Friday, April 17, 2009

April 17: Giovanni Battista Riccioli

Giovanni Battista Riccioli
April 17, 1598 – June 25, 1671

Giovanni Riccioli was an Italian astronomer. He was a Jesuit who entered the order in 1614. He was also the first person to measure the rate of acceleration of a freely falling body.

Riccioli was born in Ferrara, Italy. He devoted his career to the study of astronomy, often working with Francesco Maria Grimaldi. He wrote the important work Almagestum novum in 1651. By necessity, he opposed the Copernican heliocentric theory though praising its value as a simple hypothesis.

He and Grimaldi extensively studied the Moon, of which Grimaldi drew a map. Much of the nomenclature of lunar features still in use today is due to him and Grimaldi. He also observed Saturn, and was one of the first Europeans to note that Mizar was a double star.

Other books he wrote were: Geographiae et hydrographiae reformatae libri (1661), Astronomia reformata (1665), Chronologia reformata (1669) and Tabula latitudinum et longitudinum (published in 1689).

Despite his stated opposition to Copernicus's theory he named the prominent lunar crater Copernicus after him, and other important craters were named after other proponents of the theory: Kepler, Galileo and Lansbergius. Craters that he and Grimaldi named after themselves are in the same general vicinity, while some other Jesuit astronomers have craters named after them in a different part of the Moon, near Tycho. This is sometimes considered to be tacit sympathy for Copernican theory, which as a Jesuit he could not publicly express.

Between 1644 and 1656, he was occupied by topographical measurements, working with Grimaldi, determining values for the circumference of Earth and the ratio of water to land. Defects of method, however, gave a less accurate value for degrees of the meridian than Snellius had achieved a few years earlier. Snellius had been mistaken by approximately 4,000 meters; but Riccioli was more than 10,000 meters in error [Hoefer, 1873]. Riccioli had come up with 373,000 pes despite the fact that references to a Roman degree in antiquity had always been 75 milliare or 375,000 pes.

Image: G.B. Riccioli, Almagestum Novum (1651). The image portrays Urania, the muse of astronomy, weighing up the rival systems of Copernicus, in which the earth moves round the sun, and Riccioli himself, in which the earth remains stationary at the center of the universe. The older system of Ptolemy has already been discarded and lies on the ground alongside.

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