Moon
Moon, name given to the only natural satellite of Earth . The Moon is the second brightest object in Earth’s sky, after the Sun , and has accordingly been an object of wonder and speculation for people since earliest times. The natural satellites of the other planets in the solar system are also sometimes referred to as moons. Telescopes have revealed a wealth of lunar detail since their invention in the 17th century, and spacecraft have contributed further knowledge since the 1950s. Earth’s Moon is now known to be a slightly egg-shaped ball composed mostly of rock and metal. It has no liquid water, virtually no atmosphere, and is lifeless. The Moon shines by reflecting the light of the Sun. Although the Moon appears bright to the eye, it reflects on average only 7 percent of the light that falls on it. This reflectivity, called albedo , of 0.07 is similar to that of coal dust. The diameter of the Moon is about 3,480 km (about 2,160 mi), or about one-fourth that of Earth. The Mo