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The Moon is Earth's only natural satellite and is the fifth largest satellite in the Solar System. It is the largest natural satellite in the Solar System relative to the size of its planet, a quarter the diameter of Earth and 1/81 its mass, and is the second densest satellite after Io. It is in synchronous rotation with Earth, always showing the same face; the near side is marked with dark volcanic maria among the bright ancient crustal highlands and prominent impact craters. It is the brightest object in the sky after the Sun, although its surface is actually very dark, with a similar reflectance to coal. Its prominence in the sky and its regular cycle of phases have since ancient times made the Moon an important cultural influence on language, the calendar, art and mythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day. The Moon's current orbital distance, about thirty times the diameter of the Earth, causes it to be the same size in the sky as the Sun—allowing the Moon to cover the Sun precisely in total solar eclipses.
Taurus-Littrow is a lunar valley located on the near side at the coordinates 20°00′N 31°00′E / 20.0°N 31.0°E. It served as the landing site for the American Apollo 17 mission in December 1972, the last manned mission to the Moon to date. The valley is located on the southeastern edge of Mare Serenitatis along a ring of mountains formed between 3.8 and 3.9 billion years ago when a large object impacted the Moon, forming Mare Serenitatis and pushing rock outward and upward. Taurus-Littrow is located in the Taurus mountain range and south of Littrow crater, features after which the valley received its name. The valley's name, coined by the Apollo 17 crew, was eventually approved by the International Astronomical Union in 1973. Data collected on Apollo 17 show that the valley is composed primarily of feldspar-rich breccia in the large massifs surrounding the valley and basalt underlying the valley floor, covered by an unconsolidated layer of regolith, or mixed materials, formed by various geologic events. Taurus-Littrow was selected as the Apollo 17 landing site after the other candidates were eliminated for various reasons. The landing site was chosen with the objectives of sampling highland material and young volcanic material in the same location.