Riccioli , Giovanni Battista

(1598–1671) Italian astronomer

Born at Ferrara in Italy, Riccioli was a Jesuit priest who spent most of his life at Bologna where he was professor of astronomy. In 1651 he produced his famous work Almagestum novum (The New Almagest). It is in this work that the system of naming craters and mountains on the Moon after famous astronomers was introduced. Although the work is not Copernican – Riccioli presents no less than 77 arguments against Copernicus – it is not, despite the title, Ptolemaic either. Riccioli was a follower of Tycho Brahe, naming the largest lunar crater after him. As an observational astronomer he found that Mizar was a double star. He was also a skilled and patient experimenter who attempted to work out the acceleration due to gravity or g. He first tested Galileo's claim for the isochronicity of the pendulum and the relationship between the period and the square of the length. To measure the time a falling body takes he needed a pendulum that would beat once a second or 86,400 times per sidereal day. This led to the farce of using a team of Jesuits day after day to count the beats of his pendulum but the magic figure of 86,400 escaped them. Eventually the fathers could no longer tolerate staying up night after night counting pendulum beats and he was left with his pupil Francesco Grimaldi having to accept a less than perfect pendulum. He then performed with Grimaldi the type of experiment Galileo is supposed to have done from the leaning tower of Pisa. He dropped balls of various sizes, shapes, and weights from the 300-foot (92-m) Torre dei Asinelli in Bologna. He succeeded in confirming Galileo's results and establishing a figure for g of 30 feet (9.144 m) per second per second, which is close to the value of 9.80665 meters per second per second accepted today.