The Moon

Orbit and Rotation

  • Orbit shape: On average, the distance from Earth is 60 Earth radii [384,000 km], but the orbit is slightly elliptical (the distance varies by about 13%), with an inclination from the ecliptic of about 5°; Because the Earth's rotation is gradually slowing down, the Moon is gradually moving away from us; It might have been at 1/10 of the distance once!
  • Orbit period: The sidereal month is 27.3 days long (the synodic one 29.5: why is it longer?), and the Moon covers 12°/day [moving at almost 1 km/sec].
  • Rotation: Synchronized with the revolution around the Earth (1 day on the Moon = 1 synodic month, about 4 weeks!), so we always see roughly the same side, but not quite, because of libration; The other, "dark side", is not actually dark!


  • Unmanned Landings: The first was the Soviet Luna 2 crash-landing in 1959; Then the US Lunar Surveyor, 1966-1968.
  • Manned Missions: The Moon is the only celestial object so far visited by humans, starting with Apollo 11 in July 1969 (12 Moon walkers in 1969-1972);* Astronauts have moved around, conducted experiments, returned samples.
  • Recent Missions: In 1994 mapped in detail by Clementine; In 1998-1999 Lunar Prospector searched for ice, gas and minerals, and plunged to the surface; ESA's SMART-1 was in orbit in 2004-2006; Japan, China and India have sent orbiters; NASA's Lunar Reconnaissance Orbiter and GRAIL spacecraft are currently orbiting.

General Structure and Interior

  • Size and mass: Radius 1/4 of Earth's, elongated shape; The gravity is 1/6 of Earth's, the density similar to Earth's crust, from mass measured with data from orbiting spacecraft.
  • Core and Mantle: Slightly off-center toward us (related to locked rotation and Earth's gravitational pull); Not as dense as Earth's.
  • Crust: About 60 km on the near side, thicker on the far side because of the off-center core; Difference in thickness related to different appearance of the surface.
  • Magnetic field: Basically none now, but there is evidence for one in the past.


  • Main distinction: Large dark flat areas, maria (16%, mostly on the near side); lighter, older, crust highlands (terrae).
  • Features: Many ancient impact craters [all sizes; from 2250 km wide, 12 km deep South Pole-Aitken, to microcraters], covered with boulders and fine dust – which can be a problem for future exploration.
  • Shaping factors: No water (but some ice in craters near the poles!); erosion by meteoroid impact (constantly occurring).
  • Activity: Some volcanic activity, more than 3 Gyr ago; No life nor fossils.
  • Atmosphere: Almost none detected [so thin that its density was doubled by the exhaust from Apollo 11's Landing Module], gravity too weak to hold on to it; No sounds, rain, clouds, or blue sky; wide temperature swings [110 to -150°C]; Bust dust, charged by the Sun's radiation and wind, levitates above the surface.

Formation and Evolution

  • Facts: Rocks taken back to Earth are 3-4.5 billion years old!
  • Unlikely theories: The Earth spun up and a portion of it became the Moon; The Moon formed elsewhere and was captured as it passed near the Earth; Earth and Moon formed together side by side.
  • Impact/Ejection theory: A hybrid scenario from impact of a Mars-sized object about 4.5 billion yr ago [50-100 Myr after the formation of the Solar System] - the most likely one, based on analysis of lunar samples and simulations.
  • Evolution after formation: Evidence for cataclysm from swarm of asteroids or comets 3.9 Gyr ago; Two particularly strong impacts helped form bulges on near/far side; Molten magma surfaces through cracks in impact basins and forms maria.
* Astronauts left on the Moon objects including an American flag, a seismic detector, and a laser reflector. Humans also littered the landscape with everything from food wrappers to the backpacks that kept them alive while they roamed one small corner of Tranquility Base. They even left their boots (and a golf ball).

page by luca bombelli <bombelli at>, modified 7 oct 2013