4.9 Tycho brahe  (Page 2/6)

Tycho Brahe's contributions to astronomy were enormous. He notonly designed and built instruments, he also calibrated them and checked their accuracy periodically. He thus revolutionizedastronomical instrumentation. He also changed observational practice profoundly. Whereas earlier astronomers had been contentto observe the positions of planets and the Moon at certain important points of their orbits (e.g., opposition    , quadrature    , station), Tycho and his cast of assistants observed these bodiesthroughout their orbits. As a result, a number of orbital anomalies never before noticed were made explicit byTycho. Without these complete series of observations of unprecedented accuracy, Kepler could not have discovered thatplanets move in elliptical orbits. Tycho was also the first astronomer to make corrections for atmospheric refraction    . In general, whereas previous astronomers made observations accurateto perhaps 15 arc minutes, those of Tycho were accurate to perhaps 2 arc minutes, and it has been shown that his bestobservations were accurate to about half an arc minute.

Tycho's observations of the new star of 1572 and comet of 1577, and his publications on these phenomena, were instrumental inestablishing the fact that these bodies were above the Moon and that therefore the heavens were not immutable as Aristotle hadargued and philosophers still believed. The heavens were changeable and therefore the Aristotelian division between theheavenly and earthly regions came under attack (see, for instance, Galileo's Dialogue) and was eventuallydropped. Further, if comets were in the heavens, they moved through the heavens. Up to now it had been believed that planetswere carried on material spheres (spherical shells) that fit tightly around each other. Tycho's observations showed that thisarrangement was impossible because comets moved through these spheres. Celestial spheres faded out of existence between 1575and 1625.

If Tycho destroyed the dichotomy between the corrupt and ever changing sublunary world and the perfect and immutable heavens,then the new universe was clearly more hospitable for the heliocentric planetary arrangement proposed by NicholasCopernicus in 1543. Was Tycho therefore a follower of Copernicus? He was not. Tycho gave various reasons for notaccepting the heliocentric theory, but it appears that he could not abandon Aristotelian physics which is predicated on anabsolute notion of place. Heavy bodies fall to their natural place, the Earth, which is the center of the universe. If theEarth were not the center of the universe, physics, as it was then known, was utterly undermined. On the other hand, the Copernican system had a number of advantages, some technical (such as a better lunar theory and smaller epicycles), andothers more based on harmony (an obvious explanation of retrograde planetary motion    , a strict demonstration of the order and heliocentric distances of the planets). Tycho developed asystem that combined the best of both worlds. He kept the Earth in the center of the universe, so that he could retainAristotelian physics (the only physics available). The Moon and Sun revolved about the Earth, and the shell of the fixed starswas centered on the Earth. But Mercury, Venus, Mars, Jupiter, and Saturn revolved about the Sun. He put the (circular) path ofthe comet of 1577 between Venus and Mars. This Tychonic world system became popular early in the seventeenth century amongthose who felt forced to reject the Ptolemaic arrangement of the planets (in which the Earth was the center of all motions) butwho, for various reasons, could not accept the Copernican alternative.