Venus - Orbit and Rotation

Orbit and Rotation

Venus orbits the Sun at an average distance of about 0.72 AU (108,000,000 km; 67,000,000 mi), and completes an orbit every 224.65 days. Although all planetary orbits are elliptical, Venus's orbit is the closest to circular, with an eccentricity of less than 0.01. When Venus lies between the Earth and the Sun, a position known as inferior conjunction, it makes the closest approach to Earth of any planet at an average distance of 41 million km. The planet reaches inferior conjunction every 584 days, on average. Owing to the decreasing eccentricity of Earth's orbit, the minimum distances will become greater over tens of thousands of years. From the year 1 to 5383, there are 526 approaches less than 40 million km; then there are none for about 60,158 years. During periods of greater eccentricity, Venus can come as close as 38.2 million km.

All the planets of the Solar System orbit the Sun in a counter-clockwise direction as viewed from above the Sun's north pole. Most planets also rotate on their axis in a counter-clockwise direction, but Venus rotates clockwise (called "retrograde" rotation) once every 243 Earth days—the slowest rotation period of any planet. The equator of the Venusian surface rotates at 6.5 km/h, while on Earth rotation speed at the equator is about 1,670 km/h. Venus's rotation has slowed down by 6.5 minutes per day since the Magellan spacecraft visited it 16 years ago. A Venusian sidereal day thus lasts longer than a Venusian year (243 versus 224.7 Earth days). Because of the retrograde rotation, the length of a solar day on Venus is significantly shorter than the sidereal day, at 116.75 Earth days (making the Venusian solar day shorter than Mercury's 176 Earth days); one Venusian year is about 1.92 Venusian (solar) days long. To an observer on the surface of Venus, the Sun would rise in the west and set in the east.

Venus may have formed from the solar nebula with a different rotation period and obliquity, reaching to its current state because of chaotic spin changes caused by planetary perturbations and tidal effects on its dense atmosphere, a change that would have occurred over the course of billions of years. The rotation period of Venus may represent an equilibrium state between tidal locking to the Sun's gravitation, which tends to slow rotation, and an atmospheric tide created by solar heating of the thick Venusian atmosphere. A curious aspect of the Venusian orbit and rotation periods is the 584-day average interval between successive close approaches to the Earth is almost exactly equal to five Venusian solar days. However, the hypothesis of a spin–orbit resonance with Earth has been discounted.

Venus has no natural satellite, though the asteroid 2002 VE68 presently maintains a quasi-orbital relationship with it. In the 17th century, Giovanni Cassini reported a moon orbiting Venus, which was named Neith and numerous sightings were reported over the following 200 years, but most were determined to be stars in the vicinity. Alex Alemi's and David Stevenson's 2006 study of models of the early Solar System at the California Institute of Technology shows Venus likely had at least one moon created by a huge impact event billions of years ago. About 10 million years later, according to the study, another impact reversed the planet's spin direction and caused the Venusian moon gradually to spiral inward until it collided and merged with Venus. If later impacts created moons, these also were absorbed in the same way. An alternative explanation for the lack of satellites is the effect of strong solar tides, which can destabilize large satellites orbiting the inner terrestrial planets.

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