The Moon is the only natural satellite of the Earth. It orbits our planet in 27.3 days at a mean distance of 384,400 km. Its diameter of 3,474 km makes it the fifth largest satellite in the solar system.
It is thought that the Moon was formed at the same time as the Earth, 4.55 billion years ago, from the debris of a giant collision between the Earth and a Mars-size object.
The Moon is in on a synchronous orbit around the Earth, which means that it makes one turn around itself as it makes one turn around the Earth. So, it is always showing the same face to us. All we know about the ‘hidden’ face of the Moon comes from the records taken by astronauts or automatic probes that went around on to the other side. There is no rising or setting of the Earth on the horizon of the Moon. If you were standing on the near side of the Moon, the Earth would appear immobile in the sky.
During its revolution around the Earth, the disk of the Moon is not always completely lit by the Sun. This variation in the appearance of the lunar disk is called the “phases of the Moon”, and is characterized by a well-known cycle.
The apparent diameter of the Moon as seen from Earth is almost exactly that of the Sun. This is why the Moon can sometimes completely mask the solar disk and produce total solar eclipses. Another important influence of the Moon on Earth is the phenomenon of tides, which is due to the Moon’s gravitational pull on the seas and oceans of Earth.
The Moon is the only celestial object that has been visited by human beings. The first visit was on 21 July 1969 when two members of the Apollo 11 mission set foot on our satellite: they were Neil Armstrong and Edwin Aldrin. As many as 12 astronauts walked the surface of the Moon between 1969 and 1972, and they returned with 382 kg of lunar soil to be analysed on Earth.
(while you explore the tactile schematic image)
Mountains and Maria
The visible surface of the Moon shows bright and dark areas. The bright areas are generally hills or mountains (materialized by thick fabric on the tactile image), while the dark ones are flat lands, called “mare” (materialized by thin fabric in relief on the tactile image). These low altitude areas were filled with lava during an ancient period of volcanic activity, around 3 billion years ago. Most of them were named by ancient astronomers after common phenomena encountered on terrestrial seas and oceans: Oceanus Procellarum (‘Ocean of Storms’), Mare Imbrium (‘Sea of Rains’), Mare Serenitatis (‘Sea of Serenity’), Mare Tranquilitatis (‘Sea of Tranquility’), etc.
The whole face of the Moon is dotted with craters, with diameters from a few meters to hundreds of kilometres (curved sequins). They are the result of impacts by asteroids, since the Moon does not have any atmosphere to prevent them from reaching the surface (a few of them are materialized by buttons on the tactile image: Plato and Aristoteles at the top, Aristarchus on the left, Copernicus near the centre, and Clavius at the bottom). Copernicus has a diameter of 93 km and is located in Mare Imbrium at the end of a chain of mountains called Apennins (materialized by the thick fabric on top of the thin fabric on the tactile image).