How Big is the Moon?
SPACE.com reader George Garcia sent in his photo of the September 2012 harvest moon taken on Sept. 29, 2012, in Montebello, CA.
Credit: George Garcia

Earth's moon is the brightest object in our night sky. It appears quite large, but that is only because it is the closest celestial body. The moon is a bit more than one-fourth (27 percent) the size of Earth, a much smaller ratio (1:4) than any other planets and their moons. Earth's moon is the fifth largest moon in the solar system.

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

"If Earth were the size of a nickel, the moon would be about as big as a coffee bean," according to NASA.

The moon's surface area is about 14.6 million square miles (38 million square kilometers), which is less than the total surface area of the continent of Asia (17.2 million square miles or 44.5 million square km).

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

The moon's gravitational force is only about 16.6 percent of Earth's gravity. A 45-kilogram person would weigh 100 lbs. on Earth but only 16.6 lbs. on the moon. A person who can jump up 10 feet on Earth would be able to jump almost 60 feet on the moon.

Like most of the solar system worlds, the moon's gravity varies based on its surface features. In 2012, NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission mapped the moon's gravity in unprecedented detail.

"What this map tells us is that more than any other celestial body we know of, the moon wears its gravity on its sleeve," GRAIL principle investigator Maria Zuber of the Massachusetts Institute of Technology said in a statement. "When we see a notable change in the gravity field, we can sync up this change with surface topography features such as craters, rilles or mountains."

While the moon is the closest and one of the longest studied astronomical objects, scientists continue to press the celestial body for details.

"The moon is the Rosetta Stone by which we understand the rest of the solar system," Noah Petro, deputy project scientist for NASA's Lunar Reconnaissance Orbiter (LRO) mission, said in a statement.

Because the moon's orbit is not circular, it is sometimes closer than at other times. Perigee is the term for when the moon is closest to Earth. When a full moon coincides with perigee, we get a supermoon, which appears 14 percent larger and 30 percent brighter than usual. First applied by astrologer Richard Nolle in 1979, "supermoon" originally referred to a new or full moon that occurs when the moon is within 90 percent of its closest approach to Earth. The astronomical term for the phenomena is perigee-syzygy moon.

The difference isn't always obvious; according to NASA, "A 30 percent difference in brightness can be easily masked by clouds or the competing glare of urban lights." [Infographic: How the 'Supermoon' Looks]

"The main reason why the orbit of the moon is not a perfect circle is that there are a lot of tidal, or gravitational, forces that are pulling on the moon," Petro told Space.com, adding that the gravity of the Earth, sun and planets of our solar system all impact the orbit of the moon. "You have all of these different gravitational forces pulling and pushing on the moon, which gives us opportunities to have these close passes."

A supermoon occurs about every 414 days. That's an average, however; the year 2016 boasted not one but three supermoons. The moon won't get as close to Earth as it got during the November 2016 supermoon until November 25, 2034.

The year 2017 only has one supermoon, which will occur on December 3, 2017. The year 2018 will have two supermoons, both in January, with the first on January 2 and the second on January 31. (The January 31 supermoon will also occur during a lunar eclipse.)

A little-understood optical effect can make the moon seem bigger when rising behind distant objects on the horizon. This trick of the brain — known either as the moon illusion or the Ponzo illusion — has been observed since ancient times, but still has no generally accepted explanation.

One theory holds that we're used to seeing clouds just a few miles above us, while we know that clouds on the horizon can be tens of miles distant. If a cloud on the horizon is the same size as clouds normally are overhead despite its great distance, we know it must be huge. And because the moon near the horizon is the same size as it normally is overhead, our brains automatically tack on a similar size increase.

But not everyone thinks clouds have worked their magic on our brains to such a great extent. One alternative hypothesis holds that the moon seems larger near the horizon because we can compare its size to nearby trees and other objects on Earth — and it looms large in comparison. Overhead, amid the vast expanse of outer space, the moon seems diminutive. 

One way to test whether it's just an illusion is to hold your thumb up next to the moon and compare the moon's size with your thumbnail. When the moon is higher in the sky, look at it again; the moon will be the same size compared to your thumbnail.

Additional reporting by Nola Taylor Redd, Space.com contributor

Editor's note: This article was corrected to give Io back to its proper planet, Jupiter.