The Growth of Astrophysical Understanding

© 2003 American Institute of Physics, S-0031-9228-0311-020-4 P 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? Around 250 BC, the Greek natural philosopher Aristarchus of Samos 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, that Earth revolves around the Sun.1 Aristarchus’s theory was largely discredited, especially by Claudius Ptolemaeus of Alexandria. 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 wasn’t 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.