A body that is by now geologically dead, a rocky fragment that can no longer change and, for this very reason, is full of information about a faraway past of which the Earth has no memory and which instead, was common to both. This is a general overview of the Moon that emerged following the most specific exploration project ever made on an extraterrestrial body: the American Apollo program. The exploration program basically ended on December 19, 1972: after 301 hours of flight and 22 hours and 5 minutes on the lunar soil, Apollo 17, the last of the missions that bore this name, returned to the surface of the Earth. It carried another load of moon rocks which brought the total to 600 kilos - the booty from six space missions, starting on July 20, 1969, on the seas and upland plains of our satellite. One member of the crew was a certain doctor H.H. Schmitt, the first and only geologist that has ever had the pleasure of studying the rocks - on site - of a celestial body other than the Earth. The Moon now has more than twenty rather complex man-made objects: some have been reduced to a shapeless heap of scrap metal from the violent impact with the surface, while others are tiny but perfect laboratories which analyzed the structure and behavior of our natural satellite for years. What made it possible to explore the Moon was the relative closeness of the two celestial bodies. In fact, for an observer in space, the Earth-Moon system may seem to be more similar to a double planet than a planet with a single satellite. In effect, even if the other planets of the solar system have their own satellites, they usually are very small if compared to the dimensions of their planet. This is not the case with the Moon: in fact, with a volume equal to a fiftieth of the Earth, a mass that is the eightieth part of the Earth and a diameter only four times smaller, it is as if it were at the end of a rod that connects it with the planet. In other words, the system acts like a gigantic dumbbell with two unequal spheres at the end whose point of equilibrium, or center of gravity is not in space, but 1 700 kilometers under the surface of the Earth. In any event, the distance between the Moon and our planet is destined to change over time. In fact, the Earth's motion during its rotation slows down by a fraction of a second each century: that 's not very much for us if we consider the average lifespan of a single man, but on a cosmic scale we would discover that in 100 000 000 years, the Earth day would be more than half an hour longer. This slowing down of the Earth does not allow the Moon to follow a closed trajectory around our planet, but rather a type of spiral that brings it closer and farther away by about three centimeters each year: this means that in 100 000 000 years it will have increased its distance by 3 000 kilometers. There is another peculiar feature of the movements of the Moon in relation to the Earth which for us is very important: since the Moon's period of rotation is equal to 27.3 days, it is equal to a revolution around our planet; therefore our satellite always shows us with the same face, the only one that we have ever known prior to the space age.
A body that is by now geologically dead, a rocky fragment that can no longer change and, for this very reason, is full of information about a faraway past of which the Earth has no memory and which instead, was common to both. This is a general overview of the Moon that emerged following the most specific exploration project ever made on an extraterrestrial body: the American Apollo program. The exploration program basically ended on December 19, 1972: after 301 hours of flight and 22 hours and 5 minutes on the lunar soil, Apollo 17, the last of the missions that bore this name, returned to the surface of the Earth. It carried another load of moon rocks which brought the total to 600 kilos - the booty from six space missions, starting on July 20, 1969, on the seas and upland plains of our satellite. One member of the crew was a certain doctor H.H. Schmitt, the first and only geologist that has ever had the pleasure of studying the rocks - on site - of a celestial body other than the Earth. The Moon now has more than twenty rather complex man-made objects: some have been reduced to a shapeless heap of scrap metal from the violent impact with the surface, while others are tiny but perfect laboratories which analyzed the structure and behavior of our natural satellite for years. What made it possible to explore the Moon was the relative closeness of the two celestial bodies. In fact, for an observer in space, the Earth-Moon system may seem to be more similar to a double planet than a planet with a single satellite. In effect, even if the other planets of the solar system have their own satellites, they usually are very small if compared to the dimensions of their planet. This is not the case with the Moon: in fact, with a volume equal to a fiftieth of the Earth, a mass that is the eightieth part of the Earth and a diameter only four times smaller, it is as if it were at the end of a rod that connects it with the planet. In other words, the system acts like a gigantic dumbbell with two unequal spheres at the end whose point of equilibrium, or center of gravity is not in space, but 1 700 kilometers under the surface of the Earth. In any event, the distance between the Moon and our planet is destined to change over time. In fact, the Earth's motion during its rotation slows down by a fraction of a second each century: that 's not very much for us if we consider the average lifespan of a single man, but on a cosmic scale we would discover that in 100 000 000 years, the Earth day would be more than half an hour longer. This slowing down of the Earth does not allow the Moon to follow a closed trajectory around our planet, but rather a type of spiral that brings it closer and farther away by about three centimeters each year: this means that in 100 000 000 years it will have increased its distance by 3 000 kilometers. There is another peculiar feature of the movements of the Moon in relation to the Earth which for us is very important: since the Moon's period of rotation is equal to 27.3 days, it is equal to a revolution around our planet; therefore our satellite always shows us with the same face, the only one that we have ever known prior to the space age.
