Tide Goes In, Tide Goes Out

The Classical World and the Middle Ages wrestled with the cause of the tides for centuries. Although one early scholar (Alpetragius, who flourished in the late 1100s) felt it was caused by some general motion of the world/celestial spheres that ran from east to west, most others (such as Bede and Gerald of Wales) felt it had a stronger connection to the movement of the Moon.
Alpetragius died in 1204, and his theory on the motion that caused the tides was translated into Latin by Michael Scot. This brought it to the attention of Robert Grosseteste (c.1175-1235), who had an explanation for the tides that relied on his theories of light. (The following is from the Questio de fluxu et refluxu maris, attributed to Grosseteste, although that attribution is disputed.)

Remember that there was no working theory of gravity yet; just a feeling that substances could be heavier or lighter depending upon their composition and gravitate (see? in this enlightened age, the concept of gravity pervades even our language) toward like substances: solids fall to earth; liquid (containing more of the element of water) flows to a lower spot to find its kind; fire yearns upward through air, because fire is even "lighter" than air.

For Grosseteste, light imparted force. Rays of light could carry with them the power to generate heat, for instance (see his theory on the sun). He postulated that, when the Moon rose above the horizon, its rays impressed against the waters and pushed them ahead of it, toward the west. This was not as simple and direct as a physical object pushing against water, and so water didn't rush to the shore as soon as the Moon rose. The rays of the Moon started pushing against the sea closest to it, pushing that water toward the observer. When the Moon was overhead, its rays had pushed as much water as it could at that time. Once the Moon passed the zenith and was over land, then the waters started to recede. The Moon then passes west and under the earth, at that point causing (somehow) the tides again.

Grosseteste admits that we don't know everything about this process, and my summary is a radical simplification of his detailed analysis. He notes the changes in tides as the Moon changes its declination, and theorizes that the Sun also "helps" the Moon in some manner.

For more detail, find the Question on the flow and re-flow of the sea (available in Isis, Vol. 57, No. 4 (Winter, 1966), pp. 455-474 in an article by Richard C. Dales) and enjoy.

Nicholas Oresme

Does the Earth rotate?

Born in  the 1320s, Nicholas (or Nicolas) Oresme likely came from humble beginnings; we assume this because he attended the College of Navarre, a royally funded and sponsored college for those who could not afford the University of Paris. He had his master of arts by 1342, and received his doctorate in 1356. ne of his published works was:

Livre du ciel et du monde
(The Book of Heaven and Earth)
In this work he discussed the arguments for and against the rotation of the Earth.

• He dismissed the notion that a rotating Earth would leave all the air behind, or cause a constant wind from east to west, pointing out that everything with the Earth would also rotate, including the air and water.
• He rejects as figures of speech any biblical passages that seem to support a fixed Earth or a moving sun. (Keep in mind even today we unanimously speak about the beauty of the sun setting when it's really the Earth rising!)
• He points out that it makes more sense for the Earth to move than for the (presumably more expansive and massive) heavenly spheres and Sun to move.
• He assures his readers that all the movements we see in the heavens could be accounted for by a rotating Earth.
• Then he assures the reader that everyone including himself thinks the heavens move around the earth, and after all he has no real evidence to the contrary!