviness Recovery and also Interior laboratory (GRAIL) mission mapped the moon"s gravity in unprecedented detail.

"What this map tells us is that much more than any kind of other celestial body we recognize of, the moon put on its gravity on the sleeve," GRAIL rule investigator Maria Zuber of the Massachusetts academy of technology said in a statement. "When we see a notable change in the gravity field, we can sync up this adjust with surface topography attributes such as craters, rilles or mountains."

While the moon is the closest and one the the longest studied expensive objects, scientists continue to push the celestial body because that details.

"The moon is the Rosetta stone by i beg your pardon we recognize the remainder of the solar system," young name Petro, deputy project scientist because that NASA"s Lunar reconnaissance Orbiter (LRO) mission, said in a statement.

Super moon

Because the moon"s orbit is no circular, that is periodically closer 보다 at various other times. Perigee is the hatchet for when the moon is closest to Earth. As soon as a full moon coincides with perigee, we acquire a supermoon, which appears 14 percent larger and also 30 percent brighter 보다 usual. Very first applied through astrologer Richard Nolle in 1979, "supermoon" initially referred to a brand-new or full moon that occurs once the moon is within 90 percent the its closest method to Earth. The huge term for the phenomena is perigee-syzygy moon.

The difference isn"t constantly obvious; according to NASA, "A 30 percent difference in brightness have the right to be quickly masked by clouds or the completing glare of urban lights."

"The key reason why the orbit of the moon is no a perfect circle is that there room a many tidal, or gravitational, pressures that are pulling top top the moon," Petro said, adding that the gravity of the Earth, sun and also planets of our solar system all influence the orbit of the moon. "You have every one of these various gravitational forces pulling and also pushing on the moon, which gives us avenues to have actually these close passes."

A supermoon occurs about every 414 days. That"s an average, however; the year 2016 boasted no one however three supermoons. The moon won"t gain as close to earth as it got throughout the November 2016 supermoon until November 25, 2034.

The year 2017 only has actually one supermoon, i beg your pardon will happen on December 3, 2017. The year 2018 will have actually two supermoons, both in January, with the very first on January 2 and also the 2nd on January 31. (The January 31 supermoon will additionally occur throughout a lunar eclipse.)

Horizon illusion

A little-understood optical effect can do the moon seem bigger once rising behind remote objects ~ above the horizon. This trick of the brain — recognized either together the moon illusion or the Ponzo illusion — has been it was observed since ancient times, yet still has no generally welcomed explanation.

One theory holds that we"re used to see clouds just a couple of miles over us, when we understand that clouds top top the horizon deserve to be 10s of mile distant. If a cloud top top the horizon is the exact same size together clouds typically are overhead in spite of its good distance, we recognize it need to be huge. And also because the moon near the horizon is the same size together it typically is overhead, our brains immediately tack ~ above a comparable size increase.

But not everyone thinks clouds have functioned their magic top top our brain to together a an excellent extent. One alternative hypothesis holds the the moon seems bigger near the horizon since we deserve to compare its size to adjacent trees and also other objects on earth — and also it looms big in comparison. Overhead, between the huge expanse of external, the moon seems diminutive.

One means to test whether it"s simply an illusion is to hold your thumb up beside the moon and also compare the moon"s dimension with her thumbnail. Once the moon is greater in the sky, look in ~ it again; the moon will be the very same size compared to your thumbnail.

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Additional report by Nola Taylor Redd, contributor

Additional resources

Editor"s note: This article was corrected to give Io earlier to its appropriate planet, Jupiter.

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