Principal Characteristics Of The Earthmoon System

In addition to its nearness to Earth, the Moon is relatively massive compared with it—the ratio of their masses is much larger than those of other natural satellites to the planets that they orbit. The Moon and Earth consequently exert a strong gravitational influence on each other, forming a system having distinct properties and behaviour of its own.


Mascons are regions where particularly dense lavas rose up from the mantle and flooded into basins. Lunar mascons were first identified by the observation of small anomalies in the orbits of Lunar Orbiter spacecraft launched in 1966-67. As the spacecraft passed over certain surface regions, the stronger gravity field caused the craft to dip slightly and speed up. Apollo space program scientists used the data to correct for the observed gravity irregularities in order to improve the targeting accuracy of manned Moon landings. Later scientific study of these anomalies supported the interpretation that the Moon had a complex history of heating, differentiation (sinking of denser materials and rising of lighter ones to form a deep mantle and overlying crust), and modification by impacts and subsequent huge outflows of lava. Tracking of the velocities of the Clementine, Lunar Prospector, and Kaguya spacecraft (launched 1994,1998 and 2007, respectively) by their Doppler-shifted radio signals as they orbited the Moon provided detailed gravity maps, including mascon characteristics, of most of the lunar surface.

The Moon's larger mascons coincide with circular, topographically low impact basins where particularly dense—and thus more massive and gravitationally attractive—magma upwelled from the mantle and solidified to form dark mare plains. Examples are the Imbrium, Serenitatis, Crisium, and Nectaris basins (maria), all of which are visible at full moon with the unaided eye from Earth. The survival, over the three billion years since they were formed, of these gravity anomalies testifies to the existence of a thick, rigid lunar crust. This, in turn, implies that the Moon's initial heat source is extinct.

Although the Moon is commonly described as orbiting Earth, it is more accurate to say that the two bodies orbit each other about a common centre of mass. Called the barycentre, this point lies inside Earth about 4,700 km (2,900 miles) from its centre. Also more accurately, it is the barycentre, rather than the centre of Earth, that follows an elliptical path around the Sun in accord with Kepler's laws of planetary motion. The orbital geometry of the Moon, Earth, and the Sun gives rise to the Moon's phases and to the phenomena of lunar and solar eclipses.

The distance between the Moon and Earth varies rather widely because of the combined gravity of Earth, the Sun, and the planets. For example, in the last three decades of the 20th century, the Moon's apogee—the farthest distance that it travels from Earth in a revolution—ranged between about 404,000 and 406,700 km (251,000 and 252,700 miles), while its perigee—the closest that it comes to Earth—ranged between about 356,500 and 370,400 km (221,500 and 230,200 miles). Tidal interactions, the cyclic deformations in each body caused by the gravitational attraction of the other, have braked the Moon's spin such that it now rotates at the same rate as it revolves around Earth and thus always keeps the same side facing the planet. As discovered by the Italian-born French astronomer Gian Domenico Cassini in 1692, the Moon's spin axis precesses with respect to its orbital plane; i.e., its orientation changes slowly over time, tracing out a circular path.

In accord with Kepler's second law, the eccentricity of the Moon's orbit




approximate ratio (Moon to Earth)

mean distance from Earth (orbital radius)

384,400 km

period of orbit around Earth (sidereal period of revolution)

27.3217 Earth days

inclination of equator to ecliptic plane (Earth's orbital plane)





approximate ratio (Moon to Earth)

inclination of



equator to body's

own orbital plane

(obliquity to orbit)

inclination of orbit


to Earth's equator

eccentricity of orbit


around Earth

recession rate from

3.8 cm/year


rotation period

synchronous with orbital period

23.9345 hr

equatorial radius

1,738 km

6,378 km


surface area

37,900,000 km2

510,066,000 km2 (land area, 148,000,000 km2)



0.0735 * 1024 kg

5.976 x 1024 kg


mean density

3.34 g/cm3

5.52 g/cm3


mean surface

162 cm/sec2

980 cm/sec2



escape velocity

2.38 km/sec

11.2 km/sec


mean surface

day, 380 K (224 °F,

288 K (59 °F, 15 °C)


107 °C); night, 120 K (-244 °F, -153 °C)


396 K (253 °F, 123

331 K (136 °F, 58 °C) to


°C) to 40 K (-388 °F, -233 °C)

184 K (-128 °F, -89 °C)

surface pressure

3 * 10-15 bar

1 bar

1:300 trillion


day, 104 molecules/

2.5 * 1019 molecules/

about 1:100

molecular density

cm3; night, 2 * 105 molecules/cm3

cm3 (at standard temperature and pressure)


average heat flow

29 mW/m2

63 mW/m2


results in its traveling faster in that part of its orbit nearer Earth and slower in the part farther away. Combined with the Moon's constant spin rate, these changes in speed give rise to an apparent oscillation, or libration, which over time allows an observer on Earth to see more than half of the lunar surface. In addition to this apparent turning motion, the Moon actually does rock slightly to and fro in both longitude and latitude, and the observer's vantage point moves with Earth's rotation. As a result of all these motions, more than 59 percent of the lunar surface can be seen at one time or another from Earth.

The orbital eccentricity also affects solar eclipses, in which the Moon passes between the Sun and Earth, casting a moving shadow across Earth's sunlit surface. If a solar eclipse occurs when the Moon is near perigee, observers along the path of the Moon's dark inner shadow (umbra) see a total eclipse. If the Moon is near apogee, it does not quite cover the Sun; the resulting eclipse is annular, and observers can see a thin ring of the solar disk around the Moon's silhouette.

The Moon and Earth presently orbit the barycentre in 27.322 days, the sidereal month, or sidereal revolution period of the Moon. Because the whole system is moving around the Sun once per year, the angle of illumination changes about one degree per day, so that the time from one full moon to the next is 29.531 days, the synodic month, or synodic revolution period of the Moon. As a result, the Moon's terminator—the dividing line between dayside and nightside—moves once around the Moon in this synodic period, exposing most locations to alternating periods of sunlight and darkness each nearly 15 Earth days long.

The sidereal and synodic periods are slowly changing with time because of tidal interactions. Tidal friction occurs between water tides and sea bottoms, particularly where the sea is relatively shallow, or between parts of the solid crust of a planet or satellite that move against each other. Tidal friction on Earth prevents the tidal bulge, which is raised in Earth's seas and crust by the Moon's pull, from staying directly under the Moon. Instead, the bulge is carried out from directly under the Moon by the rotation of Earth, which spins almost 30 times for every time the Moon revolves in its orbit. The mutual attraction between the Moon and the material in the bulge tends to accelerate the Moon in its orbit, thereby moving the Moon farther from Earth by about 3 cm (1.2 inches) per year, and to slow Earth's daily rotation by a small fraction of a second per year. Millions of years from now these effects may cause Earth to keep the same face always turned to a distant Moon and to rotate once in a day that is about 50 times longer than the present one and equal to the month of that time. This condition probably will not be stable, because of the tidal effects of the Sun on the Earth-Moon system.

View over the lunar north pole, in a mosaic made from images collected by the Galileo spacecraft as it flew by the Moon on December 7,1992. In this image, the north pole lies just within the shadowed region about a third of the way along the terminator, starting from the top left. NASA/JPL

Extending this relationship back into the past, both periods must have been significantly shorter hundreds of millions of years ago—a hypothesis confirmed from measurements of the daily and tide-related growth rings of fossil corals. That the Moon keeps the same part of its surface always turned toward Earth is attributed to the past effects of tidal friction in the Moon. The theory of tidal friction was first developed mathematically after 1879 by the English astronomer George Darwin (1845-1912), son of the naturalist Charles Darwin.

Because the Moon's spin axis is almost perpendicular to the plane of the ecliptic (the plane of Earth's orbit around the Sun)—inclined only 1%° from the vertical—the Moon has no seasons. Sunlight is always nearly horizontal at the lunar poles, which results in permanently cold and dark environments at the bottoms of deep craters.

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