The Coriolis deflection of moving objects seen from within a rotating frame of reference—important in physics, meteorology, and oceanography—was described by Italian scientists Giovanni Battista Riccioli (1598–1671) and his assistant Francesco Maria Grimaldi (1618–63) nearly two centuries before Gaspard-Gustave Coriolis (1792–1843).
Among other things, historians attribute to Riccioli our system of lunar nomenclature and the first accurate measurement of gravitational acceleration at Earth’s surface, and to Grimaldi the discovery and naming of the phenomenon of diffraction of light. Riccioli and Grimaldi give a detailed description in Riccioli’s 1651 Almagestum Novum of how Earth’s rotation should cause a rightward deflection in a projectile fired toward the north. They write,
If a ball is fired along a Meridian toward the pole (rather than toward the East or West), diurnal motion will cause the ball to be carried off [that is, the trajectory of the ball will be deflected], all things being equal: for on parallels of latitude nearer the poles, the ground moves more slowly, whereas on parallels nearer the equator, the ground moves more rapidly.1
Grimaldi and Riccioli provide a diagram (see figure, next page) of a cannon aimed northward and eastward. They write that if the cannon is fired eastward at a target at B, then as the ball is in flight, Earth’s diurnal rotation carries the mouth of the cannon from A to C and carries the target from B to D, so the ball travels from A to D. If the cannon is aimed northward and fired at a target at E, then as the ball is in flight, the target moves from E to N. However, the ball travels along the curve AKF, not the straight line AHF, because the diurnal motion is faster at the beginning of the ball’s flight (page 426).1
Illustration from Giovanni Battista Riccioli's 1651 Almagestum Novum. Because of Earth's rotation, a projectile fired north to hit a garget at E/N follows curved path AKF, and strikes east of N at G. This effect is today referred to as the Coriolis effect, after the early 19th-century physicist.
Illustration from Giovanni Battista Riccioli's 1651 Almagestum Novum. Because of Earth's rotation, a projectile fired north to hit a garget at E/N follows curved path AKF, and strikes east of N at G. This effect is today referred to as the Coriolis effect, after the early 19th-century physicist.
The authors write that the ball will not strike the target at N squarely but will graze it or miss it. However, if another target were positioned east of N, such as at G, the ball would squarely strike it, even though the cannon is not aimed at it (page 427).1
Riccioli and Grimaldi do not calculate the size of the effect, but they suppose that since a skilled artilleryman can place a shot right into the mouth of an enemy’s cannon, the difference in shots from east to west versus those north to south should have been detected. That it had not been they interpreted as evidence that Earth is immobile (page 427).1 Riccioli and Grimaldi were geocentrists who supported the Tychonic theory—in which the Sun, Moon, and stars circled Earth while the planets circled the Sun—rather than the Copernican theory.
Christina Graney and I discovered this historic work accidentally, while researching how early astronomers interpreted telescope measurements of star diameters (which they did not understand to be spurious).2 Riccioli discusses such measurements,3 and thus we have been translating portions of his book.
Coriolis discussed motion in a rotating frame of reference in 1835. Siméon-Denis Poisson dealt with the deflection of projectiles in 1838. Earlier, in 1735, George Hadley noted the possible effect of Earth’s diurnal rotation on winds.4 Historian of science Edward Grant discussed Riccioli and Grimaldi’s use of cannon balls fired in different directions to argue against Earth’s motion, but did not mention that it describes the Coriolis effect.5
