file2-Wikipedia-Moon.txt

The Moon is Earth's only natural satellite. Its diameter is about one-quarter of Earth's (comparable to the width of Australia),[17] making it the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet. It is larger than all known dwarf planets in the Solar System.[18] The Moon is a planetary-mass object with a differentiated rocky body, making it a satellite planet under the geophysical definitions of the term. It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's at 0.1654 g—Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.

The Moon orbits Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter. Its gravitational influence is the main driver of Earth's tides and very slowly lengthens Earth's day. The Moon's orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of the Moon's Earth-facing surface that is illuminated by the Sun varies from none up to nearly 100%, resulting in lunar phases that form the basis for the months of a lunar calendar.[19] The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. Due to cyclical shifts in perspective (libration), 59% of the lunar surface is visible from Earth.