In the autumn of 1676, a young Danish astronomer in Paris discovered, while trying to solve one of the most pressing practical problems of his age, that light itself takes time to cross the void — that the universe, in other words, has a speed limit written into its very fabric. Ole Rømer was refining celestial tables for maritime navigation when he noticed that Jupiter's moon Io ran consistently late or early depending on whether Earth was drawing away from or toward Jupiter, a discrepancy that pointed to only one explanation. His willingness to commit publicly to a claim his own colleagues d