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The MOON

Size--Radius 1738 km
Distance from Earth--380,000 km
Topography--It is thought that most of the lunar surface features, including the surface layer of powdered rock (called the regolith--``blanket of rock"), were created by the high velocity impacts of solid objects with the moon. The absence of significant seismic activity within the moon suggests that its interior generates very little heat compared with that produced at the earth's core. The moon's crust, unlike the earth's, is unfractured. The moon has a lower average density than the earth. This suggests that the moon's core contains a smaller concentration of iron than does the earth's.

It is interesting that the lunar rocks brought back to earth by the Apollo astronauts have a similar composition to those of the earth's crust. One hypothesis that could explain this is that the moon was formed from the outer layers of the Earth.

A computer simulation of a collision between the young Earth and an impactor. Each is composed of an iron core and a rock mantle. The internal energy in each increases with both temperature and pressure. Increasing internal energy is shown as dark red, light red, brown, and yellow for rock, and dark blue, light blue, dark green, and light green for iron. In the first images, the impactor hits the young Earth, separates, and then falls in again, pulling out a tail of matter. At the end of this sequence, which takes six days, several large clumps and many smaller clumps are left. This material then clumps together to form the Moon.

The main surface features of the moon are: Maria, Craters, and Highlands.

AAn artist’s view of the formation of the Mare Imbrium region of the lunar surface. (A) An asteroid impact on the Moon, sometime between 3.85 and 4.0 billion years ago. (B) The shock of the asteroid impact began the Imbrium crater. (C) As the dust and heat subsided, the 1300-km Imbrium crater was left. (D) The molten rock (lava) flowed over outlying craters and cooled, leaving lunar mountains. (E) Lava welled up from inside the Moon 3.8 billion years ago. It filled the basin. (F) By 3.3 billion years ago, the lava flooding was nearly complete. (G) The final flow of thick lava came 2.5–3.0 billion years ago. (H) Subsequent cratering has left the Mare Imbrium of today’s Moon.

The far (popularly known as the "Dark") side of the Moon has more craters.

The caraters are believed to have been formed by meteoritic bombardment.

Note the small "hill" in the middle of the crater which is formed by the re-bouncing shock wave formed after the impact.

Atmosphere--The moon has no atmosphere because its gravitational force field is too weak to retain one. Any atmosphere that might have existed has long since escaped into space.

ORBIT and MOTIONS

Synchronous Rotation: the same side of the Moon always faces the Earth!!! This is a result of "mode locking" - a complicated nonlinear dynamical effect common to all the moons in the solar system.
The moon's orbit is tilted about 5 degrees relative to earth's orbital plane. This orbital tilt causes eclipses to occur twice per year, during eclipse seasons, instead of every month as would be expected were there no tilt. A solar eclipse occurs when the sun, moon and earth are perfectly aligned, in that order; a lunar eclipse occurs when the sun, earth and moon are aligned. A solar eclipse occurs when the moon blocks out the light from the sun and the moon's shadow touches the earth. A lunar eclipse occurs when the moon enters the earth's shadow.