The Moon

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The Moon
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Bulk Properties of the Moon

• Semi-major Axis 384,000 km (363-406K)
• Orbital Period 27.3 days (29.5 days)
• Rotation Period 27.3 days
• Orbital Inclination 5.2o
• Mass 0.012 Earth masses
• Radius 0.27 Earth radii
• Mean density 3.3 g/cm3
• Surface Temperature 100-400K

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Measuring the Moon

• The angular diameter of the moon is approximately
  30 arc minutes
• The distance to the moon was first measured
  precisely by bouncing radio waves off the surface
• Later the distance was corrected using lasers
  reflected off prisms erected by Apollo astronauts
• The mass of the Moon is calculated by measuring
  its gravity

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Comparing the Moon to Earth

• The moon is about ¼ the size of Earth
• This is very large compared to its parent planet
• The density and composition of the moon is very
  similar to the material that makes up Earths
  mantle
• Basalt
• Gravity on the moon is about 1/6 that of Earth
• A 180lb person would only weigh 30lb on the moon

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The Moons Atmosphere

• It doesnt have one
• The moon is not large enough to keep gases
  gravitationally bound to the surface
• The surface is exposed to the solar wind and to
  meteoroids
• Temperatures vary widely
• 400K on the daylight side
• 100K on the nighttime side

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Surface Features of the Moon

• Large dark areas that resemble Earths oceans
• Extensive flat areas
• Called maria (sing. mare) meaning seas
• Light colored areas resembling continents
• Elevated several km above the maria
• Called highlands, or terrae (land)
• Telescopic observations reveal numerous craters
• Craters are much more prevalent in the highlands
• Range from hundreds of km across to microscopic
  size

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Geography of the Moon

• Maria have fanciful Latin names
• Mare Imbrium (Sea of Showers)
• Mare Nectaris (Sea of Nectar)
• Mare Tranquilitatis (Sea of Tranquility)
• Mountain ranges share the names of ranges on
  Earth
• Alps, Carpathians, Pyrenees, etc.
• Craters are named after great scientists and
  philosophers
• Tycho, Plato, Eratosthenes, Copernicus, etc.

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Lunar Rotation

• The Moon rotates once on its axis in the same
  amount of time it takes to complete one orbit
  around the Earth
• Dubbed a synchronous orbit
• This means the same side of the Moon always faces
  the Earth
• This occurs because Earth is so much larger than
  the Moon
• Earths gravity keeps the moon tidally locked
• Most of the moons in our solar system are
  similarly locked by their parent planets

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Lunar Composition

• The Moon is unlikely to have a metallic core
• Its average density is similar to that of
  surface rocks
• The internal temperature of the Moon may be as
  low as 1500K, not hot enough to melt rock
• Seismic instruments left behind by astronauts
  suggest the core may be at least partially molten

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Lunar Composition, contd

• The Moons crust is between 60-150km thick
• It is thicker on the far side of the moon
• Earths gravity has pulled the more dense mantle
  and core off-center
• The highlands are made of rocks rich in aluminum,
  making them lighter in color and lower in density
• The mare are made of basalt
• The mare resulted in magma seeping up from the
  Moons formerly molten interior and solidifying
• There are few mare on the far side (thicker crust)

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Lunar Dust

• Meteoroid collisions are the main cause of the
  layer of dust
• Called regolith
• Covers surface to an average depth of 20m
• Consistency of talcum powder
• Thinnest in the maria (10m) and thickest on the
  highlands (100m)
• Contains no organic material and no water

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Lunar Volcanism

• Almost all lunar craters are meteoric in origin
• A few craters are likely to be volcanic craters
• Lava formed a dome, solidified, the lava receded,
  the dome collapsed
• There are also remains of ditches where lava once
  flowed
• Called rilles
• Volcanic activity ended LONG AGO

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Origin of the Moon

• Theory 1
• Co-formation
• The Moon formed simultaneously with Earth from
  the pre-planetary material surrounding the young
  Sun
• Flaw the Moons composition is different
• Theory 2
• Capture
• The Moon formed elsewhere in the solar system but
  was captured by Earths gravitational field
• Flaw the Moon is huge also has similar mantle

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Origin of the Moon

• Theory 3
• Daughter/Fission
• A young Earth spun so fast it ejected a large
  mass that became the Moon
• Flaw theres no evidence Earth ever spun fast
  enough to do that
• Theory 4
• Impact
• The early Earth was hit by a large object,
  causing a large portion of the outer Earth to be
  ejected and coalesce into the Moon
• Current accepted theory

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Lunar Cratering

• In the early Solar System, Earth and the Moon
  were bombarded by debris
• Combined actions of wind and water have eroded
  most craters on Earth
• The moon has no air, water, plate tectonics,
  volcanoes, or seismic activity to cause erosion
• Large collisions created the maria, which then
  filled with molten rock and re-solidified
• Most major impacts ended long ago (few large
  objects left in the solar system)
• Bombardment continues on a smaller scale
• Earth too, though most burn up in the atmosphere
  (meteors)

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The Moons Effects on Earth

• Gravitational attraction is causing Earths
  rotational speed to decrease
• Stabilizes Earths axial tilt
• Causes tides
• The side of Earth facing the Moon experiences a
  stronger attraction (high tide)
• The side opposite the Moon is left behind by
  the Earth being pulled to the Moon (high tide)
• The sides of Earth perpendicular experience low
  tide
• Water is easier to deform than land, though it
  bulges too

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Tides

• Earth rotates beneath this bulge
• Two high tides and two low tides each day
• The average difference in the open ocean is about
  1m
• Tidal effects are magnified by geography, such as
  river mouths, bays, or other narrow openings
• Bay of Fundy (Maine/Canada) 60 ft difference

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Tides, contd

• The Sun also has a tidal influence, but less
  because it is further away
• When the Moon and Sun line up, tidal effects are
  increased
• Spring Tide
• New Moon, Full Moon
• When the Moon and Sun are perpendicular to the
  Earth, tidal effects are decreased
• Neap Tide
• First Quarter, Third Quarter

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