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


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


Phases of the Moon

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


  • 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
  • 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