Most people know Galileo for his support for the Copernican model of the solar system (where the earth revolves around the sun), but he contributed to many other ares of science too.
Introduction to Galileo’s Tidal Theory
Beyond his work on astronomy, mathematics, and physics, Galileo was concerned with explaining the tides. His explanation of the tides was bound up with his attempt to show that the earth revolved around the sun, a theory for which he was later put under house arrest by the Catholic Church.
Explaining why the tides exist at all, and their variable pattern across the globe, was very difficult in the 16th and 17th centuries. This was in the era before Newton had proposed his explanation of gravity. Galileo tried to explain the tides, as did Johannes Kepler, Francis Bacon, René Descartes, and many other philosophers of nature. While many early theories connected the tides to the movements of the moon, Galileo explained this phenomenon in a very different way.
Galileo’s Tidal Theory
Following Nicolaus Copernicus’s geocentric model of the universe, Galileo argued that the earth rotates on its axis every day, and that the earth orbits around the sun once every year. The combination of these movements produced the tides.
The image on this web page shows the two motions in question: rotational (daily) and orbital (yearly). As you can see, the half of the earth that is closest to the sun has a rotational motion that opposes the orbital motion. The side of the earth furthest from the sun has a rotational motion that moves in the same direction as the orbital motion. Put another way, the side of the earth that is furthest from the sun is moving faster than the side of the earth that is closest to the sun.
Since the earth is always rotating, different parts of the earth will be moving at different speeds throughout the day. Galileo argued that the difference between the motions on these opposite sides of the earth cause tidal motion.
How did this difference in motions work to produce the tides? Galileo thought that the earth was like a vase filled with water. Imagine that you are walking in one direction with a vase; if you stop suddenly, then the water will begin to slosh back and forth. This is caused by the change in motion of the vase from walking to stopped. Similarly, if you are stopped and start walking quickly forward, then the water will start to slosh back and forth. In this example, then, the vase is the earth and the water is the oceans. The daily acceleration and deceleration of the earth as it rotates causes the the tides because the ocean sloshes back and forth.
Galileo’s Theory of the Tides in History
Before Galileo had explained his theory, the German mathematician Johannes Kepler theorized that the moon’s influence caused the tides. While Kepler’s theory of the tides makes sense to us today, Galileo didn’t buy it. Kepler’s theory did not explain why the moon influenced the tides. Because of this, Galileo developed his own explanation of the tides based on the movement of the earth. It was not until the end of the 17th century, after Isaac Newton published his law of universal gravitation, that Kepler’s theory made more sense. As Newton’s law of gravitation came to be accepted by the scientific community, Galileo’s theory was abandoned.
Galileo first explained his theory of the tides in a 1616 letter to Cardinal Orsini in an effort to prevent the church from banning Copernicanism. Later, in 1632, Galileo made another explanation of the tides in his book Dialogue Concerning Two Chief World Systems.
Conclusion of Galileo’s Tidal Theory
Today we know that the tides are caused by the gravitational influence of the moon, sun, and many other influences like the rotation of the earth. In Galileo’s day, this was not known. His explanation, while ultimately wrong, was an attempt to use the phenomenon of tides as evidence for the Copernican model of the universe.
Sources:
The Galileo Project. galileo.rice.edu Accessed March 2011
Mark Ulett - Mark Ulett is a graduate student at Arizona State University. He studies the history of biology and evolution.
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