Riccioli Measures the Stars: Observations of the telescopic disks of stars as evidence against Copernicus and Galileo in the middle of the 17th century

Abstract

G. B. Riccioli's 1651 Almagestum Novum contains a table of diameters of stars measured by Riccioli and his associates with a telescope. These telescopically measured star diameters are spurious, caused by the diffraction of light waves through the circular aperture of the telescope, but astronomers of the time, including Riccioli and Galileo Galilei, were unaware of this phenomenon. They believed that they were seeing the physical bodies of stars. In the Almagestum Novum Riccioli uses these telescopically measured disks to determine the physical sizes of stars under both geocentric (or geo-heliocentric - Tychonic) and heliocentric (Copernican) hypotheses. The physical sizes obtained under the Copernican hypothesis are immense - dwarfing the Earth, the Sun, and the Earth's orbit; even exceeding the distances to the stars given by Tycho Brahe. Thus Riccioli felt that telescopic observations were an effective argument against the Copernican system.