Defeated by the tides

The Copernican worldview has prevailed - not, however, Galileo’s theory of the tides.

February 11, 2014

Clearly inspired by the behaviour of water when boats come to a halt, Galileo Galilei concluded that the ebb and flow of the tides resulted, similarly, from the acceleration and deceleration of the oceans. This, in turn, was caused by the movement of the Earth around the Sun, and its rotation on its own axis. However, Galileo was completely mistaken in this theory.

Text: Jochen Büttner

After the discoveries he made with his telescope, Galileo Galilei embarked on an increasingly urgent quest for proof of the Copernican worldview, which placed the Sun rather than the Earth at the centre of the universe. In his explanation of the tides, he believed that he had finally confirmed that the Copernican view was correct: his theory of the tides also rounded off the arguments in support of Copernicus, which he presented in his controversial work Dialogue Concerning the Two Chief World Systems in 1632.

Together with his friend Paolo Sarpi, Galileo had observed what happens when the barges that transported freshwater to Venice were brought to a sudden halt for docking: “keeping its impetus”, the water in the hull – as opposed to the boat itself – “will run forward toward the prow where it will rise perceptibly”. And thereafter slosh forward and back for some time. Accordingly, in Galileo’s view, high and low tides could result from the forward and backward flow of water in sea basins when the latter are decelerated or accelerated. But by which means?

As a staunch Copernican, Galileo had an explanation at hand: the tides are propelled by the dual motion of the Earth around the Sun and on its own axis. Because the direction of rotation of the Earth’s annual and daily movements are the same, their speeds accumulate on the side of the Earth turned away from the Sun. The opposite happens on the side facing the Sun, he argued. Between these points, the seawater is either accelerated or decelerated, just like the water in the barge.

The student of nature elaborated further. To establish how the water flows backwards and forwards in the sea basins, he adopted the comparison with a giant pendulum. The National Library in Florence houses a 200-page-thick bundle of manuscripts containing Galileo’s notes on questions concerning motion, including calculations, tables and sketches.

An unusual thought experiment can be found on page 154r: Galileo imagines a gigantic pendulum, the length of which corresponds to the Earth’s radius. He assumes that this Earth pendulum swings back and forth in six hours and examines whether this assumption can be reconciled with the quantifiable period of oscillation of a ten-metre-long pendulum, for example a swinging light that hangs from the ceiling of a church.

As the manuscripts show, his attempt to extrapolate the period of the tides through the comparison with a pendulum came to nothing. In any case, Galileo did not find any conclusive explanation for the regularity with which high and low tide recur at a delay of around 50 minutes day by day.

Nonetheless, he published the Dialogue - and doubts were quickly expressed about his theory of the tides. Worse still: Although the Roman censor had initially authorised the printing of the Dialogue, in late 1632, Galileo was accused of having contravened the anti-Copernican Decree of 1616 with this publication. The scientist was ordered to deny Copernican teachings and was condemned to lifelong house arrest.

In the centuries that followed, the Dialogue became a symbol of Galileo’s commitment to the freedom of scientific thought. It was not until 1992 that Pope John Paul II described Galileo’s conviction by the Inquisition as a “painful misunderstanding”.

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