Tuesday, October 24/4:15 PM

Special Expository Lecture

If Copernicus Had a Computer

If you watch Mars against the backdrop of the fixed stars, then night after night you'll see rather steady progress across the zodiac. But every so often, the planet appears to "back-up" before continuing on its forward trek. This periodic, retrograde motion wreaks havoc with a model of the solar system that places each planet on a steadily rotating circle with Earth at the center. Ptolemy did a pretty good job patching up the model by placing each planet on a small rotating circle whose center is on the rim of a larger rotating circle. The path traced out is called an epicycle and it offers some explanation for Mars' orbital wanderings.

The epicycle model lasted for centuries until Copernicus set the record straight by suggesting that the Earth revolved around the sun along with the other planets. But would he have been so bold a scientist if he had access to 1995 computers? Or would he have just mouseclicked his way into fame, developing a simulation package that supported further tinkering with the Ptolemaic model? After all, it is not hard to imagine a computational environment where arbitrarily deep nestings of epicycles could be tried out, producing ever better fits to the data. Where would it have all led?

Musing on these questions gives us the chance to think about the interplay between mathematical model building and the computer, and what computational science is all about.

Charles Van Loan
Department of Computer Science
Cornell University

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