Galileo, Scheiner and the Earth's rotation

Disquisitiones mathematicae

From 1610 to 1615 Galileo publicly advocated Copernican theory. In February 1615 he received a dispatch of a small work from Ingolstadt by his rival, the Jesuit priest Christoph Scheiner: external page "Disquisitiones mathematicae de controversiis et novitatibus astronomicis" (1614). It contains countless arguments against Copernican theory and Scheiner's invitation to Galileo to take a stand on this.

The author of the work, published in 1614 in Ingolstadt, was in reality Johannes Gregorius Locher, a pupil of Scheiner’s; but the Disquisitiones contain Scheiner's body of thought, which Galileo could see perfectly well and which Scheiner himself later admitted.

An ever growing resentment developed between the two scientists. To this day we don’t know whether Scheiner played a role in Galileo's condemnation. In any case Galileo berated his antagonist after the trial in the fiercest terms and called him among other things a "swine" and a "spiteful ass" (Galileo Opere XVI 391).

Galileo's commentary on the Disquisitiones

Galileo's reply to the Disquisitiones is first contained in the Dialogue of 1632. In a text passage from the Second Day (II 604–609), Galileo trashes with bitter irony an illustration in Scheiner's work with which a counterargument to the daily rotation of the Earth is explained.

Scheiner was imagining what would actually happen if it was the Earth that was rotating. What effect this would have on a body that fell from the Moon or on the flightpath of a rocket, the flight of a bird, hail, etc., that falls from the sky. In Scheiner's work, the illustration was enriched with explanatory texts but Galileo ignored them in his scathing review.