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Apr 17 The Priest who Mapped the Moon

Today we look at the remarkable life of Giovanni Battista Ricciolo. A priest, professor of astronomer who was the fist person to map the moon, naming many of the craters and features than can be seen on a bright night. Also one of the first to discover and record a double star. His use of pendulums to try and measure the speed of gravity was one of many things he became famous for.


A Jesuit priest, at the age of 53 he published a massive treatise on astronomy (the Almagestum Novum), which became the standard reference work for astronomers throughout Europe for many decades. In this work he introduced the system of naming craters and mountains on the Moon after famous astronomers. With his fellow Jesuit, Francesco Grimaldi, he made the first map of the moon’s surface, establishing the names still used for its main features. Beginning a tradition which still sees Jesuits sitting on the Working Group for Planetary System Nomenclature, a committee of the International Astronomical Union (IAU). Ricciolo was an enthusiastic follower of the Danish nobleman, astronomer, and alchemist Tycho Brahe, and as a token of his esteem named the largest lunar crater after him. Having studied at the Jesuit College of Parma as a young man, he was profoundly influenced by one of his teachers who had demonstrated to him the existence of lunar mountains and the fluid nature of the heavens.

Ricciolo is also credited to being one of the first to discover a double star. A double star is a pair of stars that appear close to each other as viewed from Earth, usually with the aid of optical telescopes. This occurs because the pair either forms a binary star (i.e. stars that are in mutual orbit that are gravitationally bound to each other) or as an optical double, which describes merely a chance line-of-sight alignment of two stars at different distances from the observer. Ricciolo observed the only "binary star" whose two components are separately visible to the naked eye, Mizar and Alcor on the handle of the Plough asterism in the constellation of Ursa Major or the Great Bear. As they are about 83 light-years away from the Sun, it is still not known for sure whether or not they are gravitationally bound. Riccioli recorded Mizar as appearing as a double Mizar and is one of the most visible asterisms in the night sky of the Northern Hemisphere. Around the same time the Benedictine monk, Benedetto Castelli asked Galileo to investigate it. The monk Castelli was a long-time friend and supporter of Galileo, and in turn teacher to Galileo's son.

Ricciolo was also a skilled and patient experimenter who attempted to work out the acceleration due to gravity two hundred years before Isaac Newton. Because we were still awaiting the invention of the digital stopwatch, to measure the time a falling body takes, Ricciolo needed a pendulum that would beat once a second or 86,400 times per day. This led to him 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 with only his pupil Francesco Grimaldi left, he had to accept a less than perfect pendulum. He performed with Grimaldi the type of experiment Galileo is supposed to have done from the leaning tower of Pisa, dropping balls of various sizes, shapes, and weights from the 300-foot high Torre dei Asinelli in Bologna. He succeeded in confirming Galileo's results.

His partnership with Grimaldi was scientifically very fruitful, for instance the history of Science credits Grimaldi with the discovery and naming of the phenomenon of diffraction of light. Riccioli and Grimaldi also discovered and analyzed the important "Coriolis Effect" in physics two hundred years before Coriolis did. This described the deflection of moving objects when seen from within a rotating frame of reference (i.e the planet). This is why aeroplanes or currents of air travelling long distances around the Earth appear to move at a curve as opposed to a straight line and is important in physics, meteorology, and oceanography. He was awarded a prize by Louis XIV in recognition of his activities and their relevance to contemporary culture. He also kept up a rich correspondence with leading scientists of his time, including Huygens, Cassini, and Kircher. He continued to publish on both astronomy and theology up to his death and died aged 73 in Bologna


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