In geophysical terms, the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is 3,474 km (2,159 mi), roughly one-quarter of Earth's (about as wide as the contiguous United States).
Moon, Earth’s sole natural satellite and nearest celestial body. Known since prehistoric times, it is the brightest object in the sky after the Sun. Its name in English, like that of Earth, is of Germanic and Old English derivation.