8. The Moon
How the Moon Looks from Earth The Moon subtends about one half a degree in the sky (angular
size), goes through phases, has the same side always facing the Earth, has a surface dominated by
maria (dark “seas”) and impact craters.
Basic Lunar Characteristics Size: one quarter Earth’s diameter (3500 km); Distance: 30 Earth
diameters away (380,000 km); Density: surprisingly low—revealing very little iron content; Surface:
covered in rock and rock dust (regolith); no surface activity; Atmosphere: none; Temperature: hot days,
cold nights. No weather; Gravity: one sixth Earth’s gravity; Orbit: not aligned with Earth’s equator!!!
Lunar History: 1. The Moon’s Formation
Consensus accepts that the Moon was formed by a collision
between the Earth and a Mars-sized planetoid, soon after the Earth had formed. The impact would
have ejected some of the Earth’s surface material into orbit. This material (along with some of the
planetoid) coalesced to form the Moon.
This impact formation explains why: the density of the Moon is
so low (little iron), the Moon’s orbit is not aligned with the Earth’s equator—the orbit is aligned with the
angle of impact, so much lunar rock is identical to Earth rock, the lunar rock is nearly devoid of water.
Lunar History: 2. The Moon Slows Down, and Recedes When first formed, the Moon was much closer
to the Earth than it is now, and the tidal forces were much stronger than now. Tidal motion uses up
energy. This loss of energy causes the Moon’s spin to slow down, and the Moon to move further from
the Earth. Eventually, the spin matches the orbit, called synchronous rotation, and tidal locking, and oneto-one spin-orbit resonance.
Lunar History: 3. Same Side of Moon Always Faces the Earth
As the Moon has slowed down, the
Earth’s gravity has pulled the Moon’s small core closer toward the Earth, giving the Moon a “heavy
side.” This side has settled into the “down” position (toward the Earth). Simultaneously, the Moon’s
crust was pulled thinner on the side toward Earth.
Lunar History: 4. Maria are Formed
After a large impact, liquid rock from below seeps upward and
fills in a basin with lava. This darker basaltic rock cools, forming a mare (pronounced MAR-ay). The
lack of craters demonstrates the maria’s young age.
Moon’s Near Side versus Far Side
The Moon’s far side has almost no maria, because the crust is
thicker on the far side, so it’s harder for liquid basalt to flow upward.
Lunar History: 5. Impacts Continue
Continued impacts produce lunar craters. Lunar rays are lines of
material ejected from an impact. Rock hitting rock produces fine powder that covers the surface—
regolith (Moon dust).
Moon’s Atmosphere
The Moon is so small (low mass), and so close to the Sun (hot), that the Moon
can retain no atmosphere. Gasses escape into space (boil away). Temperatures range from: -280 °F
(-170 °C) at night to 240 °F (120 °C) during the day.
Water on the Moon?
Sunlight does not shine onto the bottom of deep craters at the lunar poles.
Comets which impact the Moon deliver ice into these craters. Without sunlight, the buried ice can
remain permanently. Ice could be the key to long-term lunar bases.
LCROSS Mission The Lunar Crater Observation and Sensing Satellite blasted a projectile into the
bottom of crater Cabeus A, at the lunar south pole, in October 2009 to gather conclusive data on the
existence of water-ice on the Moon. Spectral analysis indicates the presence of water.
The Production of Tides
The force of gravity decreases with distance, so the Moon’s gravitational pull
is less on one side of the Earth than on the other. This produces a stretching effect on the Earth, which
is most noticeable on the water. Since the Earth spins, the depth of water changes during the day—
tides.
Lunacy
Is there a correlation between lunar phases and crime? No. Is there a correlation between
lunar phases and birth? No. Do people remember more of what happens during the full moon?
Probably.
Is the distance to the Moon still increasing? Yes. We can measure the distance to the Moon very
precisely by bouncing laser light off of reflectors that were placed on the Moon in 1969. The Moon is
currently moving away from Earth at a rate of 1.5 inches (3.8 centimeters) a year.
Questions to be Answered:
•
How does the Moon appear from Earth?
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What are the Moon’s basic characteristics?
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How was the Moon formed? What evidence shows this?
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Why does the same side of the Moon always face the Earth?
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What are the lunar maria, and how were they formed?
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How is the far side of the Moon different from the near side?
•
What are other lunar surface features, and how were they formed?
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Why does the Moon have no atmosphere?
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Can there be water on the Moon?
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What affect does the Moon have on people’s behavior?
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What is some of the basic history of lunar exploration?