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Session 1 - HAD I: LeRoy E. Doggett Memorial Session.
Oral session, Tuesday, January 06
Georgetown,
Historians of astronomy have generally assumed that the Ptolemaic and Copernican systems give equivalent predictions of planetary positions, but Tycho Brahe knew that in the Ptolemaic arrangement Mars' distance was always greater than the sun's, whereas in the Copernican system Mars at opposition approached to half the sun's distance. Because Tycho accepted the traditional solar distance scale, 20 times too small, he expected to measure a Martian diurnal parallax of 4.5' at opposition if the Copernican system was true. (In reality the horizontal parallax was too small to measure by naked-eye observations.) Hence, during the golden decade of the 1580s at Hven, Tycho undertook a major campaign to find Mars' parallax. Observations at the opposition of 1582-83 failed, according to a letter he wrote in 1584. The campaign at the next opposition led to frustration, but after the 1587 opposition he claimed that in fact he had already found the parallax in 1582. Was Tycho merely prevaricating because he wanted to have an observational basis for his new Tychonic cosmology? During this decade Tycho gradually became aware of the role of refraction, and much of the new instrumentation built at Stjerneborg seems to have been motivated by this problem. Using an erroneously chosen refraction table Tycho apparently convinced himself of a large parallax for Mars. He may well have discovered his error by 1592, for he never again claimed to have found the large parallax. Because of the failure of this major goal, Tycho's reputation as a very smart and program-motivated observer has suffered, but because of this particular observational campaign, there were ultimately enough astonishingly accurate Mars observations for Kepler's later studies to succeed in finding the law of areas and the elliptical form of planetary orbits.
