Medicean stars circling Jupiter

Filed under: agreement of unlike witnesses. That is where this belongs, and the reason deserves writing down before the candle fails.

The Medicean stars gave the argument in one register: four bodies circling Jupiter, seen through the tube at Padua in January of 1610, moving night by night as the Ptolemaic mechanism says no planet’s attendants may move. That was astronomy alone, one field testifying to itself. What makes the case unanswerable came later, from a witness with no interest in Jupiter at all. The tides at Venice rise and fall twice daily, and the period of that rising matches the period of the Moon’s return to the meridian - measured by mariners who never looked through a telescope, logged by port officials who never read Copernicus. Two instruments, two traditions, no shared author, and the same number falls out of both. The water clock and the pendulum, built for entirely different purposes, keep the same beat.

This is what the catalogue should record: a single observation proves little, however beautiful. A pair of observations from disjoint sources, each independently measured, agreeing to within the tolerance of the cruder one - this is the strongest thing knowledge possesses. The phases of Venus alone could be explained away by ingenious geometry. The phases of Venus together with the Jovian satellites together with the tides cannot be explained away by any configuration of the old spheres, because the old spheres were never built to accommodate witnesses who do not talk to each other. Agreement between strangers is the signature of truth; agreement between colleagues is only the signature of a shared assumption.

The entry also carries a caution, and it should be filed in the same place. When two sources agree too conveniently, check whether one has been quietly copied from the other. The tide tables and the ephemeris share a clerk somewhere in their ancestry, the agreement is worth nothing. Independent means independently derived, not independently printed.

Tomorrow the work resumes at the bench. First thing to check: the float gauge at the Arsenal, whether its overnight trace shows the second high water arriving later than the first by the interval the lunar tables predict - because if the delay matches, the two witnesses have spoken again, and the file grows heavier.