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

TL;DR

Riccioli's 17th-century star disk measurements, based on telescopic observations, were flawed due to diffraction effects, leading him to wrongly argue against Copernican heliocentrism by estimating implausibly large star sizes.

Contribution

This paper reveals how Riccioli's star size measurements were affected by optical diffraction, challenging their use as evidence against heliocentrism in the 17th century.

Findings

Riccioli's star diameters were caused by optical diffraction, not actual star sizes.

He used these measurements to argue against Copernican heliocentrism.

The estimated star sizes under Copernican hypothesis were astronomically large.

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.