This is rather tangential, but it’s something about geocentrism that has been bothering me recently. Aristarchus (c 250 BC) and Hipparchus (c 150 BC) computed that the sun has 10x the diameter of the earth and thus the earth should circle the sun. Their contemporaries said: no, heliocentrism implies that the fixed stars are very far away. That’s an OK argument against heliocentrism, but did they really engage with the intermediate step? [see update] Ptolemy agreed that the sun was 10 million kilometers away, but did he discuss its size?
And what did later astronomers do? When Kepler and his contemporaries got it right, did they think this a big deal? Did they think it relevant to heliocentrism? This larger distance makes the distance to the fixed stars proportionately larger, but I think the plausible size of the universe should increase superlinearly in observed distances, so discovering that the sun is farther away than you thought should erode the parallax argument against heliocentrism.
ETA: improved measurement of the tides should allow one correctly do Hipparchus’s calculation, and multiply the AU by 10. For fixed parallax, the (lower bound) distance to the fixed stars is measured in AU and thus increases proportionately, which is what I meant above. But later astronomers measured stars more accurately than earlier, providing evidence against parallax, and thus against heliocentrism. In fact, no one followed Hipparchus, but instead used astronomical measurements to improve on Aristarchus and Ptolemy. So I don’t have much to ask heliocentric Kepler about the discovery that the sun was even bigger (though he had to bite a bigger bullet than Hipparchus of distance to fixed stars). Still, did his predecessors really grapple with the size of the sun?
Added, 2013: the writings of Aristarchus and Hipparchus are lost, so we’re not really sure what they did, but they did both conclude that the sun is much bigger than the earth. Aristarchus was heliocentric, but Hipparchus was geocentric. Added, 2014: actually, even the claim that Hipparchus was geocentric is dubious. The claim that their contemporaries were geocentric, let alone that they made a parallax argument, is a complete fabrication for symmetry with the Renaissance arguments. My question about Ptolemy and later remains.
This is rather tangential, but it’s something about geocentrism that has been bothering me recently. Aristarchus (c 250 BC) and Hipparchus (c 150 BC) computed that the sun has 10x the diameter of the earth and thus the earth should circle the sun. Their contemporaries said: no, heliocentrism implies that the fixed stars are very far away. That’s an OK argument against heliocentrism, but did they really engage with the intermediate step? [see update] Ptolemy agreed that the sun was 10 million kilometers away, but did he discuss its size?
And what did later astronomers do? When Kepler and his contemporaries got it right, did they think this a big deal? Did they think it relevant to heliocentrism? This larger distance makes the distance to the fixed stars proportionately larger, but I think the plausible size of the universe should increase superlinearly in observed distances, so discovering that the sun is farther away than you thought should erode the parallax argument against heliocentrism.
ETA: improved measurement of the tides should allow one correctly do Hipparchus’s calculation, and multiply the AU by 10. For fixed parallax, the (lower bound) distance to the fixed stars is measured in AU and thus increases proportionately, which is what I meant above. But later astronomers measured stars more accurately than earlier, providing evidence against parallax, and thus against heliocentrism. In fact, no one followed Hipparchus, but instead used astronomical measurements to improve on Aristarchus and Ptolemy. So I don’t have much to ask heliocentric Kepler about the discovery that the sun was even bigger (though he had to bite a bigger bullet than Hipparchus of distance to fixed stars). Still, did his predecessors really grapple with the size of the sun?
Added, 2013: the writings of Aristarchus and Hipparchus are lost, so we’re not really sure what they did, but they did both conclude that the sun is much bigger than the earth. Aristarchus was heliocentric, but Hipparchus was geocentric. Added, 2014: actually, even the claim that Hipparchus was geocentric is dubious. The claim that their contemporaries were geocentric, let alone that they made a parallax argument, is a complete fabrication for symmetry with the Renaissance arguments. My question about Ptolemy and later remains.