To the Editors:

In his interesting article “The Missions of Astronomy” [ NYR, October 22], Steven Weinberg attributes something to Copernicus to which he did not really subscribe and therefore overlooks one of Johannes Kepler’s greatest insights. Weinberg writes, “Even Copernicus, because he was committed to orbits composed of circles, retained the idea of epicycles.” If by “orbits” what is meant is some kind of curve followed by, say, a planet such as a circle, then Copernicus did not believe in orbits at all. To him the planets were attached to unseen spheres that moved them.

It was Kepler who introduced orbits in the sense we usually mean them. It wasn’t that he replaced circular orbits by elliptical ones; it was that he replaced celestial spheres by curves. It was this sort of thing that caused John Donne, who met Kepler once, to write:

And new philosophy calls all in doubt,
The element of fire is quite put out,
The sun is lost, and th’earth, and no man’s wit
Can well direct him where to look for it.

Jeremy Bernstein
Aspen, Colorado

Steven Weinberg replies:

Jeremy Bernstein is correct that Copernicus accepted what in his time was a common view, that the planets are attached to unseen revolving spheres. In using the words “orbits” and “circles,” I was not intending to comment on this, one way or the other. Indeed, Copernicus himself referred to the motion of the heavenly bodies as circular (as for example in the heading of section 4 of Book One of De Revolutionibus ). I don’t think that this was an important point for Copernicus, though it had been for a few earlier astronomers, such as the Greeks Theon and Adrastus and the Arab al-Haitam.

As to Kepler, I am all for giving him plenty of credit, but let’s not slight Tycho Brahe, who earlier showed that the comet of 1577 would have penetrated any spheres to which the planets were attached.

This Issue

November 19, 2009