|MadSci Network: Astronomy|
The only planet that we know of that has a longer solar day than solar year is Earth's sister planet Venus. The Venusian solar day (the time it takes the planet to spin once on its axis) is the equivalent of 243 Earth solar days. Its year (or period of its revolution around the sun) is equal to 224.7 Earth days. As a result, the surface of Venus sees a sunrise and a sunset approximately 117 days apart. Most of the planets in our solar system have a solar day far shorter than its year, though Mercury comes close to Venus (solar day = 59 Earth days; year = 88 days). At present, it is difficult to determine the rate of rotation on planets found outside our solar system, but none have been found that has a longer day than year like Venus. All information taken from the book, "Space Encyclopedia" by Heather Couper and Nigel Henbest for DK Publishing.
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