About the Moon -Post (2 of 6) 

Moon, the lonely satellite of our planet, orbits the Earth in an elliptical path with a mean distance of about 385,000 km from the center of the Earth. With a Mean orbital velocity of 1.023 km/s, the Moon moves relative to the stars each hour by an amount roughly equal to its angular diameter, or by about 0.5°.

The Moon differs from most satellites of other planets in that its orbit is close to the ecliptic plane, not the Earth's equatorial plane. But most of the solar system's satellites orbit their planets on equatorial planes. The lunar orbit plane is inclined to the ecliptic by about 5.1°, whereas the Moon's spin axis is inclined by only 1.5°.

It is also the largest natural satellite in the Solar System relative to the size of the planet it orbits around, having 27% the diameter and 60% the density of Earth, resulting in 1⁄81 Earth’s mass. The Moon has a geo-chemically distinct crust, mantle, and core. The Moon has a low solid iron-rich inner core with a radius of only 240 kilometers and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometers. Around the core is a partially molten boundary layer with a radius of about 500 kilometers.

The radius of the core is about 20% of the radius of the Moon, in contrast to about 50% for most of the other terrestrial bodies. The relatively low size of the moon’s core is suggested due to the fact that the Moon has probably formed out of material splashed into orbit by the impact of a large body into the early Earth. Differentiation on Earth had probably already separated many lighter materials toward the surface so that the impact removed a disproportionate amount of silicate material from Earth, and the iron core of the impacting body sank into the young Earth's core, as most of its mantle and a significant portion of the Earth's mantle and crust were ejected into orbit around the Earth. This material quickly coalesced into the Moon. Estimates based on computer simulations of such an event suggest that some two percent of the original mass of the impacting bodies ended up as an orbiting ring of debris, and about half of this matter coalesced into the Moon. 

On the Moon, a distinctive basaltic material has been found that is high in "incompatible elements" such as potassium (K), rare earth elements, and phosphorus (P), (KREEP). It is also high in uranium and thorium. These elements are excluded from the major minerals of the lunar crust which crystallized out from its primeval magma ocean, and the KREEP basalt may have been trapped as a chemical differentiation between the crust and the mantle, with occasional eruptions to the surface.

This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation. Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene; after about three-quarters of the magma ocean had crystallized, lower-density plagioclase minerals could form and float into a crust on top. The final liquids to crystallize would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements. Consistent with this, geochemical mapping from orbit shows the crust is mostly anorthosite and moon rock samples of the flood lavas erupted on the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth.