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!

  Exploration

• Landings: The first was by the Soviet Luna 2 in 1959; Lunar Surveyor, 1966-68.
• 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 1969-72);* Astronauts have moved around, conducted experiments, returned samples.
• Recent Missions: In 1994 mapped in detail by Clementine; In 1998-99 Lunar Prospector searched for ice, gas and minerals, and plunged to the surface; 2003, ESA's SMART-1 has been launched, next will be the Japanese probe Lunar-A.
• Future Missions? NASA is considering a sample return mission to the South Pole-Aitken basin, and is setting up a program to send humans again to the Moon and possibly establish lunar bases, for further exploration and possible mining; India plans to send a mission by 2007, China a rover in 2012.

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.

  Surface

• 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.
• 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 yr 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; Molten rock surfaces through cracks in impact basins and forms maria; Various stages.