Theme 4: from the Greeks to the Renaissance: the Earth in Space
Theme 4: From the Greeks to the Renaissance: the Earth in Space 4.1 Greek Astronomy Unlike the Babylonian astronomers, who developed algorithms to fit the astronomical data they recorded but made no attempt to construct a real model of the solar system, the Greeks were inveterate model builders. Some of their models—for example, the Pythagorean idea that the Earth orbits a celestial fire, which is not, as might be expected, the Sun, but instead is some metaphysical body concealed from us by a dark “counter-Earth” which always lies between us and the fire—were neither clearly motivated nor obviously testable. However, others were more recognisably “scientific” in the modern sense: they were motivated by the desire to describe observed phenomena, and were discarded or modified when they failed to provide good descriptions. In this sense, Greek astronomy marks the birth of astronomy as a true scientific discipline. The challenges to any potential model of the movement of the Sun, Moon and planets are as follows: • Neither the Sun nor the Moon moves across the night sky with uniform angular velocity. The Babylonians recognised this, and allowed for the variation in their mathematical des- criptions of these quantities. The Greeks wanted a physical picture which would account for the variation. • The seasons are not of uniform length. The Greeks defined the seasons in the standard astronomical sense, delimited by equinoxes and solstices, and realised quite early (Euctemon, around 430 BC) that these were not all the same length. This is, of course, related to the non-uniform motion of the Sun mentioned above.
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