Discourse on the Tides

Discourse on the Tides (Discorso Sul Flusso E Il Reflusso Del Mare) is an essay written by Galileo Galilei in 1616 as a letter to Alessandro Orsini that explains the motion of Earth's tides as a consequence of Earth's rotation and revolution around the sun. Though conclusively misguided and eventually discredited by theories addressing gravity, the essay represents the early lineage of tidal theory.

Galileo's intrigue for tidal theory was motivated by a boat trip in 1595. While aboard a ferry carrying freshwater to Venice, Galileo noticed that the ship's cargo would undulate in accordance with the rocking of the ship.

Galileo composed Discourse on the Tides while in Rome and appealing for papal acceptance of the teaching of Copernican theory. The letter is thus not just an explanation of tidal phenomenon but also a private confirmation and defense of Galileo's ideas on heliocentrism, which are discussed completely in his Dialogue Concerning the Two Chief World Systems.

Galileo wished to present a convincing argument for heliocentrism. He was aware that all the astronomical arguments in the Dialogue were also consistent with the Tychonic system. Hence, his desire to present his theory of the tides which, he believed, provided clear evidence for the motion of the Earth.

The letter compares the ocean's waves to the disturbances in a vase of water, which move for three reasons: the slope of the vase, external forces exerted on the vase-water system, and the possible acceleration of the vase itself. Comparably, the ocean's tides are due to Earth's terrain, wind currents, and circular accelerations. In Galileo's understanding, Earth's rotation and simultaneous orbital revolution dictate that half of the Earth's rotational arc is in concordance with the direction of revolution and the other half is opposed to the direction of revolution. He reasoned that by sometimes matching and sometimes counteracting the motion of orbit, positive and negative acceleration is generated that influences bodies of water to rock back and forth, creating the tides. Though these opposing acceleration cannot be observed, large bodies of water containing points of location far away enough to experience significantly different vectors of acceleration would necessarily contour into waves.

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