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. reader George Garcia sent in his photo of the September 2012 harvest moon taken on Sept. 29, 2012, in Montebello, CA. reader George Garcia sent in his photo of the September 2012 harvest moon taken on Sept. 29, 2012, in Montebello, CA.
Credit: George Garcia

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). The moon's equatorial circumference is 6,783.5 miles (10,917 km).

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 sq mi or 44.5 million sq km).

Mass, density and gravity

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; Saturn's moon Io is denser, with 3.53 g/cm3.

The moon's gravitational force is only about 17 percent of Earth's gravity. A 100-pound (45 kg) person would weigh only 17 pounds (7.6 kg) on the moon. A person who can jump up 10 feet on Earth would be able to jump almost 60 feet on the moon.

Super moon

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. [Infographic: How the 'Supermoon' Looks]

A supermoon occurs about every 414 days. The last supermoon was on May 5, 2012, so the next supermoon will be on June 23, 2013.

Horizon illusion

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.

— Tim Sharp, Reference Editor