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Earth's Moon

The regular daily and monthly rhythms of Earth's only natural satellite,

the Moon, have guided timekeepers for thousands of years. Its

influence on Earth's cycles, notably tides, has also been charted by

many cultures in many ages. More than 70 spacecraft have been sent

to the Moon; 12 astronauts have walked upon its surface and brought

back 382 kg (842 pounds) of lunar rock and soil to Earth.

The presence of the Moon stabilizes Earth's wobble. This has led to a

much more stable climate over billions of years, which may have

affected the course of the development and growth of life on Earth.

How did the Moon come to be? The leading theory is that a Mars-sized

body once hit Earth and the resulting debris (from both Earth and the

impacting body) accumulated to form the Moon. Scientists believe that

the Moon was formed approximately 4.5 billion years ago (the age of

the oldest collected lunar rocks). When the Moon formed, its outer

layers melted under very high temperatures, forming the lunar crust,

probably from a global "magma ocean."

From Earth, we see the same face of the Moon all the time because

the Moon rotates just once on its own axis in very nearly the same

time that it travels once around Earth. This is known as "synchronous

rotation." Patterns of dark and light features on the nearside have

given rise to the fanciful "Man in the Moon" description. The light areas

are lunar highlands. The dark features, called maria, are impact basins

that were filled with dark lava between 4 and 2.5 billion years ago.

After this time of volcanism, the Moon cooled down, and has since

been nearly unchanged, except for a steady rain of "hits" by

meteorites and comets. The Moon's surface is charcoal gray and

sandy, with much fine soil. This powdery blanket is called the lunar

regolith, a term for mechanically produced debris layers on planetary

surfaces. The regolith is thin, ranging from about 2 meters on the

youngest maria to perhaps 20 meters on the oldest surfaces in the


Unlike Earth, the Moon does not have moving crustal plates or active

volcanoes. However, seismometers planted by the Apollo astronauts in

the 1970s have recorded small quakes at depths of several hundred

kilometers. The quakes are probably triggered by tides resulting from

Earth's gravitational pull. Small eruptions of gas from some craters,

such as Aristarchus, have also been reported. Local magnetic areas

have been detected around craters, but the Moon does not have a

magnetic field resembling Earth's.

A surprising discovery from the tracking of the Lunar Orbiter

spacecraft in the 1960s revealed strong areas of high gravitational

acceleration located over the circular maria. These mass

concentrations (mas-cons) may be caused by layers of denser, basaltic

lavas that fill the mare basins.

In 1998, the Lunar Prospector spacecraft team reported finding water

ice at both poles. Comet impacts deposited water on the Moon. Some

of it migrated to very dark, very cold areas at the poles.

Much remains to be learned about our Moon. Researchers continue to

study the samples and data returned by Apollo and other missions, as

well as lunar meteorites.


Earth's Moon: Facts & Figures

Discovered By: Known by the Ancients

Date of Discovery: Unknown

Average Distance from Earth

Metric: 384,400 km

English: 238,855 miles

Scientific Notation: 3.84400 x 105 km (0.00257 A.U.)

By Comparison: 0.00257 x Earth's Distance from the Sun


Perigee (closest)

Metric: 363,300 km

English: 225,700 miles

Scientific Notation: 3.633 x 105 km (0.00271 A.U.)

By Comparison: 0.00247 x Earth's Distance from the Sun


Apogee (farthest)

Metric: 405,500 km

English: 252,000 miles

Scientific Notation: 4.055 x 105 km (0.00243 A.U.)

By Comparison: 0.00267 x Earth's Distance from the Sun


Equatorial Radius

Metric: 1737.4 km

English: 1079.6 miles

Scientific Notation: 1.734 x 103 km

By Comparison: 0.2724 x Earth


Equatorial Circumference

Metric: 10,916 km

English: 6,783 miles

Scientific Notation: 1.0916 x 104 km


Metric: 21,970,000 km3

Scientific Notation: 2.197 x 1010 km3

By Comparison: 0.020 x Earth



Metric: 73,483,000,000,000,000,000,000 kg

Scientific Notation: 7.3483 x 1022 kg

By Comparison: 0.0123 x Earth



Metric: 3.341 g/cm3

By Comparison: 0.606 x Earth


Surface Area

Metric: 37,932,330 km2

English: 14,645,750 square miles

Scientific Notation: 3.793233 x 107 km2

By Comparison: 0.074 x Earth


Equatorial Surface Gravity

Metric: 1.622 m/s2

English: 5.322 ft/s2

By Comparison: 0.166 x Earth


Escape Velocity

Metric: 8,568 km/h

English: 5,324 mph

Scientific Notation: 2,380 m/s

By Comparison: 0.213 x Earth


Sidereal Rotation Period (Length of Day)

27.321661 Earth days

655.72 hours

By Comparison: Synchronous With Earth


Sidereal Orbit Period (Length of Year)

0.075 Earth years

27.321661 Earth days

By Comparison: Orbit Period = Rotation Period


Mean Orbit Velocity

Metric: 3,682.8 km/h

English: 2,288.4 mph

Scientific Notation: 1,023 m/s

By Comparison: 0.034 x Earth


Orbital Eccentricity


By Comparison: 3.285 x Earth

Orbital Inclination to Ecliptic

5.145 degrees

By Comparison: Oscillates roughly 0.15 degrees in 173 days.


Equatorial Inclination to Orbit

6.68 degrees

Orbital Circumference

Metric: 2,290,000 km

English: 1,423,000 miles

Scientific Notation: 2.290 x 106 km

Minimum/Maximum Surface Temperature

Metric: -233/123 C

English: -387/253 F

Scientific Notation: 40/396 K

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