Rashid Akmaev and Jim Clarage raise interesting questions about the work of Giovanni Battista Riccioli and Francesco Maria Grimaldi concerning the Coriolis effect (Physics Today, January 2012, page 8 and page 8), in response to my letter in August 2011, page 8). Akmaev notes that the effect is direction independent, so Riccioli and Grimaldi, who foresaw it only for projectiles directed north or south, could not possibly have compared the deflection of cannonballs fired in different directions and thereby determined whether Earth rotates. Riccioli and Grimaldi had argued, for example, that the best artillerymen, who supposedly could place a shot into the mouth of an enemy cannon, would have noticed the effect if Earth rotated.

However, Riccioli and Grimaldi’s work predated the Newtonian mechanics and advanced mathematics needed to fully understand the Coriolis effect. Absent those tools, it is difficult even to visualize the Coriolis deflection of a projectile launched east or west, let alone determine it to be identical with the deflection of a projectile launched north or south. Even today, elementary discussions of the Coriolis effect typically consider, as did Riccioli and Grimaldi’s, only projectiles launched north or south.1 The problem Akmaev notes would not likely have been recognized during their time.

Moreover, Riccioli envisioned another “Coriolis” test of Earth’s rotation: He argued in his 1651 Almagestum novum that falling bodies, with which he experimented extensively (see Physics Today, September 2012, page 36), should deflect eastward if Earth rotates. Decades later Isaac Newton, in proposing the same test to Robert Hooke, wrote that “the advance of the body from the perpendicular eastward will be very small and yet I am apt to think it may be enough to determine the matter.” Hooke attempted it unsuccessfully. That delicate but apparently straightforward experiment is bedeviled by all sorts of effects that may spring to the mind of the modern physicist but were then unknown; it wouldn’t be successfully performed until the 19th century.2 

Clarage comments that Riccioli’s work bears on significant historical matters and that his vocation as a Jesuit priest should be noted. Indeed, Riccioli was a prominent defender of Tycho Brahe’s brand of geocentrism; he opposed the heliocentric theory with scientifically robust challenges—including an astronomical anti-Copernican argument he developed with Grimaldi3—that would not be fully answered for almost two centuries. His work suggests that opposition to the ideas of Copernicus was more scientific in nature than has generally been acknowledged. Certainly, more study of the scientific work of Riccioli, Tycho, and other anti-Copernicans is called for, and it will probably yield more surprises. As Manuel López‐Mariscal points out above, no aspect of the history of physics ever seems to be as simple as the versions presented in the physics textbooks.

1.
See, for example,
E.
Gregersen
, ed.,
The Britannica Guide to Heat, Force, and Motion
,
Britannica Educational in assoc. with Rosen Educational Services
,
New York
(
2011
), p.
121
.
2.
See
C. M.
Graney
,
Phys. Perspect.
13
,
387
(
2011
).
3.
C. M.
Graney
,
J. Hist. Astron.
41
,
453
(
2010
).