FIGURE 3.3 Ptolemaic system. The basic ordering of the planets around Earth is a feature of the Ptolemaic system. SOURCE: http://abyss.uoregon.edu/~js/glossary/ptolemy.html. Reproduced by permission of the Whipple Library, University of Cambridge.

3.5.5 Next Steps: Distances Beyond the Solar System

The next step in building a picture of the universe depended on creating a distance scale that could accommodate distances beyond the solar system. The key method for doing so involved the concept of astronomical parallax (Figure 3.6). From the perspective of spatial thinking, the measurement of astronomical parallax plays an interesting role in resolving the debate between the geocentric and heliocentric models of the solar system.

The essential challenge of astronomical observation in the middle of the second millennium was the accurate measurement of the position of the planets and the stars. The master of this challenge was Tycho Brahe. At his observatory, Uraniborg, he assembled the most accurate instruments of the day for his observations, observations that made Kepler’s work possible. Tycho, who arguably had the best understanding of the relative positions of the astronomical objects of the time, opposed the Copernican view because he was unable to observe stellar parallax. His observations of stellar positions were by far the most accurate of the time and, therefore, he would have been the



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