Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" among the planets of the Solar System for its orbit being the closest to Earth's, both being rocky planets, and having the most similar and nearly equal size, mass, and surface gravity. Venus, though, is significantly different, especially as it has no liquid water, and its atmosphere is far thicker and denser than that of any other rocky body in the Solar System. The atmosphere is composed mostly of carbon dioxide and has a thick cloud layer of sulfuric acid that spans the whole planet. At the mean surface level, the atmosphere reaches a temperature of 737 K (464 °C; 867 °F) and a pressure 92 times greater than Earth's at sea level, turning the lowest layer of the atmosphere into a supercritical fluid. From Earth, Venus is visible as a star-like point of light, appearing brighter than any other natural point of light in the sky, and as an inferior planet always relatively close to the Sun, either as the brightest "morning star" or "evening star".
The orbits of Venus and Earth make the two planets approach each other in synodic periods of 1.6 years. In the course of this, Venus comes closer to Earth than any other planet, in contrast to Mercury which stays closer over the course of an orbit to Earth than any other planet, due to its orbit being closer to the Sun. In interplanetary spaceflight from Earth, Venus is frequently used as a waypoint for gravity assists, offering a faster and more economical route. Venus has no moons and a very slow retrograde rotation about its axis, a result of competing forces of solar tidal locking and differential heating of Venus's massive atmosphere. As a result, a Venusian day is 116.75 Earth days long, about half a Venusian solar year, which is 224.7 Earth days long.
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