Giovanni Battista Riccioli, SJ (17 April 1598 – 25 June 1671) was an Italian astronomer and a Catholic priest in the Jesuit order.

He is known, among other things, for his experiments with pendulums and with falling bodies, for his discussion of 126 arguments concerning the motion of the Earth, and for introducing the current scheme of lunar nomenclature.

Riccioli described himself as a theologian, but one with a strong and ongoing interest in astronomy since his student days, when he studied under Biancani.

[citation needed] Eventually his superiors in the Jesuit order officially assigned him to the task of astronomical research.

Riccioli dealt not only with astronomy in his research, but also with physics, arithmetic, geometry, optics, gnomonics, geography, and chronology.

[citation needed] He was awarded a prize by Louis XIV in recognition of his activities and their relevance to contemporary culture.

It became a standard technical reference book for astronomers all over Europe: John Flamsteed (1646–1719), the first English astronomer royal, a Copernican and a Protestant, used it for his Gresham lectures; Jérôme Lalande (1732–1807) of the Paris Observatory cited it extensively[7] even though it was an old book at that point; the 1912 Catholic Encyclopedia calls it the most important literary work of the Jesuits during the seventeenth century.

[9] Books 2 and 9 of the New Almagest Riccioli included a significant discussion of and extensive experimental reports on the motions of falling bodies and pendulums.

[10] With pendulums to keep time (sometimes augmented by a chorus of Jesuits chanting in time with a pendulum to provide an audible timer) and a tall structure in the form of Bologna's Torre de Asinelli from which to drop objects, Riccioli was able to engage in precise experiments with falling bodies.

[14] He illustrated the reliability of his experiments by providing detailed descriptions of how they were carried out, so that anyone could reproduce them,[15] complete with diagrams of the Torre de Asinelli that showed heights, drop locations, etc.

[18] Thus as D. B. Meli notes, Riccioli's accurate experiments were widely known during the second half of the [seventeenth] century and helped forge a consensus on the empirical adequacy of some aspects of Galileo's work, especially the odd-number rule and the notion that heavy bodies fall with similar accelerations and speed is not proportional to weight.

[24][25] A substantial portion of the New Almagest (Book 9, consisting of 343 pages) is devoted to an analysis of the world system question: Is the universe geocentric or heliocentric?

The historian of science Edward Grant has described Book 9 as being the "probably the lengthiest, most penetrating, and authoritative" analysis of this question made by "any author of the sixteenth and seventeenth centuries",[26] in his opinion apparently superseding even Galileo's Dialogue Concerning the Two Chief World Systems — Ptolemaic and Copernican.

Galileo offered a conjecture in his 1632 Dialogue that the apparent linear acceleration of a stone falling from a tower was the result of two uniform circular motions acting in combination – the daily rotation of Earth, and a second uniform circular motion belonging to the stone and acquired from being carried along by the tower.

So we need not look for any other causes of acceleration or any other motions, for the moving body, whether remaining on the tower or falling, moves always in the same manner; that is, circularly, with the same rapidity, and with the same uniformity.... if the line described by a falling body is not exactly this, it is very near to it... [and] according to these considerations, straight motion goes entirely out the window and nature never makes any use of it at all.

[38] However, the rightward[39] deflection actually occurs regardless of the direction the cannon is pointed (a much more developed understanding of physics than what was available in Riccioli's time is required to explain this).

[41] In the Copernican theory, the stars had to lie at vast distances from Earth in order to explain why no annual parallax was seen among them.

Riccioli and Grimaldi made numerous measurements of star disks using a telescope, providing a detailed description of their procedure so that anyone who wanted could replicate it.

In some scenarios one single star would exceed the size of the entire universe as estimated by a geocentrist like Tycho Brahe.

There were arguments concerning: whether buildings could stand or birds could fly if Earth rotated; what sorts of motions were natural to heavy objects; what constitutes the more simple and elegant celestial arrangement; whether the heavens or the Earth was the more suited for motion and the more easily and economically moved; whether the center of the universe was a more or less noble position; and many others.

However, he also rebutted certain anti-Copernican arguments, siding with the Copernicans in his assertions that rotation of the Earth would not necessarily be felt, and that it would not ruin buildings or leave birds behind.

[44] Some authors have suggested that Riccioli may have been a secret Copernican, required due to his position as a Jesuit to pretend to oppose the theory.

[45] Another prominent astronomical publication of Riccioli's was his 1665 Astronomia Reformata (Reformed Astronomy)—another large volume, although only half the length of the New Almagest.

James Gregory published a report in England in 1668 on the resulting public and personal dispute on the matter of falling objects.

This was a prelude to Robert Hooke's (1635–1703) invitation to Isaac Newton (1642–1727) to resume his scientific correspondence with the Royal Society, and to their ensuing discussion about the trajectory of falling bodies "that turned Newton's mind away from 'other business' and back to the study of terrestrial and celestial mechanics.

[53] Between 1644 and 1656, Riccioli and Grimaldi were occupied by topographical surveys, specifically an arc measurement for determining values for the circumference of Earth.

Defects of method, however, gave a less accurate value for degrees of arc of the meridian than Snellius had achieved a few years earlier.