Class 5: The Earth’s Moon
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Today’s class:
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What are the main properties of the Moon?
Some fascinating facts about the Moon’s
rotation and orbit…
How did the Moon form?
Fact Sheet: The Earth’s Moon
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Earth-Moon distance =
384,000 km (range is 363,000406,000 km).
Orbital period = 27.3 days.
Rotational period = 27.3 days.
Radius = 1738 km (0.27R).
Mass = 7.41022 kg (0.012M).
Density = 3340 kg/m3 (Earth’s
density is 5500 kg/m3).
No detectable global magnetic
field.
I : Properties of the Moon
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Oldest known rocks are 4.4-4.5 billion
years (so the Moon is at least this old!)
Moon has much less iron than Earth:
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Moon is much less dense.
No global magnetic field.
In Earth, iron is mostly in the core –
comprises 16% of the planet’s volume
Any iron core in the Moon must be less
than 3% of its volume.
… structure of the Earth
Lunar composition
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Moon rocks very similar to the Earth’s
mantle, except…
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Very few volatile elements or compounds
(e.g., water, sodium, zinc, lead…)
High boiling-point (refractory) elements in
greater abundance: aluminum, titanium,
calcium…
Recent missions that studied the Moon’s
polar region show evidence for H2O ice!
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Located in deep polar craters – the ice is in
perpetual shadow, but there’s not much.
II : Tidal effects in the EarthMoon system
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Tidal effects are responsible for…
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Twice-daily rising and falling of tidal waters.
Synchronizing the rotation and orbit of the
Moon around the Earth.
Gradually slowing down the rotation of the
Earth.
 Our day is increasing by 2 ms/century!
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Gradually increasing the Earth-Moon
distance.
 The Moon is moving away at 4 cm/year!
III : Theories of the formation
of the Moon
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Simultaneous formation theory
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Fission theory
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The Earth was spinning so fast in the past
that large fraction tore away to form Moon.
Capture theory
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Moon formed from same material as Earth,
at same time and under same conditions.
Moon formed elsewhere and was captured.
Giant impact theory
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A Mars-size body struck Earth in the past
and the Moon formed from the debris.