The Moon

The Moon – Phases, Formation, Statistics and Cratering



Notes on the above diagram:

When the sunlight area of the near side is growing it is said to be WAXING

When the sunlight area of the near side is shrinking it is said to be WANING

Moon Statistics

Apogee (when furthest from Earth) 251,970 km

Perigee (when nearest to Earth) 225,740 km

Average orbital speed 3680 km/hour

Diameter at equator 3,476km

Surface gravity 1/6 that of Earth

Surface Temperature Range 130 degrees C to -233 degrees C

Formation of the Moon

The Moon formed as a large molten mass about 4.5 billion years ago. The present scientific consensus is the ‘Collisional Ejection Theory’, whereby the proto-Earth was struck off-centre by a Mars sized object and this collision ejected debris from which the Moon was formed. Tidal forces from the Earth slightly elongated the molten Moon with the long axis pointed toward the Earth. The Moon retained this shape and orientation as it cooled and solidified and still sustains this orientation by spinning once on its axis for each orbit of the Earth. We say the Moon is ‘Tidally Locked’ with a ‘1:1 Spin Orbit Ratio’. This is why we always see the same side of the Moon from the Earth. Most of the moons in the solar system are tidally locked to their planets, including both moons of Mars - Phobos and Deimos.

Moon Craters

Impact Craters are formed when a body like an asteroid strikes the surface. An asteroid will typically be travelling at speeds of 15 to 25 kilometres per second. Rock can withstand pressures measured in kilotonnes, but an asteroid strike delivers pressures measured in megatonnes. This force vaporises both the asteroid and the rocky surface, causing the material in the impact site to behave like a liquid. The resultant shockwave travels outwards, excavating the rock and driving the displaced material to form a circular wall, giving the crater its characteristic ring appearance. The rim of a large crater can be truly mountainous and the central impact point can rebound to form a central peak, sometimes kilometres in height.