The search for stellar parallax and the discovery of the aberration of light: the observational proofs of the Earth’s revolution, Eustachio Manfredi and the ‘Bologna case’

A milestone in astronomy — proof of the revolution of the Earth around the Sun — was reached in 1729 when James Bradley presented his theory of the annual aberration of light, providing the correct interpretation of the long-observed apparent motion of stars. Meanwhile, in Bologna Eustachio Manfredi published the result of his work in two books. In the first one, published in 1729, he demonstrated that the motion observed until then was not due to parallax; in the second one, dated 1730, he presented numerous observations supporting Bradley’s hypothesis. Due to censorship of the heliocentric system, however, Manfredi deemed (or was ‘supposed’ to deem) Bradley’s hypothesis as rash. By presenting the two astronomers’ different approaches to this astronomical conundrum, we have reconstructed the debate that arose within a cultural environment in which the Copernican argument continued to cause problems. We have also examined how this influenced the theoretical reflections of Manfredi and the Italians who tackled this subject.