Galileo, Cassini, and the Great Red Spot

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Imagine a hurricane that lasted for 300 years! Jupiter's Great Red Spot indeed seems to be a giant hurricane-like storm system rotating with the Jovian clouds. Observed in 1655 by Italian-French astronomer Jean-Dominique Cassini it is seen here over 300 years later - still going strong - in a mosaic of recent Galileo spacecraft images. The Great Red Spot is a cold, high pressure area 2-3 times wider than planet Earth. Its outer edge rotates in a counter clockwise direction about once every six days. Jupiter's own rapid rotation period is a brief 10 hours. The Solar System's largest gas giant planet, it is presently well placed for evening viewing. (APOD thanks to Alan Radecki for assembling a preliminary mosaic from the Galileo imagery!)