ScienceWeek

HISTORY OF PHYSICS: ON ASTROPHYSICAL UNDERSTANDING

The following points are made by Martin Harwit (Physics Today 2003 November):

1) Perhaps the most remarkable aspect of the growth in our understanding of the Universe is that we understand anything at all. Beyond the obvious regularities of the seasons, the Assyrians noted, as early as 700 BC, that the planets appeared to move in a complex semiregular pattern and that solar eclipses were possible only at the new moon, whereas lunar eclipses occurred only at the full moon. But what did all that tell the ancients about the structure of the Universe?

2) Around 250 BC, the Greek natural philosopher Aristarchus of Samos (c.310-c.230 BC) worked out the distance of the Moon and its size. He proposed a method for determining the Sun's distance, but he was able to conclude only that the Sun was much farther away than the Moon and much larger than Earth. That led him to postulate, 18 centuries before Nicolaus Copernicus (1473-1543), that Earth revolves around the Sun.(1)

3) Aristarchus's theory was largely discredited, especially by Claudius Ptolemaeus of Alexandria (c.2nd century AD). Ptolemy's /Almagest/, which appeared in about 150 AD, dominated Western astronomical thought for a millennium and a half. Ptolemy argued that Earth could not be rotating. Rotation, he thought, would throw anything not firmly attached off the surface, and "animals and other weights would be left hanging in the air". Moreover, Earth's rotation would be so fast that "never would a cloud be seen to move toward the east."(2) That sounds quaint today, but it was not illogical. Ptolemy was a great scientist. The first lesson in astrophysics, however, is that every cosmic phenomenon is governed by competing effects -- in this case, gravity, centrifugal forces, and friction. Unless we know the order of magnitude of each, we are likely to draw wrong conclusions.

4) When Copernicus revived the notion of a heliocentric system in 1543, he could offer no observational confirmation. The ground for a final resolution had to be prepared by Tycho Brahe (1546-1601), the greatest of the pre-telescope observers. Tycho constructed astronomical instruments more precise than any previously known. Over a 20-year period, he assembled the most accurate, systematic data that had ever been compiled on the positions of the planets. The young Johannes Kepler (1571-1630), a theorist if ever there was one, dogged Tycho, intent on getting his hands on the data, which the great observer was jealously guarding so he could deduce the orbits of the planets himself. When Tycho was banished in 1597 from his island observatory in Denmark and sought political refuge in Prague, Kepler followed him. But it was not until after Tycho's death that Kepler inherited and began analyzing the data.(3-5)

References (abridged):

1. T. Heath, Aristarchus of Samos, The Ancient Copernicus, Clarendon Press, Oxford (1913), reprinted by Dover, New York (1981)

2. Ptolemy [Claudius Ptolemaeus], The Almagest, R. Taliaferro, trans., vol. 14 in the series Great Books of the Western World, Encyclopedia Britannica, Chicago (1955)

3. Kepler's interaction with Tycho and Galileo is well described in A. Koestler's The Watershed: A Biography of Johannes Kepler, Anchor Books, Garden City, N.Y. (1960)

4. C. L. Bennett et al. (WMAP collaboration), Astrophys. J. suppl. 148, 1 (2003)

5. M. Harwit, Cosmic Discovery: The Search, Scope, and Heritage of Astronomy, MIT Press, Cambridge, Mass. (1984)

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