Giant’s Shoulders #15 is up at Entertaining Research and it includes a post by Ethan Siegel at Starts with a Bang titled Learning from a Huge Past Mistake. The past mistake Ethan is referring to is the diurnal rotation of the earth or rather the fact that according to him nobody considered it. Having nicely sketched the relativity of the observed motion i.e. the rotation of the stars could equally well be actual stellar rotation or diurnal rotation he then goes historically off the deep en; I quote,
So, why was “eppur si muove” such a big deal? Didn’t we know about this second possibility? Didn’t people at least consider it?
The answer, shockingly, is no, not really. Why didn’t we consider it? Because looking up and noticing that the heavens rotate predated our idea that the Earth could be round. If the Earth is flat, then it wouldn’t make sense for it to rotate, would it?
The huge mistake is that once we discovered the Earth was round (in about 300 B.C., by the way), we didn’t reconsider this problem. We didn’t go back and say, “Hey, you know what? Instead of this celestial sphere idea, maybe it’s just us that’s spinning!” And so we held onto the idea of a fixed Earth for nearly 2,000 more years.
This is so wrong it’s difficult to know where to start. Just for the record the discovery that the world is spherical was somewhere between 500 and 600 BCE and not 300BCE. Several of Ethan’s commentators naturally draw his attention to Aristarchus of Samos who proposed a heliocentric system with diurnal rotation 1800 hundred years before Copernicus. Commentator Glen Davidson also wrote,
Aristarchus, who was mentioned above, was used by Copernicus to give credence to his ideas, since authorities were heavily relied upon by academics at that time.
This is not true he had apparently intended to do so and included a passage with this intention in his manuscript but then removed it from the printed version published in Nürnberg by Petreius in 1543.* Why he removed this passage is not known and has of course been the subject of much lively debate among the Copernicus experts. He did however use the cosmology of the Pythagoreans as a source of authority for a moving earth. The Pythagoreans who were the first to consistently argue for a spherical earth had already in the 6th century BCE, i.e. earlier than Aristarchus, claimed that the earth and the counter earth revolve around the sun. Copernicus’ use of this argument led to the early heliocentric supporters being called Pythagoreans and heliocentricity the Pythagorean hypothesis in the 16th and 17th centuries.
But what of the supporters of geocentricity in ancient Greece did they comment on or discuss the possible movement of the earth? Unfortunately we have lost almost all of the astronomical writings from before Ptolemaeus in the 2nd century CE but we do have his Syntaxis Mathematiké (The Almagest) and here we can find in Book I of his thirteen books a detailed discussion that occupies two pages of Toomer’s translation** and rejects mostly on philosophical grounds all possible forms of movement for the earth.
The same discussion with the same arguments is to be found throughout Islamic astronomy, which is based on Ptolemaic astronomy, from the 8th to the 18th centuries.
In the High Middle Ages in Europe with the return of astronomical studies we find the same arguments in shortened form in the Sphere of Sacrobosco the standard introductory book on astronomy for university students. In the period immediately preceding Copernicus Several scholars, most notably Nicolas of Cusa, openly discussed the possibility of diurnal rotation.
Ptolemaeus’ argument against diurnal rotation was that the speed necessary to rotate the earth in 24 hours, and he had a fairly good idea of its size, would be so great that the earth would violently fly apart. Interestingly Copernicus and Galileo turned this argument around and argued that the speed necessary to rotate the sphere of the fixed stars in twenty-four hours would be much greater as this sphere was immensely greater than the sphere of the earth and therefore even more violent. This argument had some success as diurnal rotation was accepted by many astronomers much earlier than annual rotation. Both Gilbert’s Ptolemaic system with diurnal rotation and Ursus’ Tychonic geo-heliocentric system with diurnal rotation were very popular in the first half of the 17th century.
Far from being ignored the possibility of diurnal rotation was a standard part of the astronomical discussion from at least 500 BCE up till its actually acceptance in the 17th century. In the end diurnal rotation could only become scientifically acceptable through the development of the concepts of force and in particular of gravitational force both of which evolved over the 17th century in the work of many astronomers and physicists between Kepler and Newton.
*See On the Revolutions, translated and commentated by Edward Rosen, Paperback ed., The Johns Hopkins University Press, Baltimore and London, 1992, p. 25
**Ptolemy’s Almagest, translated and annotated by G.J. Toomer, Paperback ed., Princeton University Press, Princeton, 1998, pp. 43 – 45