"Discourse on the Tides" ( it, Discorso Sul Flusso E Il Reflusso Del Mare) is an essay written by

Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...

in 1616 as a letter to

Alessandro Orsini Alessandro Orsini (1592–1626) was an Italian Cardinal. He was a patron of Galileo, who dedicated his 1616 work on the tides to him, and requested that he pass it on to Pope Paul V. Orsini belonged to the ducal family of Bracciano. Orsini was ...

that attempted to explain the motion of

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

tide Tides are the rise and fall of sea levels caused by the combined effects of the gravity, gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide t ...

s as a consequence of

Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in retrograd ...

and revolution around the Sun.Gigli, Rosella. "Galileo's Theory of the Tides." The Galileo Project, 1995. http://galileo.rice.edu/sci/observations/tides.html. Retrieved February 20, 2014 The same ideas form an important part of Galileo's ''Dialogue Concerning the Two Chief World Systems''. Galileo's theory was in fact erroneous, as proven by future scientific research and contemporary observations.

Background

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's ideas for tidal theory may have begun during 1595. While aboard a ferry carrying freshwater to

Venice Venice ( ; it, Venezia ; vec, Venesia or ) is a city in northeastern Italy and the capital of the Veneto Regions of Italy, region. It is built on a group of 118 small islands that are separated by canals and linked by over 400 ...

, Galileo noticed that the ship's cargo would undulate in accordance with the rocking of the ship.Tyson, Peter. "Galileo's Big Mistake." PBS, October 29, 2002. https://www.pbs.org/wgbh/nova/earth/galileo-big-mistake.html. Retrieved February 21, 2014 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 The Tychonic system (or Tychonian system) is a model of the Universe published by Tycho Brahe in the late 16th century, which combines what he saw as the mathematical benefits of the Copernican system with the philosophical and "physical" bene ...

. Hence, his desire to present his theory of the tides which, he believed, provided clear evidence for the motion of the Earth.

Summary

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 In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...

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 Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

and simultaneous

orbital revolution In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

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.

Criticism

Galileo's theory of the tides could not account for one of the two daily occurrences of high tide observed along most coastlines. He countered that the totality of Earth's water system contains an extenuating amount of variables, such as Earth's tilt, uneven surface and coastline, oceanic depth, and inclement winds, that can explain the second high tide. Also, Galileo argued that most natural scientists were examining and basing theories on the tides of the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...

, a microcosm compared to the ocean that may be idiosyncratic.Aiton, E.J. “Galileo’s Theory of the Tides.” ''Annals of Science'' 10. 1 (1954). 44-57. http://www.tandfonline.com/doi/pdf/10.1080/00033795400200054. Retrieved April 18, 2014. Historical writer E. J. Aiton states that the discourse "is among the least successful of alileo'sinvestigations and completely misrepresents the phenomena it is supposed to explain," and served mainly as an impetus for continued research by Galileo's followers. Aiton reveals that Galileo erred in choosing two different frames of reference: the tide is the motion of water relative to the Earth, but annual revolution is the motion of the Earth, and its water, relative to the Sun. Annual revolution results in a force that accelerates the Earth-water system but does not accelerate the water relative to the Earth, in the same way that an observer standing on Earth's surface does not feel the gravitational attraction of the Sun because the observer and Earth are moving together around the Sun.

Retrospect

The discourse does not include gravitational forces in its theory to explain the Earth's orbit and does not consider the relation between the ocean and gravitational forces, like that of the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

. Occurring invisibly, gravity was far too mystic for Galileo's consideration. Galileo did end the "Discourse on the Tides" with reservations that his theory may be incorrect and the hope that further scientific investigation will confirm his proposal. Still, his 1632 ''Dialogue Concerning the Two Chief World Systems'' omitted gravitational forces and included a reprisal of his tidal theory in its fourth section.

References

{{Galileo Galilei Tides Essays by Galileo Galilei 17th-century essays

Background

Summary

Criticism

Retrospect

See also

References