Galileo Galilei (1564-1642) was an Italian physicist, mathematician, engineer, astronomer, and philosopher known as the "Father of Modern Science." Some of his "popular" writings are collected in Discoveries and Opinions of Galileo. The book is a fictional four-day series of discussions between two philosophers (Salvitri, expressing Galileo's "heliocentric" position; and Sagredo, who is "neutral") and a layman rather insultingly named Simplicio, who represents the Ptolemaic/"geocentric" view endorsed by Pope and the Catholic Church. [NOTE: page numbers refer to a 496-page paperback edition.]
He wrote in his introduction "To the Discerning Reader," that "Three principal headings are treated. First, I shall try to show that all experiments practicable upon the earth are insufficient measures for proving its mobility... Secondly, the celestial phenomena will be examined, strengthening the Copernican hypothesis until it might seem that this must triumph absolutely... In the third place, I shall propose an ingenious speculation. It happens that long ago I said that the unsolved problem of the ocean tides might receive some light from assuming the motion of the earth..." (Pg. 6)
Salvitri says, "The moon certainly agrees with the earth in its shape, which is indubitably spherical. This follows necessarily from its disc being seen perfectly circular, and from the manner of its receiving light from the sun. For if its surface were flat, it would all become covered with light at once, and likewise would all be deprived of light in an instant... as from the earth we see the moon now completely lighted, now half, now more, now less, sometimes sickle-shaped and sometimes completely invisible... just so would the illumination made by the sun on the face of the earth be seen from the moon, with precisely the same period and the same alterations of shape..." (Pg. 62-63) He also suggests, "I consider the moon very different from the earth. Though I fancy to myself that its regions are not idle and dead, still I do not assert that life and motion exist there." (Pg. 99)
He argues, "First, let us consider only the immense bulk of the starry sphere in contrast with the smallness of the territorial globe, which is contained in the former so many millions of times. Now if we think of the velocity of motion required to make a complete rotation in a single day and night, I cannot persuade myself that anyone could be found who would think it the more reasonable and credible thing that it was the celestial sphere which did the turning, and the terrestrial globe which remained fixed." (Pg. 115) Later, he adds in support of the heliocentric view, "we find all the planets closer to the earth at one time and farther from it at another. The differences are so great that Venus, for example, is six times as distant from us at its farthest as at its closest, and Mars soars nearly eight times as high in the one state as in the other. You may thus see whether Aristotle was not some trifle deceived in believing that they were always equally distant from us." (Pg. 321)
He asserts about the ocean tides that "this ebb and flow itself cooperates in confirming the earth's mobility." (Pg. 416) He adds, "since the alterations in the tides at the said times consist of nothing more than changes in their sizes; that is, in the rising and lowering of the water a greater or less amount, and its running with greater or less impetus. Hence it is necessary that whatever the primary cause of the tides is, it should increase or diminish its force at the specific times mentioned." (Pg. 445-446) He points out, "The monthly periodic changes would cease if there were no variation due to the annual motion, and if the additions and subtractions of the diurnal rotation were kept always equal, then the annual periodic alterations would be missing." (Pg. 448) He rejects the notion that that the tides are caused by the motion of the earth: "a simple and uniform motion, such as the simple diurnal motion of the terrestrial globe for instance, does not suffice, and that an uneven motion is required, now accelerated and now retarded. For if the motion of the vessels were uniform, the contained waters would become habituated to it and would never make any mutations." (Pg. 461-462)
This book will be both fascinating and "must reading" for anyone interested in the history of scientific thought.