From the letter by Christopher Graney (PHYSICS TODAY, August 2011, page 8) I learned that 16th-century Italian scientists Giovanni Riccioli and Francesco Grimaldi were apparently aware of the Coriolis effect nearly two centuries before Gaspard-Gustave Coriolis. However, the description of the effect and the diagram reproduced from their book are only half correct.

For an object moving horizontally with a given speed, conservation of angular momentum, seen from a rotating planet, results in the same amount of deflection regardless of the object’s direction. Contrary to the assertion by Riccioli and Grimaldi, a cannonball fired eastward in the Northern Hemisphere will be deflected to the right—southward—by the same distance as its cousin fired northward is deflected eastward. (In the Southern Hemisphere, both will be deflected to the left.) Therefore, it would have been impossible for Riccioli and Grimaldi to determine whether Earth rotates by comparing the deflection of cannonballs fired in different directions.