The Rotation Rate Of Venus
The Venusian atmosphere is both optically thick and highly reflective. The upper cloud deck has an albedo of 0.76, meaning that it reflects 76% of the sunlight that falls on it. In addition, the low-lying Venusian clouds are so dense that they completely obscure any optical view of the planet's surface. Not being able to monitor variations in surface detail has meant that astronomers have only recently discovered the true rotation rate of Venus.
While the Venusian atmosphere presents an impenetrable shroud against optical observations, it is transparent to radio and microwave radiation. Using the giant radio telescope at the Arecibo Observatory in Puerto Rico, astronomers were able to bounce microwave signals off the surface of Venus. By analyzing the Doppler shift in the returned signals the astronomers were then able to determine the planet's rotation rate. The results were a complete surprise.
The microwave measurement of Venus's rotation rate showed that the planet was spinning on its axis in the opposite sense to which it orbits the Sun. If one could stand on the surface of Venus, sunrise would be in the west and sunset would be in the east, the exact opposite to that seen on Earth. When viewed from above, all the planets in the solar system orbit the Sun in a counter clockwise direction. Most of the planets also rotate about their spin axies in a counter clockwise sense. The only exceptions to this rule are the planets Venus, Uranus, and Pluto. When a planet spins on its axis in the opposite sense to its orbital motion, the rotation is said to be retrograde.
Venus takes 243.01 days to spin once on its axis. With this slow rotation rate it actually takes 18.3 days longer for Venus to spin once on its axis than it takes for the planet to orbit the Sun. The Venusian day, that is the time from one noon to the next, is 116.8 terrestrial days long. A curious relationship exists between the length of the Venusian day and the planet's syndic period. The syndic period of Venus, that is, the time for the planet to repeat the same alignment with respect to Earth and Sun, is 584 days, and this is five times the Venusian day (584 = 5 × 116.8). It is not known if this result is just a coincidence, or the action of some subtle orbital interaction. The practical consequence of the relationship is that, should a terrestrial observer make two observations of Venus that are 584 days apart, then they will "see" the same side of the planet turned towards Earth.
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