Galileo Galilei, who was an Italian physicist, mathematician, astronomer and philosopher, is well known for his achievements and discoveries which have made great contributions to modern astronomy. Unfortunately his tidal theory, which by the way was a very impressive and well thought out idea, was wrong. Galileo proposed an argument for the motion of the planet based on his theory of tides, which did not agree with Kepler's theory. Kepler believed that the Moon caused tides on the Earth, and Galileo's theory was based on tides caused by the Earth's orbit around our sun and the rotation of our planet. Of course we know that Galileo's theory was wrong, but given Galileo's time and his rationality, this theory was very plausible. The justification for the tides came to him while he was on a barge traveling to Venice. Galileo observed that whenever the speed of the ship changed, the water inside it splashed accordingly. At that moment he realized that the tides could not be explained without the movement of the Earth. Galileo's argument was that the Earth's rotation and orbit around the sun had the same effect on the oceans as water splashing onto the barge when the speed of the barge changed. Galileo's argument was based on the consideration of the earth's rotation with the orbital plane moving which would result in a greater speed of the surface during the night than during the day. Then, Galileo came to the conclusion that the acceleration and slowing of the Earth causes the oceans to move back and forth and causes low or high tides. The part of the Earth that is not facing the sun travels faster because it moves in the same direction as the Earth's rotation and orbit around the sun. Slow down when… half of article… of scientific biography, (New York: Charles Scribner's Sons, 1970–1980), pp. 287–288. Shea, W.J. (1970) Galileo's claim to fame: evidence that the earth moves from tidal evidence, TheBritish Journal for the History of Science, 5, 111–127. Topper, D. (1990) Galileo, sunspots, and the motions of the Earth, Isis, 90, 757–767. Trefil, J.S. (1975) Introduction to the Physics of Fluids and Solids (New York: Pergamon Press). Wallis, J. (1666) Hypotheses on the ebb and flow of the sea, Philosophical Transactions of the Royal Society of London, 1, 263–294.© 2011 John Wiley & Sons A/SCopyright of Centaurus is the property of Wiley-Blackwell and the Its contents may not be copied or emailed to multiple sites or posted to a listserv without the express written consent of the copyright holder. However, users may print, download, or email items for individual use.