The rotation period of the Moon is the same as the period of its revolution about the Earth. This arrangement keeps the same face of the Moon turned toward the Earth.
The distance to the Moon averages 384,400 km but can be as large as 406,700 km and as small as 356,400 km. The Moon's diameter is about 1⁄4 that of the Earth.
The two parts of the shadows of the Earth and the Moon are the penumbra, within which sunlight is partially blocked, and the umbra, within which sunlight is completely blocked. Total eclipses occur within the umbral shadow.
The Moon's umbral shadow is only about 150 km wide where it reaches the Earth. During a solar eclipse, the Moon's umbral shadow sweeps rapidly across the Earth, producing an eclipse track that covers less than 1% of the Earth's surface area.
Eclipses can occur only when the Moon is in its new or full phase and when the Moon is near the ecliptic. Eclipses occur in series in which successive eclipses are separated by 18 years 11 1/3 days, the saros period. Using their knowledge of the saros, ancient astronomers were able to predict eclipses.
Tides cause the day to lengthen and the Moon to recede slowly from the Earth. In the remote future, the day and month may become equal in length. When that happens, the Earth will keep the same face turned toward the Moon.
The Moon's surface is covered with craters. The meteoroids that made the craters have pulverized the Moon's surface layers and produced a fine layer of debris.
The lunar maria were produced when vast volcanic flows flooded the floors of basins produced by earlier impacts. Little or no volcanic activity has taken place for more than 3 billion years.
Analysis of lunar samples has shown that they are similar to the surface rocks of the Earth but lack water and are deficient in easily vaporized elements, such as chlorine, zinc, and lead. Lunar rocks range in age from 3.2 billion to 4.5 billion years. The terrae are about 1 billion years older than the maria.
Measurements of the crater densities of regions of the Moon that have different ages have been used to show that the Moon experienced an early period of intense bombardment. The relationship between crater density and age for the Moon has been applied to other planets and satellites to estimate their ages.
The Moon has almost no atmosphere because any gases that enter the atmosphere escape almost immediately. These gases come from the solar wind and radioactive decays in lunar surface rocks. When comets strike the lunar surface, vaporized water molecules form an atmosphere, some of which eventually condense at the lunar poles to form ice deposits.
The Moon is mostly rocky with a small metallic core. Seismic studies show that the Moon's interior is virtually inactive. The crust and lithosphere of the Moon are both very thick compared with the Earth's.
Old theories of the origin of the Moon all suffered from serious deficiencies. A new theory, which proposes that the Moon accumulated from debris from an impact of a large body with the Earth, accounts much more successfully for what we know about the history and composition of the Moon.
