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Satellites of planets

 Automatic translation  Automatic translation Updated June 01, 2013

When we look at the sky, the stars form figures do not change over the centuries.
We can therefore say that all the stars are fixed against each other, to our eye. However, a small group of 5 objects, in addition to the Moon and Sun, are nomadic and move relative to the stars, what are the planets (Greek meaning "wanderer").
These 7 will also stray their names to the seven days of the week. Moon (Monday), Mars (Tuesday), Mercury (Wednesday), Jupiter (Thursday), Venus (Friday), Saturn (Saturday), Sun (now dominus but the Anglo-Saxons have kept Sunday).
With modern telescopes we can see the "stray"from "stray".
These are the satellites of the planets, and there are many.


With the exception of Mercury and Venus, all the planets in the solar system have moons qualified moons.
In the solar system, there are over 140 moons, but the moon, although it is not the largest satellite in the solar system, is the largest compared to its planet.

Image: photograph of a bright moon, taken in Caen - France in the summer of 2009.
Credit: Jean-Louis Lerossignol

 Crescent Moon



There are more than 160 moons in the solar system, but the Moon exceptionally large relative to its planet, it is a great satellite, its diameter is 3 474 km while that of the Earth is 12,756 km, the Moon that is smaller than 4 times the Earth. Although the Moon is not the largest in the solar system, its relationship to the planet is the largest (27% compared to the Earth), Ganymede (3.7% compared to Jupiter), Titan (4.3 % by Saturn), Callisto (3.4% compared to Jupiter) and Io (2.5% compared to Jupiter). The Earth-Moon configuration is unique in the solar system outside the couple Pluto-Charon (52%). The Moon is the only celestial body on which the man is gone, and yet it is not so friendly as you might think. There is absolutely no air, so you need a pressure suit any leakiness of the combination exposes astronauts to deadly empty. It is also a world of silence, the sound can not be transported unless the use of radio transmitters and headphones.


As there is no atmosphere, sunlight is not filtered and then the sky is always black. In addition there is no color, the landscape is sad, it is the same everywhere in a more or less monotonous gray tone. Temperatures are extremely inhospitable, the transition from hot to cold is especially brutal, 132°C during the day to -151°C at night. And finally the absence of a magnetic field passes deadly solar radiation. Many meteorites collide with the lunar surface at high speed without being slowed down, for 4 billion years, they spray the moon rock, creating over a layer of fine dust, regolith. The dust is so fine that it presents a danger, it sticks on the combinations and on the instruments and would be a big problem if we were to live.

NB: The regolith means the layer of dust produced by the impact of meteorites on the surface of a planet or a satellite without atmosphere.

 Size comparison between Earth and Moon



The Moon with one L capital letter, always has was a center of major interest for the man. The origin of the Moon is the object of a scientific debate.
Several hypotheses are advanced, the capture of an asteroid, the fission of a part of the Earth by the centrifugal energy, the co-accretion of the original matter of the solar system.
Given the slope of the lunar orbit, it is not very probable that the Moon forms at the same time as the Earth, or that this one captured the Moon.
The best accepted hypothesis is the one of the huge impact: a collision between the young Earth and the object of the size of Mars would have ejected some matter around the Earth, which would have eventually formed the Moon which we know today.

Image: This photo of December 2010, is a mosaic of 1300 images from the visible face of the Moon, taken by the lunar probe LRO.
Credit: NASA / GSFC / Arizona State Univ. / Lunar Reconnaissance Orbiter.

 La Lune satellite de la Terre 
Moon satellite of the earth
Diameter 3 474,6 km
Masse 7,34×1022 kg
Escape velocity 2.38 km/s
Gravity  1.62 m/s²
Albedo 0.12
Temperature 123°C-77°C-233°C
Synodic period The synodic period of a planet is the time put by this planet to return to the same configuration Earth-planete-Sun, that is to the same place in the sky with regard to the Sun, seen by the Earth. This duration differs from the period of sidereal revolution of the planet because the Earth moves itself around the Sun. As a consequence, it is about the period of visible revolution, the duration between two conjunctions planet-sun, such as observed since the Earth.  29 days 12 h
44 min 12,8 s
Orbital periodThe period of revolution, is the time put by a celestial body to carry out its trajectory, or revolution, around another celestial body. As a planet around the Sun, or a satellite around a planet. The necessary time to carry out this movement can be estimated by return at the same position with regard to a fixed star, or at the same position with regard to the equinox point. In that case, it is called period of sidereal revolution. 27 days 7 h
43 min 11,5 s
Slope in the ecliptic 5.145° = 5°8'24"
Apogee 405 500 km
Perigee 363 300 km
Average surface temp. equator: 220 K
85° N: 130 K

Both most brilliant celestial bodies


The most brilliant both celestial bodies of the night-sky seemed to touch in the evening of May 19th, 2007, the Moon and the planet Venus were visible in the same part of the sky, and at the time of their biggest nearness were separated only from a degree of bow. The conjunction appears here such as it was visible since the neighborhood of the city of Quebec in Canada. Venus is situated in the western South of the Moon.
The arrows that seems to hurl Venus are in fact egrets of diffraction provoked by the optics of the camera. The image is besides so net as it reveals the presence of the lunar craters.


The distance which really separates these two celestial bodies was about 250 million kilometers and this visible nearness, called a conjunction, is only an illusion of perspective. Although the Moon so exceeds Venus every month, a visible passage so moved closer in the evening is relatively rare.

Image: Photo of Venus and the distance of the Moon illuminated by the Sun in the foreground.

 Vénus et la Lune

A theory which reminds coupling of two living cells


The various theories on the origin of the Moon in competition up to the edge of the spatial era were worsened by the results of the analyses of the lunar rocks.
The analysis of the lunar rocks advances an interesting theory on the origin of the Moon. These materials result from the coat of two global accomplices. A gigantic shock has of to shake the quite young Earth. The partner of our nights could arise from the accretion of materials ejected during a tremendous collision.


Let us imagine that at the beginning of the creation of the Earth, a planet very big of the size of Mars (half of the size of the Earth), him too differentiated (metal nucleus, coat) which under a particular angle of impact and a relative speed came to collide the Earth. This collision provokes the fusion of both metal nuclei, whereas the materials of the coat of both objects of the solar system are ejected but remain bound to the Earth by the force of gravity.


Most of its materials are slowly going to gather together in what is going to become our Moon. All this takes place at the very beginning of the history of the Earth, which kept it no track. Nevertheless, the numeric simulations succeed in representing well enough the stages. On the other hand, this hypothesis is contradictory not at all with that we know at present about the Moon.

Moon formationMoon formationMoon formationMoon formation

Theia, hypothetical planet


The simulations of the formation of the solar system continue to give more weight to the above scenario for the formation of Moon.
There are 4.533 billion years, about 34 million years after the formation of Earth, a terrestrial planet called Theia or Orpheus, of 6 500 km in diameter, would have struck the Earth.
The impact would have pulled tangentially part of the mantle Land. The violence of the impact would have caused fragmentation complete and a large spray of Theia. Probably different, the iron core of Theia would incorporated into the Earth and a magma ocean would have covered our planet. Part of the material of the mantle, and Theia debris would remain in orbit around the Earth eventually form the Moon, our satellite.
This theory still needs to be supported by observations on the geological nature of the Moon. This is one reason why the Japan Aerospace


Exploration Agency (JAXA) was launched and the mission Selenological Engineering Explorer (Selene), later renamed Kaguya. Scientists are inclined to this theory as compared to moons of other planets in the solar system, ours is much more larger than average and its orbital distance lower than other moons. What makes that our moon can not have been captured in from nearby, like the other moons. The most probable theory is that Theia, before the collision, was follow an orbit similar to Earth at Lagrange point A Lagrange point (denoted Li), or libration point is a position in space where the gravitational fields of two bodies in orbit around each other, and substantial masses are combined so provide a balance to a third body of negligible mass, as the relative positions of the three bodies to be fixed.  Earth-Sun.

NB: Theia in Greek mythology was one of the original Titans who gave birth to Selene, goddess of the moon and sister of Helios (the Sun) and Eos (Dawn).

Image: simulated impact of the 2 objects, there are 4.533 billion years or about 34 million years after the formation of Earth.

 Théia, la planète hypothétique qui entre en collision avec la Terre.

Craters of the Moon


The hidden side of the Moon is perpetually invisible from the Earth, so it is impossible to take photographs of the face without the use of space probes. Curiously, the hidden face is different from the visible face as well in terms of its average altitude than the rate of cratering. Regularly, images and spectacular videos of the surface of the Moon and especially its dark side, we are sent by U.S. probes and Japanese probes. Clementine mission had not seen the full moon, but the probe Kaguya-Selene the most important lunar mission since the Apollo program, launched in 2007, has sent us photographs of a high precision in the order of 10 meters per pixel.


Which helped detect new small craters and thus to refine the measurement of the lunar cratering rate land.

Image: the large crater Leibnitz 245 km diameter, located on the face of the Moon. In the right corner, below the image, the crater Finsen 72 km diameter.
Right in the middle of the image, the crater Davisson 87 km in diameter. Credit: JAXA, NHK

 Cratère leibnitz de 250 km sur la Lune

Water in small quantities on the Moon


The French and American researchers have discovered water, small quantities of course but almost everywhere on the surface of the Moon, at latitudes above 10 degrees. This result has been published in the journal Science on 25/09/2009.
The moon is dry, the analysis of data provided in 2009 by the probe Chandrayaan-1 and EPOXI indicate that the lunar surface contains the equivalent of a half liter of water per hectare on average, according to one researcher, the French Groussin Olivier, Laboratory of Astrophysics of Marseille.
This discovery is due to the signature of water molecules in the infrared region has been obtained by the instruments spacecraft flying over the Moon, Moon Mineralogy Mapper equipped Chandrayaan-1. This discovery reinforces
the idea that the Moon can store water in the bottom of craters.
"The water is detected on the surface of the Moon is adsorbed water," said Olivier Groussin.
NASA has discovered large quantities of water ice near the south pole of the Moon.


The U.S. probe LCross (Lunar Crater Observation and Sensing Satellite) that hit the moon on friday 9 October 2009 in 11 30 hours UT, has enabled scientists to detect the presence water at south pole of the Moon.
"We have found water and not Only a little, but significant quantities, "said Anthony Colaprete, chief scientist of the LCROSS mission, November 13, 2009.

Image: The discovery of water on the moon is confirmed by 2 instruments: instrument M3 Indian Chandrayaan 1 mission and instrument VIMS of the Cassini-Huygens mission (ESA / NASA).

 de l'eau sur la Lune

Mare Orientale


This gigantic crater drawn on the surface of the Moon is called Mare Orientale is one of the most impressive lunar crater basins, by its large size.
It is located in the extreme west of the face, it is therefore difficult to see from Earth.
This mosaic of images of the ringed structure of the large lunar impact basin reveals in stunning detail.
This mosaic is composed of images taken by the Wide Angle Camera of the lunar probe LRO (Lunar Reconnaissance Orbiter).
These images do not overlap all completely, which explains the black lines mainly visible on the right of the image. Partly submerged by lava flows, Mare Orientale is older than 3 billion years and is about 950 km in diameter.
Its formation is due to the fall of a large asteroid that was waving the lunar crust concentrically like a stone falling into water.
Although the Moon is barren and devoid of atmosphere, these dark regions on the surface of the Moon were named mare, meaning "sea " in Latin.


Early astronomers thought that these regions could indeed be vast expanses of water.
Plutarch (46-125) thought that the moon was a celestial earth, and dark areas and the plains were regularly filled with water, called Maria (Latin term meaning sea plural). The highlands, colored Terrae were baptized.

Image: mosaic of images of the ringed structure of the large lunar impact basin Mare East. Credit: NASA / GSFC / Arizona State Univ. / Lunar Reconnaissance Orbiter

 Mare orientale, Eastern Sea, on the Moon taken by RSO

Influence of the Moon


The moon has a definite influence on the lives of Earth, the gravitational influence is due to its proximity. The tides known, is due to the influence of the moon on the oceans. It causes a rise followed a drop of water level in waterfront in cycles of about 12 hours. The tide is produced by the motion of revolution around the Moon Earth. The moon's gravitational effect on Earth's oceans and seas, causes a local rise in water level on the surface of the Earth toward the Moon but also on the side opposite the moon.
On Earth the tidal wave is lagging behind in movement of the Moon because of its friction on the seabed. This causes a gradual slowing of movement Earth rotation, and a gradual isolation of the Moon.
The soil also undergoes the effect of lunar attraction because the rocks its component are elastic. Thus, when the new moon or Full moon, the crust rises by some 25 cm in the Geneva region as a result of these "tidal land".


Seismic activity, the magma of the mantle, now under the solid crust, also suffers because of its viscous state movements, corresponding to the passage of the satellite.
To some scientists, the fragmentation of the crustal plates is a consequence the presence of the Moon as it was much closer to Earth to its origin.
Life on Earth has kept track of that influence, for example, nautilus has a spiral shell composed of rings and each day an additional ring is created. All month a new internal partition formed. This is related to spawning instinct, which makes up near the surface at each full moon. By observing the nautilus shell fossils are indirectly confirms the gradual increase of the distance Earth-Moon. The axis of the Earth varies between 21 and 24 ° approximately relative to the equator Heavenly. That of Mars is not as big satellite varies between 20 and 60 °. The moon stabilizes the Earth in its motion.

 le nautile vivant

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