THE MOON

An image of the whole lunar disc taken during the Apollo 17 mission in 1972. You can see the Maria Serenitatis, Tranquillitatis, Nectaris, Foeconditas, and Crisium. (NASA)

The Moon is the nearest celestial body to the Earth and the only one, to the present day, that was partially explored by man during the Apollo missions. It is, for us, the brightest object in the sky after the Sun, although it is reflected solar light. The apparent luminosity of the Moon is 450,000 times smaller than that of the Sun.  The Moon has always fascinated man thanks to its luminosity and the periodic variations of its aspect, the so-called phases.

The Lunar Module of Apollo 11 photographed from the command module, during the orbit around the Moon. The Earth is rising on the lunar horizon. (NASA)

Edwin Aldrin Jr, the pilot of the Lunar Module, poses for a photograph with the American flag during the extravehicular activity of the Apollo 11 mission in 1969. (NASA)

The lunar surface

The surface of the Moon is perfectly visible due to the absence of an atmosphere. Its mass, in fact, is 73.5 billion billion tons (7,35 10²⁵ g), which is insufficient to retain the gas molecules.
This causes big changes of temperature on the surface of the satellite: it ranges from -233 °C at night to +123 °C during the day. 
 

The aspect of the Moon testifies the great importance of the impact of meteorites in the past of our Solar System. Even with the naked eye it is possible to distinguish light and darker regions. In the past, the former were improperly called "continents", the latter "seas", as an analogy with the terrestrial surface.
In actual fact, the seas are flat areas, which are darker and with a lower altitude than the lighter zones. The largest of the lunar seas is Oceanus Procellarius (ocean of tempests), twice as large as the Mediterranean sea. The seas were probably formed by the collapse of the rocky plates underneath.
On the contrary, the continents are flat zones in relief, with various morphology.

Mosaic of the Moon in false colours, taken by the Galileo spacecraft in 1992. You can see the uplands in red, the volcanic rocks of the Seas in blue/orange, while the small purple red zones are sediments formed during explosive volcanic eruptions. At the bottom you can see the young crater Thyco. (NASA/JPL)

Both the seas and the continents are covered with many craters, circular structures with a flat bottom and the edges in relief, with the diameter of 30-40 Km. The larger craters have diameters reaching 240 Km, with a depth of up to 5 Km.  The craters originated from the impact of meteorites on the lunar surface or, less likely, from volcanic phenomena. On the Moon, like on other satellites and on Mercury, the impacts were particularly violent because the meteorites did not meet the friction of an atmosphere. They still occur, even though with a smaller frequency compared to the past.

 

Image of the Copernicus large crater taken during the Apollo 17 mission. (NASA)

Image of the hidden side of the Moon, taken in 1969 during the Apollo 11 mission. The land on this side of the Moon is very rough and rugged.  (NASA)

 
 

Besides the craters, faults and ridges of up to dozens of Km can be distinguished on our satellite; these are fractures of the crust with the flow of rocky masses vertically and horizontally, that were maybe formed during the cooling of the Moon. Besides, there are real mountain ranges that constitute the walls of the seas, formed by the accumulation of material at the edges of the impact sites of large meteorites. The highest peaks reach the altitude of 9,000 metres.

The footprint of an astronaut on the lunar surface, photographed during the Apollo 11 mission. (NASA)

The lunar surface is covered with "regolith", a mixture of dust and rocky debris produced by the disintegration of meteorites, with a thickness between 2 and 8 metres.  Recently, the Clementine probe revealed the presence of frozen water in some of the craters around the lunar South Pole.

 
 

Logo of the Apollo 11 mission. (NASA)

The Lunar Roving Vehicle of Apollo 15. On the right, you can see the slopes of Mount Hadley. (NASA)

 
 
 
 
 

Structure and composition of the Moon

The Moon has a diameter of 3,476 metres and a mean density of 3.34.
The lunar crust has an average thickness of 68 Km, and is composed of rocks of effusive origin, mainly aluminium silicates, calcium, magnesium and oxides.
This testifies that in the past our satellite must have had a remarkable volcanic activity, which is over today. 

These vitreous fragments are the smallest lunar particles ever brought to Earth; their diameter measures 20-45 microns. They are composed of Titanium (8%), iron oxide (22%), and they are rich in Zinc. They probably have a volcanic origin. (NASA)

The crust covers a rocky mantle with the thickness of approximately 980 Km, and a hot nucleus with the radius of 700 Km. The moon quakes, seismic waves with a low intensity, originate within the mantle.
The Moon does not have a magnetic field, but could have had in the past, as testified by the magnetism of some of the rocks collected on its surface.
 

A view of the Earth that rising above a large lunar rock, photographed during the Apollo 17 mission. (NASA)

Many of the lunar rocks seem to have an age between 3 and 4.6 billion years.  The origin of our satellite is not known yet. It was postulated in the past that it originated at the same time as the Earth from the protoplanetary cloud, or by scission of terrestrial material, or even that the Moon originated somewhere else and was later captured by the gravitational force of the Earth.   According to the modern theory, a gigantic impact on the terrestrial surface caused the emission of some material from within; such material later condensed and formed the Moon.

 
 

The lunar motions

The main motions of the Moon, besides the translation around the Earth and the sun, are its revolution and rotation; in actual fact its motions are innumerable and very complex, due to the irregular shape and the variation of the position in time of the Earth and the Sun.
 

Revolution
 

A spectacular view of the Earth rising on the lunar horizon, photographed during the apollo 17 mission.  (NASA)

The orbit of the Moon has a mean radius of 384,400 Km and an eccentricity equal to 0.05; it lies on a plane with the inclination of 5° 8' on the ecliptic, that it intersects in two points called "nodes"; when the Sun, the Earth and the Moon are aligned along the line of the nodes, the result is an eclipse.

The orbit of the Moon has a mean radius of 384,400 Km, and its eccentricity is 0.05; it lies on a plane with an inclination of 5° 8' on the  ecliptic, that it intersects in two points called "nodes". The line that joins the nodes is called "line of the nodes"; when the Sun, the Earth and the Moon are aligned along such line, the result is an eclipse.
The orbital period of the Moon is called "month". It refers to the interval necessary to the Moon to travel back to the same position, relatively to the Earth, and to a given point in space.
With respect to a fixed point on the sky, for example a distant star, the Moon completes a revolution in 27 days, 7 hours and 43 minutes; such interval is called "sidereal month".
If on the other hand the lunar orbit is referred to the Moon, the month has a duration of 29 days, 12 hours and 44 minutes ( "sinodic month " or "lunation"). This happens because in the meantime, due to the revolution of the Earth around the Sun, our star travelled the apparent distance of 27 degrees on the sky.
 

Rotation
 

The motion of rotation of the Moon around its axis has the same duration of its revolution: as in the case of many other planet-satellite systems, the two motions are synchronized in time. For this reason, our satellite always shows the same side to the Earth. Nevertheless, for Kepler's second law, the revolution is slower at the apogee and is faster at the perigee, while the speed of the rotation is uniform. As a consequence, the Moon shows apparent oscillations called "librations" to a terrestrial observer, therefore it is possible to observe most of its surface, approximately 57% of the total. 
 

Consequences of the lunar motions 

The phases
 

The Moon appears completely or partially illuminated, or dark, according to its position with respect to the Earth of the Sun. 
If the Moon is between the Earth and the Sun (in "conjunction"), there is the "new moon" (position A).
If the line between the Earth and the Moon is perpendicular to the line between the Earth and the Sun (that is in the "quadrature"), we have the phases of the first and last quarter (positions B and D). 
On the other hand, if the Moon is on the opposite side of the Sun with respect to the Earth (that is in "opposition"), we have the "new moon" (position C). 

The eclipses
 

The word eclipse means "hiding" and indicates the darkening of a celestial body by an other body that is travels in front of it, with respect to an observer from the Earth. The phenomenon is connected with the position of the Earth and the two bodies in space.
When the Earth, the Sun and the Moon are aligned, that is when the line of the nodes coincides or is very close to the line that joins the Earth and the Sun, we can have the three following cases:
 

Lunar eclipse: in this case the Earth is between the Sun and the Moon and projects an umbra on the Moon, 1,376,000 Km long, surrounded by a zone of semi-darkness. If the Moon is completely contained in the umbra, the result is a total eclipse; if it travels only through the zone of semi-darkness, there is a penumbra eclipse; if it travels only partially through the umbra, a partial eclipse.

The Apollo 11 mission,  AVI, 5.8 Mb (NASA/JPL)

Astronauts with the American flag,  MPEG, 118 Kb

The Moon and Venus,  MPEG, 83 Kb (Naval Research Laboratory)

Encounter between the Galileo spacecraft and the Moon, AVI, 8 Mb (NASA/JPL)

---