Adding Epicycles

Tuesday, February 19th, 2008

Under the old Ptolemaic system of astronomy, the earth is at the center of the universe, and the sun and planets revolve around it in circular orbits — but the planets obviously don’t revolve around the earth in perfect circular orbits, because the occasionally seem to move in the wrong direction. This retrograde motion was explained via epicycles — the planets supposedly moved along a small circle that itself moved in a circular orbit around the earth.

As early astronomers made more and more observations, they needed more and more epicycles on epicycles to explain the planets’ paths:

In part due to sometimes fantastic attempts to make the failed earth-centered model work, “adding epicycles” has come to be used as a derogatory comment in modern scientific discussion. If one continues to try to adjust a theory to make its predictions match the facts, when it has become clear that the basic premise itself should be questioned, one is said to be “adding epicycles”.

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