Sarpi and Galileo Were Wrong About the Tides
Galileo Attacks Kepler
In a recent post entitled Paolo Sarpi on Local Motion, I stated, quoting his Pensiero 116, that Paolo Sarpi held that everything in the Universe ultimately is caused by local motion. Consistent with this idea, he wrote in 1591, the year before Galileo arrived in Padua, his Pensieri 568-571, exposing his theory of the tides as the composition of the two principal motions of the Earth — the annual motion of revolution around the Sun and the daily motion of the rotation around its own axis — with consequence that “every point on the [earth's] surface is now fast, now medium, now late.”
Galileo’s position on the tides was identical to Sarpi’s, and Libero Sosio is inclined to attribute the paternity of this theory to Sarpi.1 However, unlike Sarpi, Galileo wrote extensively on the matter, first in his 1616 Discourse on the Tides (Discorso Sul Flusso E Il Reflusso Del Mare), then in the fourth day of his 1632 Dialogue (this is the book comparing the Ptolemaic and Copernican systems that got him in trouble with the Holy Inquisition).
This Sarpian-Galilean theory of the tides completely ignored the fact that it had been known since Antiquity that the tides fluctuated with the lunar cycle. Furthermore, the theory in no way offered an explanation for the fact that in much of the world, there are two high tides and two low tides every day, rather than one of each.
Even more problematic was that Johannes Kepler had already published in 1609 his Astronomia Nova (seven years before Galileo’s Discourse on the Tides), with an explanation of the tides based on the motion of the Moon:
The orb of the attractive power in the moon is extended all the way to the earth, and calls the waters forth beneath the torrid zone, in that it calls them forth into its path wherever the path is directly above a place. This is imperceptible in enclosed seas, but noticeable where the beds of the ocean are widest and there is much free space for the waters’ reciprocation. It thus happens that the shores of the temperate latitudes are laid bare, and to some extent even in the torrid regions the neighboring oceans diminish the size of the bays. And thus when the waters rise in the wider ocean beds, it can happen that in its narrower bays, if they are not too closely surrounded, the moon being present, the water might even seem to be fleeing the moon, though in fact they are subsiding because a quantity of water is being carried off elsewhere.
But the moon passes the zenith swiftly, and the waters are unable to follow so swiftly. Therefore, a westward current of the ocean arises beneath the torrid zone, which, when it strikes upon the far shores, is thereby deflected. But when the moon departs, this congress of the waters, or army on the march towards the torrid zone, because it is abandoned by the traction that had called it forth, is dissolved. But since it has acquired impetus, it flows back (as in a water vessel) and assaults its own shore, inundating it. In the moon’s absence, this impetus gives rise to another impetus until the moon returns and submits to the reins of this impetus, moderates it, and carries it around along with its own motion. So all shores that are equally exposed are flooded at the same time, while those more remote are flooded later, some in different ways because of the different approaches of the ocean.2
And then in 1619, Kepler published his most significant work, the Harmonices Mundi, a full 13 years before Galileo published his Dialogue. (I mentioned the Harmonices Mundi, with Arthur Koestler’s laudatory words, in an earlier post, entitled Kepler, Shakespeare and Plato.)
There is no question that Galileo had access to Kepler’s works. Did he back down? No, he doubled down in the Dialogue, having his representative Salviati insult Kepler, calling him puerile!
Likewise it is completely idle to say (as is attributed to one of the ancient mathematicians) that the tides are caused by the conflict between the motion of the earth and the motion of the lunar sphere, not only because it is neither obvious nor has it been explained how this must follow, but because its glaring falsity is revealed by the rotation of the earth being not contrary to the motion of the moon, but in the same direction. Thus everything that has been previously conjectured by others seems to me completely invalid. But among all the great men who have philosophized about this remarkable effect, I am more than astonished at Kepler than at any other. Despite his open and acute mind, and though he has at his fingertips the motions attributed to the earth, he has nevertheless lent his ear and his assent to the moon's dominion over the waters, to occult properties, and to such puerilities.3
The word “puerilities” is not a mistranslation. The word that Galileo used in the original was “fanciullezze”.4
Kepler theorized that magnetism played a rôle in the relative motions of the bodies of the solar system, and I shall return to this theme in future posts. However, it should be noted that both Sarpi and Galileo knew of William Gilbert’s De Magnete, as they exchanged correspondence on the topic.5
The debate between proponents of local motion and proponents of action-at-a-distance is crucial, and over the centuries, many of the most important philosophers and scientists, including in Antiquity, have participated in this debate. In future posts, I will repeatedly address this topic.
If you wish to donate to support my work, please use the Buy Me a Coffee app.
Liberio Sosio. Galileo Galilei e Paolo Sarpi, Galileo Galilei e la cultura veneziana, Venezia: Istituto Veneto di Scienze, Lettere ed Arti, 1995, pp.307-308.
Johannes Kepler, Astronomia Nova. New Revised Edition. Translated by William H. Donahue. Santa Fe, New Mexico: Green Lion Press, 2015, p.25.
Galileo. Dialogue Concerning the Two Chief World Systems --- Ptolemaic and Copernican. Translated, with revised notes, by Stillman Drake. University of California Press, 1967, p.462.
Galileo. Opere VII:486, Firenze: G.Barbera, 1897.
Liberio Sosio, op.cit., pp.282-283.