The trouble with common sense

Hanging in my office is a framed photograph of an armillary sphere, which resides in the Whipple Museum of the History of Science in Cambridge, England. An armillary sphere is a celestial globe. It is made up of a spherical model of the planet Earth (the sort we all played with as children), but the model is surrounded by an intricate skeleton of graduated rings, representing the most important celestial circles. Armillary spheres were devised in ancient Greece and developed as instruments for teaching and astronomical calculation. During the same period, heavenly bodies were widely conceived as fixed to the surfaces of concentrically arranged crystalline spheres, which rotate around the Earth at their centre.

This particular armillary sphere has, I expect, many fascinating historical stories to tell, but there is a specific reason I framed the picture. Once upon a time, astronomers speculated about the causes and mechanisms of the motions of the planets and stars, and their ontology of crystalline spheres was a central feature of astronomical theory for hundreds of years. But crystalline spheres are not the sorts of things one can observe, at least not with the naked eye from the surface of the Earth. Even if it had turned out that they exist, it is doubtful one would have been able to devise an instrument to detect them before the days of satellites and space shuttles.