The Moon is the only natural satellite of the Earth and a unique member of the solar system in several respects. With a radius of 1,738 km (1,080 mi), it is approximately one-quarter of the size of the Earth and 81.3 times less massive. Although the solar system contains both larger and more massive satellites than the Moon, none except Pluto's moon differs so little from its planet in mass or size. Indeed, the Earth-Moon and Pluto-Charon systems constitute veritable double planets.

Astronomical Data
The Moon moves around the Earth in an elliptical orbit of small eccentricity, inclined by 5° 8' 43'' .4 to the plane in which the Earth revolves around the Sun. Its distance from the Earth varies between 356,000 and 407,000 km (221,000 and 253,000 mi) in the course of each month; the average distance is 384,400 km (238,900 mi), less than 1% of the distance to Venus and Mars, even at the time of their closest approach. The lunar globe appears in the sky as a disc of a little over half a degree (31' 5''.2) in apparent diameter.

The period in which the Moon completes an orbit around the Earth and returns to the same position in the sky - the sidereal month - is 27 days, 7 hr, 43 min, and 11.6 sec. Because the Earth is moving in its orbit around the Sun in the same direction as the Moon, the time needed to return to the same phase - the synodic month - is longer: 29 days, 12 hr, 44 min, and 2.8 sec. This period - the time interval that, for example, elapses between two successive full moons - was known within a second even in ancient times. The Moon's mean angular velocity in the sky is about 33 min of arc per hour, a little greater than the apparent diameter of the Moon.

In addition to its motion through space, the Moon also rotates about its axis in a period of one sidereal month, so that it keeps approximately the same side toward the Earth at all times. Nonuniformities in its orbital motion, however, together with the inclination of the orbit to the ecliptic, cause "optical librations" that allow 59% of the entire lunar surface to be seen from the Earth at one time or another. The remaining 41% was hidden until the Soviet Luna 3 spacecraft photographed the far side in October 1959. The Moon has since been thoroughly mapped.

Internal Structure. The average density of the Moon, calculated from its size and mass, is 3.34 g/cm3 (208.51 lb/ft3). The range of densities of the lunar surface rocks brought back during the Apollo program did not vary greatly from this figure. This means that materials in the Moon's interior probably do not vary much from those on the surface. The pressure at the center of a more or less uniform Moon would be about 47.1 kilobars. This value is well in excess of the crushing strength of typical lunar rocks. Thus, when the materials from which the Moon was formed began to accrete, the Moon would have formed into a globe even though the materials have remained solid throughout its mass.

Seismometers installed on the Moon by Apollo astronauts provided further indications that the lunar interior is rigid, as well as showing that the Moon is seismically much quieter than the Earth. That is, the records of the quakes - whose centers were located 600 to 900 km (375 to 560 mi) below the surface - indicated that both pressure and shear elastic waves occurred during moonquakes. This would not have been the case if the waves had to pass through fluid or semifluid layers in the Moon's interior. In addition, the quake records implied, through the very long decay time of the disturbances, that the Moon's surface layers must be highly fractured.

The rigidity of the lunar interior was further evidenced by the interaction of the Moon with the solar wind. Data indicate that the Moon exhibits an electrical conductivity consistent with that of silicate rocks cool enough to act as solids. Finally, the virtual absence of a dipole magnetic field on the Moon, attested to by many spacecraft, indicates that if the Moon has any core at all it must be a small one. Studies in the late 1990s seemed to confirm the existence of a core, but one that is at most only about 400 km (250 mi) in radius and contains only about 4% of the Moon's total mass. This possibility supports one concept of the Moon's formation: that it is a "chip" broken off from the ancient Earth's mantle by a collision with another large planetary body.