The Moon

• 384,000 kilometers from the Earth
• Diameter is 3476 kilometers
  • About $1/4$ the size of the Earth
• Moves about 12˚ in the sky each day
  • ~24 times its diameter
• Rotates on its axis at the same time it revolves around Earth
  • This is why we only see one side of it
  • Synchronous rotation
  • Tidally-locked rotation
• Far side of the Moon more heavily cratered
  • Easier to hit because the other side is always facing Earth
• Regolith on the surface of the Moon
  • Layer of fine powder and rock fragments
• $\frac{1}{80}$ the mass of Earth
• $\frac{1}{6}$ the gravity of Earth
  • Too low to retain an atmosphere
• Made mostly of silicate rock

Moonths

• Takes one month (moonth) to revolve around the Earth
• Synodic month / solar month
  • 29.5306
• Sidereal month
  • 27.3217 days

Phases

• Full cycle every solar month
• New moon --> in front of the Sun (dark)
• Full moon --> opposite the Sun

Origin

• The fission theory
  • Moon was once part of Earth
  • Somehow separated in early history
• The sister theory
  • Moon formed together with (but independent of) Earth
  • Similar to moons of out planets
• The capture theory
  • Moon formed elsewhere in the Solar System
  • Captured by Earth
• The giant impact hypothesis
  • Most likely
  • Earth was struck with a massive object
  • The impact ejected a vast amount of material into space
    • Ejected material cooled and formed the Moon

Space Astronomy Moon