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Galileo Galilei
Dialogue Concerning the Two Chief World Systems
Squashed down to read in about 15 minutes
"Does the Earth Move?"

Wikipedia - Full Text - Print Edition:

INTRODUCTION TO Galileo's Dialogue

Galileo Galilei, born in Pisa to a family of musicians, was an Italian philosopher, mathematician, astronomer, and pioneer physicist who Albert Einstein called the 'Father of Modern Science'.

As a child he constructed mechanical toys, and as a young man discovered the time-keeping properties of the pendulum. He invented the hydrostatic balance to discover the density of solids. At the age of 24 a treatise on centres of gravity led to a lectureship at Pisa University, from where he was later driven by the enmity of the traditionalist followers of Aristotle. He went to Padua University, where he invented a (rather poor) type of thermometer, a proportional compass and a microscope.

In 1609, while visiting the 'stupendous city' of Venice, Galileo heard of the telescope and constructed one for himself. In 1610 he published his telescopic discoveries, including the first observations of the moons of Jupiter, in The Starry Messenger. In 1613 Galileo wrote a rather public letter to the Grand Duchess Christina, in reply to certain questions of hers, in which he declared that the Bible teaches how to go to heaven, not how the heavens go.

It was this direct challenge to biblical authority which brought him into conflict with the Roman Church, and led to him being investigated by The Inquisition. Galileo's response was to produce this book, the Dialogue Concerning the Two Chief World Systems. The 'two systems' being discussed are ostensibly the Copernican idea that the world revolves around the sun and the older one of Ptolemy that the sun goes round the earth, but it was fairly plain that 'Two World Systems' also meant whether we should base our knowledge on the Church or on Science. This was an attack on Church intellectuals, and rather alienated those among them who had previously supported him, including Pope Urban VIII. Galileo was tried by the Inquisition, found "vehemently suspect of heresy", forced to recant, to recite penitential psalms once a week, and to spent the rest of his life under house arrest. His books were banned, including any he might write in the future, though that did not stop him getting a treatise on Two New Sciences, about mechanics, published in Holland.

In 1758 the Church dropped the prohibition on books advocating the sun as centre of the solar system, but Galileo's work was not allowed until 1835.


This condensed edition of 2000 words is extracted from the original 600,000, based on the translation by Thomas Salusbury and the earlier abridged version edited by Sir John Hammerton. It is a VERY brief summary of just a part of the huge whole, which also ranges over more general scientific subjects.

Like This?

Have a look at The Squashed Version of Copernicus' Revolutions.
Pope John Paul II on Galileo: http://www.disf.org/en/documentation/12-791110_PASC.asp
Not everyone thinks The Church was wrong to condemn Galileo: http://www.traditioninaction.org/History/A_003_Galileo.html


Galileo Galilei, 1632
Dialogue Concerning the Two Chief World Systems
"Does the Earth Move?"

Not done this bit yet - You can e-prod me if you really need it - glynhughes@btinternet.com

The Squashed Philosophers Edition of...

Dialogue Concerning the Two Chief World Systems
Galileo Galilei
Squashed version edited by Glyn Hughes © 2012

To the Most Serene Grand Duke of Tuscany

The difference between Men and other living Creatures hath dependence upon the different abilities of their Intellectuals; which I reduce to their being, or not being, a Philosopher. Philosophy being the proper food of such as live by it, distinguisheth a Man from the common Essence of the Vulgar.

The Constitution of the Universe being the greatest standard by which all things are known, no person can there challenge the intellects of Ptolomey and Copernicus, who had the honour to see farthest into the Worlds Systeme. My work about these men I conceived it my duty to dedicate to your highness.

Praying for Your Prosperity, I humbly kiss Your Hands;
Your Most Serene Highnesses Most Humble and most devoted Servant and Subject

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?

Galileo Galilei
Galileo died on 8 January 1642, aged 77.
The Church refused the Grand Duke of Tuscany, Ferdinando II, permission to bury Galileo in a marble mausoleum in the Basilica of Santa Croce, and he was interred instead in a side chapel. In 1737, the marble mausoleum was completed and permission granted. Galileo's remains were transferred there, but three fingers and a tooth were retained. The middle finger from his right hand is now displayed in the Museo Galileo in Florence.

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