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The Moon



The Moon


Orbit: 384,400 km from Earth
Orbital period: 27 days
Gravity: 1.622 m/s²
Circumference: 10,917 km
Diameter: 3476 km
Mass: 7.35e22 kg
Age: 4.527 billion years


The Moon is the only natural satellite of the Earth and the fifth largest moon in the Solar System. Called Luna by the  Romans, Selene and   Artemis   by the Greeks, and many other names in other mythologies.

The Moon, of course, has been known since prehistoric times.  It is the  second brightest object in the sky  after the Sun. As the Moon orbits around the Earth once  per month, the angle between the Earth, the Moon and the Sun changes; we see  this as the cycle of the  Moon’s  phases. The time between successive new moons is 29.5 days (709 hours), slightly  different from the Moon’s orbital period (measured against the stars) since the  Earth moves a significant distance in its orbit around the Sun in that time.

Due to its size and composition, the Moon is sometimes classified as  a  terrestrial “planet” along with  Mercury,   Venus,  Earth and  Mars.

moon foot print

The Moon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only   extraterrestrial body to have been  visited by humans.   The first  landing was on July 20, 1969  (do you remember where you were?);   the  last was in December 1972.  The Moon is also the only body from which   samples have been returned to Earth.  In the summer of 1994,   the Moon was  very extensively mapped by the little spacecraft  Clementine and again in 1999 by Lunar Prospector.

The gravitational forces between  the Earth and the Moon  cause some interesting effects.  The most obvious is  the tides.  The Moon’s gravitational attraction is stronger on the  side of the Earth nearest      to the Moon and weaker on the opposite side.   Since the Earth, and particularly  the oceans, is not perfectly rigid it is  stretched out along the line toward      the Moon.  From our perspective on the  Earth’s surface we see two small bulges,  one in the direction of the Moon  and one directly opposite.  The effect is much stronger in the ocean water  than in the solid crust so the  water bulges are higher.  And because  the Earth rotates much faster than the Moon moves in its  orbit, the bulges  move around the Earth about once a day giving two high tides per day.   (This is a greatly simplified model; actual tides, especially near the  coasts, are much more complicated.)

Earth and the Moon

But the Earth is not completely fluid, either.  The  Earth’s rotation carries the Earth’s bulges slightly ahead of the point  directly beneath the Moon.  This means that the force between the Earth and  the Moon is not exactly along  the line between their centers producing a  torque on the Earth and an  accelerating force on the Moon. This  causes a net transfer of rotational energy from the Earth to the Moon, slowing down the Earth’s rotation by about 1.5 milliseconds/century  and  raising the Moon into a higher orbit by about 3.8 centimeters per year.   (The opposite effect happens to satellites with unusual orbits such as  Phobos and Triton).

The asymmetric nature of this gravitational  interaction is also responsible for the   fact that the Moon rotates synchronously,  i.e. it  is locked in phase with its orbit so that the same side is always facing toward the Earth.  Just as the Earth’s rotation is now being  slowed by the Moon’s influence so in  the distant past the Moon’s rotation  was slowed by the action of the Earth,   but in that case the effect was much  stronger.  When the Moon’s rotation rate  was slowed to match its orbital  period (such that the bulge always faced toward the Earth) there was no  longer an off-center torque on the Moon and a stable   situation was  achieved.  The same thing has happened to most of the other satellites in  the solar system.  Eventually, the Earth’s rotation will be slowed to match  the Moon’s period, too,  as is the case with Pluto and  Charon.

Actually, the Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few degrees of  the far side can be seen from time to time, but the  majority of the  far side (left)   was completely unknown until the Soviet spacecraft  Luna 3 photographed it in 1959.  (Note:  there is no “dark side” of the Moon; all parts of the Moon  get   sunlight  half the time (except for a few deep craters near the poles).  Some uses of  the term “dark side” in the past may  have referred to the far side as “dark”  in the sense of “unknown”  (eg “darkest Africa”) but  even that meaning  is no longer valid today!)

The far side of the Moon

The Moon has no atmosphere. But evidence from  Clementine  suggested that there  may be water ice in some deep craters near the  Moon’s south pole which are  permanently shaded. This has now been  reinforced     by data from Lunar Prospector.  There is apparently ice at the north pole as well.

The Moon’s crust averages 68 km thick and varies from  essentially 0  under Mare Crisium to 107 km north of the crater Korolev    on the lunar far  side.  Below the crust is a mantle  and probably a small core (roughly  340 km radius and 2% of the Moon’s mass).  Unlike the Earth, however, the  Moon’s interior is no longer active. Curiously, the Moon’s center of mass  is offset   from its geometric center by about 2 km in the direction toward  the Earth.  Also, the crust is thinner on the near side.

moon and half moon

There are two primary types of terrain on the Moon: the heavily  cratered  and very old    highlands  and the relatively smooth and younger Maria,   The Maria (which comprise about 16% of the Moon’s surface)   are huge  impact craters that were later flooded by molten lava.  Most of the surface  is covered with regolith,   a mixture of fine dust  and rocky debris produced by meteor impacts. For some unknown reason,  the Maria are concentrated on the near side.

Most of the craters on the near side are named for famous figures in the  history of science such as Tycho, Copernicus,  and Ptolemaeus.  Features  on the far side have more modern references such as Apollo, Gagarin and Korolev  (with a distinctly Russian bias since the first images were obtained by Luna  3). In addition to the familiar features on the near  side, the Moon also has the huge craters  South Pole-Aitken on the far side which is 2250 km in diameter and 12 km deep making it the largest impact basin in the solar system and Orientale  on the  western limb (as seen from Earth; in the center of the image at left) which is a  splendid example of a multi-ring crater.

Igneous moon rock on display

A total of 382 kg of rock samples were returned to the Earth by  the Apollo and  Luna programs. These provide most of our  detailed knowledge of the Moon.  They are particularly valuable in that they can be dated.  Even today, more  than 30 years after the last Moon landing,       scientists still study these  precious samples.

Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion  years  old.  This is a fortuitous match with the oldest terrestrial  rocks which are rarely  more than 3 billion years old. Thus the Moon  provides evidence about the  early history of the Solar System not  available on the Earth.

Lander Scene Apollo 17

Prior to the study of the Apollo samples, there was no  consensus about the origin of the Moon.  There were three principal  theories: co-accretion which  asserted that the Moon and the Earth  formed at the same time from the Solar Nebula;    fission which asserted that the Moon split off of the Earth; and   capture which held that the Moon formed elsewhere and was  subsequently  captured by the Earth. None of these work very well.  But the new and detailed information from the Moon rocks  led to the  impact theory: that  the Earth collided with a very large object (as  big as Mars or more) and that  the Moon formed from the ejected material.   There are still details to be  worked out, but the impact theory is now widely accepted.

The Moon has no global magnetic field.  But some of its surface rocks exhibit   remanent magnetism indicating that there may have been a global magnetic  field  early in the Moon’s history.

Earth rise view from the Moon

With no atmosphere and no  magnetic field, the Moon’s surface is exposed directly  to the solar  wind.  Over its 4 billion year lifetime many  ions from the solar wind  have become embedded in  the Moon’s regolith.  Thus samples of regolith  returned by the Apollo       missions proved valuable in studies of the solar  wind.

For more info on the Phases of the Moon and Percent of the Moon Illuminated visit;
The Moon

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