Star Points for August, 2001; by Curtis Roelle Titius-Bode: For Planets It's the Law Last month there was a star party at the backyard observatory. About 30-40 amateur astronomers climbed the hill on a clear cool Summer night in July, some with telescopes of their own, and other with binoculars or simply a blanket. The hand full of observers who remained until the morning light succeeded in viewing all nine major planets, a project that concluded with the rising of Mercury in bright morning twilight just prior to sunrise. Besides the nine major planets star party participants were treated to some other telescopic objects located within our solar system. These included the newly discovered Comet Linear and the "minor planet" Ceres. Minor planet is just another word for "asteroid." The six brightest major planets nearest to the Sun have been known since antiquity. On the other hand Uranus, Neptune, and Pluto -- the three faintest and farthest away -- were discovered by astronomers in 1781, 1846, and 1930, respectively. The asteroids in turn were totally unknown until the first and brightest, Ceres, was discovered 200 years ago in 1801. Ceres' orbits around the Sun between the orbits of Mars and Jupiter in what we now refer to as the main asteroid belt. (Recent evidence indicates that other belts might lie beyond Pluto, but that's another story.) Ironically, the existance of a body at Ceres' distance from the Sun had been predicted in a theory first proposed by Johann D. Titius in 1766 and later publicized by Johann E. Bode in 1772. The Titius-Bode Law attempted to predict the distance of each planet from the Sun. First a bit of background on measuring distance in the solar system. The distance from the Sun to the Earth is a standard unit of measure known as an "astronomical unit" or A.U. Thus the Sun-Earth distance is 1 A.U. or approximately 93 million miles. According to Titius-Bode there should be a planet 0.4 A.U. from the Sun. Coincidentally, Mercury's distance is 0.39 A.U. Likewise, the law predicts the next planet out is 0.7 A.U. from the Sun. Venus is 0.72 A.U. At the far end Saturn's predicted distance of 10.0 A.U. compared favorably with its actual distance of 9.54. Titius-Bode also predicted a planet out at 19.6 A.U. from the Sun where none was known to be. However, the discovery of Uranus at 19.18 A.U. renewed great interest in the law. Titius-Bode law was an empiracle law, based on observation but with no theory to support it. Why should the planet Jupiter be 5.2 A.U. from the Sun? There was no logical reason for it, but according to the law a planet should be there and indeed there was a huge planet sitting there at the expected position. Critics of the law were fond of pointing one major glaring fatal flaw. According to Titius-Bode there ought to be a planet between Mars and Jupiter 2.8 A.U. from the Sun. If the law had legs then a known planet should have been found at that location. And thus a search for the missing planet ensued. The search finally concluded on the night of January 1, 1801 when the Italian astronomer Piazzia Guiseppe Piazzi discovered a slow moving object that he first interpreted as a new comet. Instead he had discovered the 600 mile wide body Ceres, 2.77 A.U. from the Sun in close agreement with the distance predicted by Titius-Bode. During the next two centuries over 10,000 asteroids have been discovered. Yet, even though Ceres is just a fraction of the size of our own Moon, no larger asteroids have ever been found in the belt. Although visible in binoculars, Ceres simply looks like an unimpressive yellow star. If you feel like giving it a try a finder chart may be found in the July issue of Sky & Telescope magazine, available at the Carroll County Library. Detailed finder charts and information for locating the outer planets (Uranus, Neptune, and Pluto) may be found in the April issue, or on the magazine's web site at http://www.skypub.com/sights/moonplanets/outerplanets01.html.