The Earth has only one natural satellite: the Moon.
A natural satellite is an astronomical object that orbits a planet . It is a celestial body that is generally smaller than the planet it accompanies.
The idea of a natural satellite is usually contrasted with the concept of an artificial satellite : in the latter case, it refers to an artifact designed by humans that is intentionally placed into orbit for some type of purpose (transmitting television or radio signals, monitor weather conditions, perform astronomical observations, etc.).
It should be noted that an orbit is a path that one body traces around another due to the influence of the force of gravity or another central force. In a planetary system, comets , asteroids and planets develop an orbit around a main star (such as the Sun in the solar system). In the case of satellites, they make an elliptical orbit close to the planet with which they maintain a gravitational link.
The Moon, the only natural satellite of planet Earth
The Moon is the only natural satellite of planet Earth . In the entire solar system, the Moon is the fifth largest satellite, with an equatorial diameter that reaches 3,478.8 kilometers.
From Earth the same side of the Moon is always observed. This is due to the phenomenon known as synchronous rotation , synchronous rotation , captured rotation or tidal coupling , which implies that the Moon's rotation on itself takes the same time as its translation around our planet.
The lunar hemisphere that cannot be seen from Earth is referred to as the hidden side of the natural satellite. It is also called the dark side , a name that is not correct since it receives light from the Sun.
It is important to note that the lunar disk carries out various oscillation movements before an observer located on our planet. Due to these movements, which constitute the lunar libration , the far side of the Moon represents approximately 41% of the satellite 's surface.
The first manned space flight to reach the Earth's natural satellite occurred in 1969.
The lunar phases
The lunar phases are the different states that the illuminated and visible area of the Moon exhibits due to changes in its position with respect to our planet and the Sun. The succession of these phases constitutes a cycle known as lunation that extends for 29.53 days.
Lunation is equivalent to what is known as a synodic month , which is the time it takes for a phase to appear again. In other words: the synodic month is the period it takes for the Moon to go through the same phase again. The sidereal month , on the other hand, is the period it takes for the satellite to return to the same position considering the stars that are fixed on the celestial sphere or vault. Its approximate duration is 27.32 days.
The phases of the Moon are nine. The first is the new moon (or black moon ), when the satellite is invisible to the naked eye because it is hidden by the sun's glare. Then the lunar illumination begins to grow, passing through the crescent moon ; the crescent quarter ; and the waxing gibbous moon until reaching the full moon , with one of its faces completely visible.
From then on, the illumination begins to decrease and the phases of the waning gibbous moon are crossed; the last quarter ; and the waning moon ( waning crescent or old moon ) until returning again to the new moon.
When a perfect alignment occurs between the Moon, the Earth and the Sun, eclipses appear. These phenomena imply that the light arriving from an astronomical object is blocked by another body.
During the full moon phase, a lunar eclipse can occur, which occurs when our planet is located between the Sun and the satellite, obscuring it. On the new moon, meanwhile, a solar eclipse can occur: in this case, the Moon is located between the Sun and the Earth.
Lunar mining is a still unexploited possibility offered by our planet's natural satellite.
The exploration of our natural satellite
Exploration of our natural satellite began in 1959 , when the Union of Soviet Socialist Republics ( USSR ) achieved the first lunar impacts by sending the Luna 2 space probe. Prior to that year, the only study method was observation from the Earth's surface using a telescope.
NASA 's Apollo missions , meanwhile, managed to take humans to the surface of the Moon six times. The first of the lunar landings took place in 1969 and the last in 1972 .
The moon landings were made possible by a type of space vehicle referred to as a lunar module . The astromobile or lunar rover is another key vehicle as it allows the exploration of the Moon by transporting people or functioning as autonomous robots.
With exploration, knowledge about lunar geography expanded. The scientific discipline called selenography specializes in the study of craters, lunar mountains and lunar seas.
It should be noted that, although lunar colonization (the creation of a permanent human settlement on the satellite through a lunar base) has not yet been achieved, there are those who believe that it is a step that humanity will complete sooner or later, just like the development of lunar tourism .
Moons of other planets
Although the Earth has only one natural satellite, the moons of other planets are numerous. Ganymede ( Moon of Jupiter ), Titan (Moon of Saturn ), Triton (Moon of Neptune ), Phobos (Moon of Mars ), Charon (Moon of Pluto ) and Titania (Moon of Uranus ) are some of them.
Ganymede , for example, is not only the largest natural satellite of Jupiter: it is also the largest satellite in the solar system. Another peculiarity is that, unlike the rest, it has a magnetic field .
Behind Ganymede in terms of size is Titan , Saturn's largest satellite. It is also notable for its atmosphere and for presenting liquid methane and ethane lakes on its surface. Titan and Earth, thus, are the only astronomical objects in the solar system with liquid bodies stable at room temperature.