Editing Gersonides (section)

===Estimation of stellar distances and refutation of Ptolemy's model===
Gersonides appears to be the only astronomer before modern times to have surmised that the fixed stars are much further away than the planets. Whereas all other astronomers put the fixed stars on a rotating sphere just beyond the outer planets, Gersonides estimated the distance to the fixed stars to be no less than 159,651,513,380,944 earth radii,<ref>Albert Van Helden, ''Measuring the Universe: Cosmic Dimensions from Aristarchus to Halley'' (Chicago: The University of Chicago Press, 1985), p.&nbsp;40.</ref> or about 100,000 lightyears in modern units.

Using data he collected from his own observations, Gersonides refuted Ptolemy's model in what the notable physicist [[Yuval Ne'eman]] has considered as "one of the most important insights in the history of science, generally missed in telling the story of the transition from epicyclic corrections to the geocentric model to [[Copernican heliocentrism|Copernicus' heliocentric model]]". Ne'eman argued that after Gersonides reviewed Ptolemy's model with its epicycles he realized that it could be checked, by measuring the changes in the apparent brightnesses of Mars and looking for cyclical changes along the conjectured epicycles. These thus ceased being dogma, they were a theory that had to be experimentally verified, "à la Popper". Gersonides developed tools for these measurements, essentially pinholes and the [[camera obscura]].

The results of his observations did not fit Ptolemy's model at all. Concluding that the model was inadequate, Gersonides tried (unsuccessfully) to improve on it. That challenge was finally answered, of course, by [[Nicolaus Copernicus|Copernicus]] and [[Johannes Kepler|Kepler]] three centuries later, but Gersonides was the first to falsify the Alexandrian dogma - the first known instance of modern falsification philosophy. Gersonides also showed that Ptolemy's model for the lunar orbit, though reproducing correctly the evolution of the Moon's position, fails completely in predicting the apparent sizes of the Moon in its motion. Unfortunately, there is no evidence that the findings influenced later generations of astronomers, even though Gersonides' writings were translated and available.<ref>[[Yuval Ne'eman]]: Astronomy in Sefarad [http://wise-obs.tau.ac.il/judaism/sefarad.html]</ref>