# 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
• 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