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Neptune and VulcanFrom MicaNeptune and Vulcan: Dark Matter in the Solar System The cosmological Dark Matter is one example of a more general thing that we might call "dark matter" with lowercase letters-- that is, something that has not been observed directly, but whose existence we infer because of its gravitational effects. In the 19th century, there was dark matter in the Solar System: Uranus was showing deviations in its orbit that could not be explained by the action of the Sun and the other known planets. That led to the discovery of Neptune. Similarly, deviations in the orbit of Mercury led to the postulated planet Vulcan inside Mercury's orbit. However, it turned out that Vulcan didn't exist at all, and that the answer to the question of Mercury's orbit was a modification of our theory of gravity. MediaAudio Recording: http://www.mica-vw.org/wiki/images/6/69/R_Knop_18_April_2009.mp3 Photo: http://www.flickr.com/photos/64176199@N00/3453309254/ |