The Moon

Saturday 31 December 2016 - 13:07:22 Posted by  Bobby

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 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.

Gravitational Pull


 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.)

A History of Violence


 In the very earliest stages of its formation, roughly 4.5 billion years ago, the satellite was 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.

Lunar Maria

 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.

Orbiting Partners

 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.

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.

 Moon Statistics

Discovered Known from antiquity
Average distance from Earth 238,855 miles (384,400 km)
Equatorial diameter 2,160 miles (3,476 km)
Axial tilt 1.5424° to ecliptic, 6.687° to orbit plane
Axial rotation period (sidereal) 656 hours (27.32)
Mass (Earth=1) 0.012
Volume (Earth=1) 0.020
Surface gravity (Earth=1) 0.165
Average density (water=1) 3.35
Escape velocity 1.5 miles/s (2.4 km/s)
Orbital eccentricity 0.055
Highest suface temperature 253°F (123°C)
Lowest surface temperature -387°F (-233°C)
Sunlight strength 100% Earth's
Albedo (reflectivity) 11%

 Where to?

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