On a January night in 1610 a professor of mathematics in Padua pointed a homemade glass at Jupiter and saw three small stars beside it. Other men had looked. He wrote down where they were and came back the next night to see whether they had moved. They had, and a fourth appeared a week later.

Within a year he had a court title, a philosopher's salary and no students to teach. Within six he had been told to stop. Within twenty-three he was on his knees before the Inquisition, and the charge was not the moons and not the sun. It was a book.

Rome had let him write it on conditions. One was that the pope's own argument had to be in it: God could produce the world we see by means no man could guess, so nothing we observe can ever prove how it is really put together. Galileo obeyed. He put it on the last page, in the mouth of Simplicio, the plodding Aristotelian who has lost every exchange for four hundred pages. Nobody in Rome had to be told whose argument it was.

And under all of it the question he sharpened for everyone who came after and then answered badly for himself. What counts as proof? The glass gave him proofs, of moons and of phases. The proof he staked his life on was something else.

A short narrative history, read in an evening. Every quotation is sourced, and the notes say where each one comes from. A companion novel of the trial, Galileo's Inquisitor, publishes the same day.