moon taking notes

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Moon, name given to the natural satellite of the Earth, and sometimes applied to the satellites of the other planets in the solar system. The diameter of the moon is about 3480 km (2160 mi), or about one-fourth that of earth, and the moon’s volume is about one-fiftieth that of the earth. The mass of the earth is 81 times greater than the mass of the moon. Thus the average density of the moon is only three-fifths, and the pull of gravity at the lunar surface only one-sixth that of the earth. The moon has no free water and essentially no atmosphere, and no weather exists to change its surface; yet it is not totally inert.

BRIEF SURVEY OF THE MOON

Mean Distance from earth: 384,403 km (238,856 mi)Diameter: 3480 km (2160 mi)Period of revolution: 27.322 earth daysEccentricity of orbit: 0.055Inclination of orbit (to ecliptic): 5°9´Inclination of orbit (to equator): 28°36´–18°18´Rotation period (sidereal day): 27.322 earth daysPeriod of phases: 29.53 earth daysMass (earth = 1):0.012Mean density (water = 1): 0.605The moon moves about the earth at an average distance of 384,403 km (238,856 mi), and at an average speed of 3700 km per hr (about 2300 mph). It completes one revolution in an elliptical orbit about the earth in 27 days, 7 hr, 43 min, and 11.5 sec with reference to the stars. For the moon to go from one phase to the next similar phase, or one lunar month, requires 29 days, 12 hr, 44 min, and 2.8 sec. The moon rotates on its axis once in about the same period of time that elapses for its sidereal period of revolution, accounting for the fact that virtually the same portion of the moon is always turned toward the earth. Although the moon appears bright to the eye, it reflects into space only 7 percent of the light that falls on it. The reflectivity, or albedo, of 0.07 is similar to that of coal dust.

Appearance from Earth

At any one time, an observer can see only 50 percent of the moon’s entire surface. However, an additional 9 percent can be seen from time to time around the apparent edge because of the relative motion called libration. This is caused by slightly different angles of view from the earth at different relative positions of the moon along its inclined elliptical orbit.