Among the physical arguments of the Almagestum Novum against the diurnal rotation of the Earth is one that modern physicists read with a start of recognition. If the Earth turned, Riccioli argued, then a cannonball fired toward the north would be carried by the greater eastward speed of the latitude it left — and should visibly deflect from its mark; since gunners observe no such deflection, the Earth stands still.
The deflection is real. It is the effect named, two centuries later, for Gaspard-Gustave de Coriolis — too small for seventeenth-century gunnery to detect, but exactly the effect that governs artillery tables, trade winds, and cyclones today. Historians of science have singled out Riccioli’s discussion as among the earliest clear statements of it.
Read it in the first complete English translation of Book IX:
- Chapter XXI — fourteen arguments from the motion of elemental bodies, including the arguments from cannon shot toward the north and toward the east;
- Chapter XIX and Chapter XX — the companion arguments from falling bodies and their increase of velocity, where Riccioli reports his own tower experiments at Bologna.
The whole of Book IX is here, free.