Spanel Planetarium

The Moon

distance away
    384,400 km (225,745 miles)
    3,476 km (2,160 miles)
temperature in sunlight
    133.9°C (273°F)
temperature in shadow
    -153.3°C (-244°F)
gravity at the surface
    0.1667g (1/6 Earth)
orbital speed
    1.023 km/sec (2,287 mph)
oldest rocks collected
    4.5 Billions yrs.
total rocks collected
    382 kg (842 lbs)
widest crater
    225.3 km (140 miles)
deepest crater
    4572 m (15,000 ft)
highest mountain
    4876.8 m (16,000 ft)
synodic period
    29.5 days
sidereal period
    27.3 days
Lunar Formation Theories

Impact Theory
Currently the leading theory for the origin of the Moon involves a collision with Earth by an object slightly larger than Mars. If such an object struck at a glancing blow, most of it and some of the Earth would have rose up as a cloud of material to above 22,000 kilometers (13,700 miles), and into particles that orbited the Earth. Eventually they would have accreted into moonlets, which further combined to form the Moon. Tidal action between the Earth and the Moon slowly moved it farther away. It is currently receding at about 3.8 centimeters per year.

The impact theory implies that a collsion happened very early in the formation of the solar system, perhaps when the Earth was less than 500 million years old. Several facts contribute to the confidence that many astronomers have in this theory.

  • The Earth has a large iron core, but the moon does not. The debris from the impact would mostly have come from the iron-depleted mantles of the two bodies, according to computer models.
  • Earth has a mean density of 5.5 grams/cubic centimeter, but the moon has a density of only 3.3 g/cc. The reason is that the moon lacks iron.
  • The moon has exactly the same oxygen isotope composition as the Earth, whereas material from other parts of the solar system have different oxygen isotope compositions. This is easiest to explain if the moon formed from material in Earth's neighborhood.
First Lunar Footprint

Neil Armstrong's footprint.
First on the Moon, July 20, 1969.

Audio Clip: "One small step..."

Dual Accretion Theory: This explanation is that the moon is a sort of sister planet that formed in orbit around the Sun in the same general area that the Earth formed. This theory's weakness is that it does not explain why the moon lacks iron, which the Earth has in abundance.

Capture Theory: This idea is that the moon formed elsewhere in the solar system where there was little iron, and then was captured into orbit around Earth. The major weakness of this theory is that lunar rocks have the same oxygen isotope composition as rocks on the Earth, which is easier to explain if they both formed at about the same distance from the Sun.

Spin Off Theory: The idea behind this theory is that the newborn Earth was spinning so fast that it spun off the material that formed the moon. This might produce a moon similar to Earth's mantle (low iron), but computer models fail to show how such high angular momentum and energy could have evolved to the current values.