The Moon (Detail)

Moon Properties

Distance to Earth: 238,900 miles

Equatorial radius: 1,079 miles

Equatorial circumference: 6,786 miles

Escape Velocity: 2.38 km/s

Sidereal rotation period: 27.3 days

Equatorial rotation velocity: 4.627 m/s

Axial tilt: 1.5424° to ecliptic, 6.687° to orbit plane

 Because Earth's moon - the planet's only natural satellite - lacks erosion and an active geology, it preserves a record of the solar system's early days. It's the closest astronomical body to do so, making it a beacon for space travel.

The Earth's Moon
 The Moon isn't the largest satellite in the Solar System, but it is the biggest relative to the size of its parent planet, measuring fully one-quarter the size of Earth. Orbiting 384,400 kilometers (238,900 miles) away, on average, it exerts a strong gravitational pull on our planet. This is evident not only in Earth's tides but also in Earth's fairly stable orientation in space. Without the steady tug of the nearby moon, Earth's axis, now tilted at 23.5 degrees, might wobble between 0 degrees and 85 degrees, with catastrophic effects on seasons and climate. The Earth-moon gravitational relationship also explains one of the moon's most obvious characteristics: the same side always faces Earth. Tidal forces between Earth and the moon have tied them together in their solar system dance so that the duration of the moon's rotation is exactly the same length of time as its orbit. Because of this, until the space age, we had never been able to see the far side of the Moon. (And note that the far side is not the "dark" side, Pink Floyd notwithstanding. The Sun shines on the moon's far side during every orbit - we just can't see the hemisphere.)

 Astronauts from Apollo 17, the last manned mission to the moon, photographed a boulder field at their Taurus-Littrow landing site in 1972. The Apollo missions brought back hundreds of kilograms of moon rocks, which are still being studied.

A history of violence
 Just where did Earth require its massive companion? Several theories have been advanced in the past, each with its own problems. The simplest explanation is that the moon formed at the same time as Earth from the coalescing debris of the early solar system. However, measurements of the moon's density, less than that of Earth, don't support this little twin scenario. Moreover, rocks brought back from manned missions lack the water-bearing minerals found in Earth's stones. Another theory holds that Earth captured the wandering moon as it floated past. This, however, would have been a very difficult task given the moon's size. A third scenario has the spinning earth somehow spitting out the material that made the moon, leaving the Pacific Ocean Basin as evidence. But this, too, is physically implausible.

 The current model of the moon's origin is a dramatic one, but it accounts for the similarities and differences between Earth and its satellite. In this hypothesis, a giant object the size of Mars struck Earth a glancing but titanic blow about 4.5 billion years ago, soon after its formation. In the heat of the catastrophic impact, the impactor's metallic core merged with that of Earth, while great chunks of Earth's crust and mantle were ejected into space. The intense heat vaporized water and most volatile elements from the cast-off material, which clump together and reformed into the orbiting moon within about a century. Because it contained relatively little iron, the moon had a small iron core and was therefore less dense than the earth. The reeling Earth, meanwhile, had been knocked askew by some 23 degrees. It was not long before the mother and daughter world settled into the close orbital partnership we know today.

Land and Seas
The dark and light patches on the full moon's piebald surface may look like a man's face, if you are not too picky about faces, but even a small telescope will reveal the bright mountains, dark plains, and thousands of giant craters that tell of the long history of violence against Earth's satellite.

Galileo, viewing the moon through his newly built telescope in 1609, likened its variable surface to that of Earth and named the lighter areas terrae, "lands," and the smooth dark regions maria, "seas." His maria labels survive on some of the Moon's largest features: Mare Tranquillitatis, for instance, the Sea of Tranquility, or Mare Imbrium, the optimistically named Sea of Rains.

The notion of watery seas on the moon did not survive into modern times, and thanks to space age studies and Apollo missions, we know that the terrae are actually lunar highlands, ancient regions of the Moon's crust containing rocks dating back almost to the Moon's birth, roughly 4.5 billion years ago. These heavily cratered, mountainous areas cover more than 80% of the moon's surface. The maria, two to five kilometers (1.2 to 3 miles) lower than the highlands, are huge impact basins filled with cooled dark lava.

Tides
The pull of the Moon on Earth causes of tides, a fact known since ancient times. But why are there high tides on the side of Earth opposite the moon?

It helps to think of the Earth as a solid ball surrounded by a liquid envelope. The moon's gravitational force pulls strongly on the water nearest it, causing it to bulge toward the moon in a high tide, it also tugs on the center of Earth itself, pulling the solid Earth toward it within its watery covering and away from the ocean's surface on the far side. That "left behind" water thus raises a high tide.

The Sun's gravitational pull contributes to tides as well, although more weakly than that of the moon. When the Sun, Moon, and Earth are aligned at the new and full moons, earth experiences unusually high, or "spring" tides.

The moon's gravitational pull is also slowing Earth's rotation and lengthening our day by two milliseconds per century. To balance the gravitational equation, the energy lost as Earth's momentum slows is gained by the moon, which moves slowly away from Earth. Laser measurements from Earth's surface confirm that our satellite is pulling away by 3.8 centimeters (1.5 in) per year.

Craters
Craters abound on the moon, at least 30,000 of them boasting a diameter greater than one kilometer (0.6 miles). When rocky bodies smacked into the moon, the energy of their motion was converted into heat and sent out shock waves through the crust. The impacting object was vaporized upon collision, while the pulverized debris from the surface was ejected outward into a circular rim 10 to 20 times as large as the impactor. The bigger craters typically have a central peak, where the crater floor rebounded after the shock of the impact, as well as surrounding carpets of debris known as an ejecta blanket. Many of the ejected rocks were large enough and fell from the sky hard enough to create their own craters in turn. The biggest craters of all are called impact basins. The Moon's far side has the solar system's largest impact basin, the continent-size South Pole-Aitken Basin, which measures about 2,500 kilometers (1,500 miles) across.

The Moon's History
In the very earliest stages of its formation, roughly 4.5 billion years ago, the satellite was the largely molten. Denser, heavier material sank inward towards its center, while its lighter elements rose to form the surface crust as the moon gradually cooled.

Then, sometime between 3.9 billion and 3.8 billion years ago, debris from the early solar system bombarded the poor moon, huge rocks blasting crater after crater out of its surface with the force of multiple hydrogen bombs. Even as the bombardment began to slow, volcanism took over. Heat from the decay of radioactive elements within the moon pushed molten rock through the thin crust beneath the biggest impact basins, where it spread out and cooled to form the lunar maria.

Lunar Anatomy
Beneath its soil, called regolith, is the lunar crust, thinner on the near side, particularly under impact basins, and thicker on the far side. Under that is a mantle, cool, dense, and semi-rigid, surrounding a partially molten zone. The Moon's iron-rich core is small, perhaps 700 kilometers (430 miles) in diameter, reflecting the Moon's birth from the Earth's lighter, outer regions.

The moon is relatively cold and quiet, geologically speaking. It does, however, experience moonquakes, relatively gentle but long-lasting tremors that ring the moon like a bell.

The Moon:

The Planets

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