What causes the tides of the sea? Today we say they are caused by gravity — by the gravitational action of the sun and the moon. Isaac Newton developed the theory of universal gravitation, in which gravity is a force between objects that decreases as the distance between them increases, such that doubling that distance decreases the force by a factor of four.
Galileo did not think in such terms. Newton’s work came many decades after Galileo, so the idea of gravity causing the tides was beyond the physics of Galileo’s time. Galileo theorized that tides were caused by the double motion of the Earth proposed by Copernicus (yearly revolution about the sun, plus daily rotation). This double motion would provide a daily “push-pull” that caused the seas to rise and fall in their basins — the tides.
A daily “push”, however, implies high tides occurring but once per day in some seas (likewise for low tides). In a 1616 essay, Galileo claimed that this occurs in Lisbon, Portugal. It does not.
Galileo was soon informed of his error. But he argued again in his 1632 Dialogue for his tides theory as evidence favoring Copernicus. This time he omitted mention of Lisbon. He cited no other location where single daily tides occurred. This left him vulnerable to criticism, both in his own time and by scholars today, that he pursued his tidal theory despite its obvious inadequacies. Moreover, omitting Lisbon was omitting known information that contradicted his theory — very improper conduct for a scientist.
Recently, I tracked down correspondence between Galileo and Giovanfrancesco Buonamici in 1629-30, and found that Buonamici correctly informed Galileo about where single tides do indeed occur, in what today is known as Indonesia. Buonamici also provided Galileo with a reference for this information — the 1606 Regimiento de Navegación by Andrés García de Céspedes (a section of the title page of Regimiento is shown at top). Therefore, Galileo had the supporting example for his theory that he needed, but in Indonesia, not Lisbon. Why would Galileo omit this example from the Dialogue? It seems to me likely to have been an oversight on his part — an oversight of great consequence.
This was all published earlier this year in the Journal for Astronomical History and Heritage:
- Christopher M. Graney, “Galileo and Buonamici on the tides of the sea: was something omitted from the Dialogue?”, Journal of Astronomical History and Heritage vol. 27, pg. 200 (2024). https://doi.org/10.3724/SP.J.1440-2807.2024.01.11
I encourage readers of Sacred Space Astronomy to take a look at this paper (click here for it). I think you will find it quite readable, in part because it grows out of stuff I have written for this blog. (Yes, you were thinking all this sounded familiar, weren’t you?)
This specific research on Galileo and the tides got its start when I was at Castel Gandolfo last March, visiting the Vatican Observatory HQ and attending a conference at the Angelicum in Rome. I came down with COVID-19 (which I blame, fairly or not, on some white-haired guy who was seated near me during the flight to Rome from the US and who was coughing his head off for the entire flight). I was stuck quarantining with my wife in the VO’s guest apartment. I was truly sick — fever, etc. — despite my vaccines, but I was never feeling absolutely horrible. I did get horribly bored, however, so much so that I was reduced to scrubbing perfectly clean floors, simply to remove bits of tile grout spillover that remained from the installation of the floor tiles (below — I did a good job, too).
In looking for less manual things to do, I picked up from the VO library shelves a copy of the book Galileo: For Copernicanism and for the Church (Third Edition), written by Annibale Fantoli and translated by Fr. George V. Coyne, S.J. (who served as VO Director during the papacy of JPII), and published in 2003 by the Vatican Observatory (the first edition was published in 1994). Reading the book, I found on page 242 a mention of Galileo’s exchange with Buonamici regarding tides. That was the genesis of this research project.
So this new paper with the Journal of Astronomical History and Heritage is truly a Vatican Observatory project across the decades, with the work of the VO’s Fr. Coyne and Fantoli in the 1990s and 2000s leading me, thanks to being COVID-stuck in the VO in 2023, to uncover something new and important in the Galileo story today. If you have been a supporter of the VO, especially through the VO Foundation (which is specifically mentioned in Galileo: For Copernicanism and for the Church, in regards to both the 1994 and 2003 editions), this is your support at work.
