|Merrill Physics. Ohio: Glencoe/McGraw-Hill, 1995: 160.||"The moon has a period of 27.3 days."||27.3 days|
|The moon. Space Exploration. NASA Marshall Spaceflight Center.||"the moon goes around the earth, on average, in 27 days, 7 hours 43 minutes.||27.3215 days|
|Go Figure. moon-Watch.com.||"The moon makes one rotation and completes a revolution around the earth every 27 days, 7 hours, and 43 minutes."||27.3215 days|
|Moon Fact Sheet. NASA Official: J.H. King. NASA, 1968-2000.||"Orbital perimeters (for orbit about the earth)… revolution period (days) 27.3217"||27.3217 days|
|Autumn's moons. New Science. 10/29/99. Space Exploration. NASA Marshall Spaceflight Center.||"A complete orbit of the moon about the earth, and hence a complete cycle of the moon's phases takes 29.5 solar days."||29.5 days|
The moon is the natural satellite of earth. The diameter of earth's moon is about 3,480 km (about 2,160 mi), or about one-fourth that of earth, and the moon's volume is about one-fiftieth that of earth. The mass of earth is 81 times greater than the mass of the moon. Compared to the earth, the average density of the moon is only three-fifths, and the gravitational pull at the lunar surface only one-sixth. The moon has no liquid water and essentially no atmosphere, so no weather exists to change its surface; yet it is not totally inert.
The moon moves about earth at an average distance of 384,403 km (238,857 mi), and at an average speed of 3,700 km/h (2,300 mph). It completes one revolution in an elliptical orbit about earth in 27 days 7 hours 43 minutes 11.5 seconds. For the moon to go from one phase to the next similar phase, or one lunar month, requires 29 days 12 hours 44 minutes 2.8 seconds. The moon rotates once on its axis in about the same period of time that elapses for its period of revolution. There are different types of periods. The synodic period is the interval for a planet or moon to return to the same position as seen from earth. Sidereal time is based on the apparent motion of the distant, "fixed" stars across the sky. It has various astronomical purposes, such as predicting locations of objects in outer space.
This is why 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.
Many theories have been developed regarding the origin of the moon. Some provide that the moon was created far from the earth and it was pulled by gravity, and some suggest that it was created in an explosion involving the earth itself. Although the origin of the moon is still uncertain, much new information has been collected over the last century due to space exploration. Thanks to dozens of manned shuttle missions and space probes launched by NASA, we now have a vast knowledge of the moon. While we still do not know everything about the moon, space exploration of the moon continues and remains an important part of science.
Marc Epstein -- 2001