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Galileo’s Dialogue

Frontispiece of Galileo Galilei's Dialogue Concerning the Two Chief World Systems,1632

Frontispiece of Galileo Galilei’s Dialogue Concerning the Two Chief World Systems,1632.

Galileo’s Dialogue Concerning the Two Chief World Systems was first published in 1632, and the following year he was suspected of heresy by the Inquisition and sentenced to house arrest for the remainder of his life. He was ordered to recite Psalms each week for three years and, of course, to forget the notion that the sun was the center of the solar system. His book was placed on a forbidden list until 1835, and it took 300 years for the Roman Catholic Church to admit he was correct and clear his name.

Human progress and the expansion of knowledge still face such obstacles around the world from religious dogma, political and economic interests, and willful ignorance. Recently, however, the scientific community and the interested world at large were rewarded when proof finally came that gravitational waves, predicted a hundred years ago by Albert Einstein in his general theory of relativity, actually exist. So, with this new knowledge, Galileo, considered by Einstein to be the father of modern physics and science in general, is once more vindicated.

The following section of Galileo’s Dialogue is taken from The World’s Greatest Books, Vol. XV, Science edited by Arthur Mee and J.A. Hammerton, published 1910. The dialogue is between three men, with Salviatus acting as the voice of Galileo himself who explains the solar system to his two colleagues.

Does the Earth Move

Salviatus: Now, let Simplicius propound those doubts which dissuade him from believing that the earth may move, as the other planets, round a fixed centre.

Simplicius: The first and greatest difficulty is that it is impossible both to be in a centre and to be far from it. If the earth move in a circle it cannot remain in the centre of the zodiac; but Aristotle, Ptolemy and others have proved that it is in the centre of the zodiac.

Salviatus: There is no question that the earth cannot be in the centre of a circle round whose circumference it moves. But tell me what centre do you mean?

Simplicius: I mean the centre of the universe, of the whole world, of the starry sphere.

Salviatus: No one has ever proved that the universe is finite and figurative; but granting that it is finite and spherical, and has therefore a centre, we have still to give reasons why we should believe that the earth is at its centre.

Simplicius: Aristotle has proved in a hundred ways that the universe is finite and spherical.

Salviatus: Aristotle’s proof that the universe was finite and spherical was derived essentially from the consideration that it moved; and seeing that centre and figure were inferred by Aristotle from its mobility, it will be reasonable if we endeavour to find from the circular motions of mundane bodies the centre’s proper place. Aristotle himself came to the conclusion that all the celestial spheres revolve round the earth, which is placed at the centre of the universe. But tell me, Simplicius, supposing Aristotle found that one of the two propositions must be false, and that either the celestial spheres do not revolve or that the earth is not the centre round which they revolve, which proposition would he prefer to give up?

Simplicius: I believe that the Peripatetics——

Salviatus: I do not ask the Peripatetics, I ask Aristotle. As for the Peripatetics, they, as humble vassals of Aristotle, would deny all the experiments and all the observations in the world; nay, would also refuse to see them, and would say that the universe is as Aristotle writeth, and not as Nature will have it; for, deprived of the shield of his authority, with what do you think they would appear in the field? Tell me, therefore, what Aristotle himself would do.

Simplicius: To tell you the truth, I do not know how to decide which is the lesser inconvenience.

Salviatus: Seeing you do not know, let us examine which would be the more rational choice, and let us assume that Aristotle would have chosen so. Granting with Aristotle that the universe has a spherical figure and moveth circularly round a centre, it is reasonable to believe that the starry orbs move round the centre of the universe or round some separate centre?

Simplicius: I would say that it were much more reasonable to believe that they move with the universe round the centre of the universe.

Salviatus: But they move round the sun and not round the earth; therefore the sun and not the earth is the centre of the universe.

Simplicius: Whence, then, do you argue that it is the sun and not the earth that is the centre of the planetary revolutions?

Salviatus: I infer that the earth is not the centre of the planetary revolutions because the planets are at different times at very different distances from the earth. For instance, Venus, when it is farthest off, is six times more remote from us than when it is nearest, and Mars rises almost eight times as high at one time as at another.

Simplicius: And what are the signs that the planets revolve round the sun as centre?

Salviatus: We find that the three superior planets—Mars, Jupiter, and Saturn—are always nearest to the earth when they are in opposition to the sun, and always farthest off when they are in conjunction; and so great is this approximation and recession that Mars, when near, appears very nearly sixty times greater than when remote. Venus and Mercury also certainly revolve round the sun, since they never move far from it, and appear now above and now below it.

Sagredus: I expect that more wonderful things depend on the annual revolution than upon the diurnal rotation of the earth.

Salviatus: YOU do not err therein. The effect of the diurnal rotation of the earth is to make the universe seem to rotate in the opposite direction; but the annual motion complicates the particular motions of all the planets. But to return to my proposition. I affirm that the centre of the celestial convolutions of the five planets—Saturn, Jupiter, Mars, Venus, and Mercury, and likewise of the earth—is the sun.

As for the moon, it goes round the earth, and yet does not cease to go round the sun with the earth. It being true, then, that the five planets do move about the sun as a centre, rest seems with so much more reason to belong to the said sun than to the earth, inasmuch as in a movable sphere it is more reasonable that the centre stand still than any place remote from the centre.

To the earth, therefore, may a yearly revolution be assigned, leaving the sun at rest. And if that be so, it follows that the diurnal motion likewise belongs to the earth; for if the sun stood still and the earth did not rotate, the year would consist of six months of day and six months of night. You may consider, likewise, how, in conformity with this scheme, the precipitate motion of twenty-four hours is taken away from the universe; and how the fixed stars, which are so many suns, are made, like our sun, to enjoy perpetual rest.

Sagredus: The scheme is simple and satisfactory; but, tell me, how is it that Pythagoras and Copernicus, who first brought it forward, could make so few converts?

Salviatus: If you know what frivolous reasons serve to make the vulgar, contumacious and indisposed to hearken, you would not wonder at the paucity of converts. The number of thick skulls is infinite, and we need neither record their follies nor endeavour to interest them in subtle and sublime ideas. No demonstrations can enlighten stupid brains.

My wonder, Sagredus, is different from yours. You wonder that so few are believers in the Pythagorean hypothesis; I wonder that there are any to embrace it. Nor can I sufficiently admire the super-eminence of those men’s wits that have received and held it to be true, and with the sprightliness of their judgments have offered such violence to their senses that they have been able to prefer that which their reason asserted to that which sensible experience manifested. I cannot find any bounds for my admiration how that reason was able, in Aristarchus and Copernicus, to commit such a rape upon their senses, as in despite thereof to make herself mistress of their credulity.

Sagredus: Will there still be strong opposition to the Copernican system?

Salviatus: Undoubtedly; for there are evident and sensible facts to oppose it, requiring a sense more sublime than the common and vulgar senses to assist reason.

Sagredus: Let us, then, join battle with those antagonistic facts.

Salviatus: I am ready. In the first place, Mars himself charges hotly against the truth of the Copernican system. According to the Copernican system, that planet should appear sixty times as large when at its nearest as when at its farthest; but this diversity of magnitude is not to be seen. The same difficulty is seen in the case of Venus. Further, if Venus be dark, and shine only with reflected light, like the moon, it should show lunar phases; but these do not appear.

Further, again, the moon prevents the whole order of the Copernican system by revolving round the earth instead of round the sun. And there are other serious and curious difficulties admitted by Copernicus himself. But even the three great difficulties I have named are not real. As a matter of fact, Mars and Venus do vary in magnitude as required by theory, and Venus does change its shape exactly like the moon.

Sagredus: But how came this to be concealed from Copernicus and revealed to you?

Portrait of Galileo Galilei by Justus Sustermans (1597–1681), oil on canvas,1636, National Maritime Museum, Greenwich, London.

Portrait of Galileo Galilei by Justus Sustermans (1597–1681), oil on canvas,1636, National Maritime Museum, Greenwich, London.

Further reading:
Galileo Galilei: When the World Stood Still by Atle Næss
Einstein: His Life & Universe by Walter Isaacson