The cosmology of Galileo in brief
«... SALV. For the three superior planets: Mars, Jupiter and Saturn, this is inferred from their being always found closest to the Earth when they are in opposition to the Sun, and farthest when they are near conjunction; this variation in distance is so significant, that when Mars is closest it appears 60 times greater than when it is farthest. Then, in regard to Venus and Mercury, we are certain of their revolving around the Sun from their never receding much from it and from our seeing them sometimes beyond it and sometimes in between; the latter is conclusively proved by the changes in the apparent shape of Venus. For the case of the Moon, it is indeed true that it cannot be separated from the Earth, for reasons which will be given more clearly as we proceed. 

SAGR. I expect to hear more marvellous things that depend on this annual motion of the Earth than was the keys for those that depend on the diurnal rotation. 

SALV. You are absolutely right. For, the action of the diurnal motion on the heavenly bodies, was and could be nothing but to make the universe appear to us to be hastily running in the opposite direction; but this annual motion, by mixing with the particular motions of all the planets, produces very many oddities, that so far have made all the greatest men in the world lose their bearings. Now, returning to the first general considerations, I repeat that it is the Sun which is the center of the heavenly revolutions of the five planets: Saturn, Jupiter, Mars, Venus and Mercury; and it will also be the center of the Earth’s motion if we can manage to place it in the heavens.  

Then, as regards the Moon, it has a circular motion around the Earth, from which (as I said) it cannot be separated in any way; but this does not mean that it fails to go around the Sun together with the Earth in the annual motion. [...] » 
 
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Simplicio, through reasoning, outlines the Copernican universe
« [...] SIMP. I still do not comprehend this arrangement too well; perhaps by drawing a diagram we will understand it better and be able to discuss it more easily. 

SALV. So be it. Indeed, for your greater satisfaction and amazement, I want you to draw it yourself, and see that you understand it very well, even though you think you do not grasp it; by merely answering my questions, you will draw it to the last detail. So, take a sheet of paper and a compass, and let this white paper be the immense expanse of the universe where you have to locate and arrange its parts in accordance with the dictates of reason. First, without my teaching it to you, you firmly believe the Earth to be located in this universe; so, take a point of your own choosing around which you understand it to be located and mark it with some symbol. 

SIMP. Let this, which is marked A, be the location of the terrestrial globe. 

SALV. Very well. Second, I know you know very well that the Earth is neither located inside the Solar body, nor contiguous to it, but is separated from it by a certain distance; so, assign to the Sun some other place of your choice, as far from the Earth as you wish and mark this too. 

SIMP. It is done; let the location of the Solar body be this, marked O. 
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Simplicio positions the inferior planets
SALV. Having fixed these two, I want us to think about accommodating the body of Venus in such a way, that its state and motion can satisfy what sensory appearances show us about them. So, recall what, from previous discussions or your own observations, you understand to occur in regard to this star; then assign to it the position you deem appropriate. 

SIMP. Let us suppose the truth of the appearance which you related, and which I also read in the booklet of conclusions: that is, that this star never recedes from the Sun more than a determinate interval of little more than 40 degrees, so that it not only never reaches opposition to the Sun, but not even quadrature, nor so much as the sexstile configuration; further, that it appears sometimes 40 times larger than at other times, namely, largest when it is in retrograde motion, and approaches evening conjunction with the Sun, and smallest when it is in direct motion and approaches morning conjunction; moreover, that it is true that when it appears largest it shows a horned shape, and when it appears smallest, it is seen perfectly round. Given that all these appearances are true, I do not see how we can escape the conclusion that this star revolves in a circle around the Sun; for this circle cannot in any way be said to enclose or to contain within it the Earth, nor to be below the Sun, namely, between it and the Earth, nor to be above the Sun.  

This circle cannot enclose the Earth, because then Venus would sometimes come into opposition to the Sun; it cannot be below the Sun, because then Venus would appear sickle-shaped at both conjunctions with the Sun; and it cannot be above the Sun, because then it would appear always round and never horned. So, for its position I will mark the circle CH around the Sun, without making it enclose the Earth. [...] »
« [...] SALV. Having accommodated Venus, you should think about Mercury; as you know, the latter stays always near the Sun, and recedes from it much less than Venus does: as so consider what place is best for it. 

SIMP. There is no doubt that, since it imitates Venus, a very appropriate place for it will be a smaller circle inside that of Venus and also around the Sun; a very conclusive argument or reason for this, especially for its vicinity to the Sun, is the vividness of its shining, which is greater than that of Venus and the other planets. So, on this basis we can draw its circle, marking it with letters BG.
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Simplicio positions the superior planets
SALV. Where, then, shall we place Mars? 

SIMP. Because Mars reaches opposition to the Sun, it is necessary that its circle enclose the Earth. But I see that it must necessarily enclose the Sun as well; for when this planet reaches conjunction with the Sun it would appear horned, like Venus and the Moon, if it were not beyond the Sun but rather in between; however, it always appears round. Therefore, its circle must enclose both the Earth and the Sun. Moreover, I remember your having said that when it is in opposition to the Sun it appears 60 times larger than when it is near conjunction: so, I think these appearances will agree very well with a circle around the center of the Sun and enclosing the Earth, which I am now drawing and marking DI. Here, at the point D, Mars is closest to the Earth, and in opposition to the Sun; but, when it is at the point I, it is in conjunction with the Sun and farthest from the Earth. Finally, the same appearances are observed in regard to Jupiter and Saturn, although with much less variation for Jupiter than for Mars, and still less with Saturn than with Jupiter; so, I think I understand that these two planets will also be very adequately accommodated by means of two circles also around the Sun. The first one is for Jupiter and is marked EL; the other larger one is for Saturn and is labelled FM. 

SALV. So far you have conducted yourself splendidly. Now (as you can see) the variation in distance for the three superior planets is measured by an amount twice the distance between the Earth and the Sun; hence, the variation is greater for Mars than for Jupiter, since Mars’s circle DI is smaller than Jupiter’s circle EL; similarly, because EL is smaller than Saturn’s circle FM, the variation is even less for Saturn than for Jupiter; this corresponds exactly to observations. What remains for you now is to think about the place to assign to the Moon. 
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Considerations on the Moon
SIMP. Let us use the same argument, which seems to me to be very conclusive. Because we see the Moon reach both conjunction and opposition with the Sun, it is necessary to say that its circle encloses the Earth; but we must not say that it encloses the Sun, because then near conjunction it would not appear horned but always round and full of light; furthermore, it could never produce, as if often does, an eclipse of the Sun by coming between it and us. Therefore, it is necessary to assign to it a circle around the Earth, such as this marked NP; thus, when positioned at P, from the Earth A it appears in conjunction with the Sun, and so can eclipse it sometimes; and when located at N it is seen in opposition to the Sun, and in this configuration it can come into the Earth’s shadow and eclipse itself. [...] »
« [...] SALV. What shall we do now with the fixed stars, Simplicio? Do we want to spread them in the immense space of the universe, at different distances from any determinate point? Or do we want to place them on a surface extending spherically around its center, such that each of them is equidistant from the same center? 

SIMP. I would rather follow an intermediate path. I would assign them an orb constructed around a determinate center and contained between two spherical surfaces, namely, a very high concave one and another convex one below it; and I would place the countless multitude of stars between them, but at different heights. This could be called the sphere of the universe, and it would contain inside it the orbs of the planets we have already drawn. 
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Last are the Earth and the fixed Stars
SALV. So far, then Simplicio, the heavenly bodies have been arranged jus as in the Copernican system, and you have done this yourself. Moreover, you have assigned individual motions to all except the Sun, the Earth and the stellar sphere; to Mercury and Venus you have attributed a circular motion around the Sun, without enclosing the Earth; you make the three superior planets, Mars, Jupiter and Saturn move around the same Sun, encompassing the Earth inside their circles; then the Moon can move in no other way but around the Earth, without enclosing the Sun; and in regard to these motions you again agree with Copernicus.  

Three things now remain to be assigned to the Sun, Earth and stellar sphere; that is, rest which appears to belong to the Earth; the annual motion along the zodiac, which appears to belong to the Sun; and the diurnal motion, which appears to belong to the stellar sphere, and to be shared by all the rest of the universe, except the Earth.  

Since it is true that all the orbs of the planets namely Mercury, Venus, Mars, Jupiter and Saturn move around the Sun as their center, it seems much more reasonable that rest belongs to the Sun than to the Earth, in as much as it is more reasonable that the center of moving spheres rather than any other point, away from this center, is motionless: therefore, leaving the state of rest for the Sun, it is very appropriate to attribute the annual motion to the Earth, which is located in the middle of moving parts; that is between Venus and Mars, the first of which completes its revolution in nine months and the second in two years.  

If this is so, then it follows as a necessary consequence that the diurnal motion also belongs to the Earth: for if, the Sun were standing still and the Earth did not rotate upon itself but only had the annual motion around the Sun, then the cycle of night and day would be exactly one year long; that is, we would have six months of daylight and six months of night, as we have stated other times.  

So you see how appropriately the extremely rapid motion of 24 hours, is taken away from the universe, and how the fixed stars, which are so many Suns enjoy perpetual rest like our Sun.  

Notice also how elegant this first sketch is for the purpose of explaining why such significant phenomena appear in the heavenly bodies. [...] »
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Selected passages from: Galilei, Galileo, «Le opere di Galileo Galilei : edizione nazionale sotto gli auspici di sua maestà il re d'Italia». Volume VII, see: Liber Liber. 

From: "Progetto Manuzio", an initiative of Liber Liber. A non-profit cultural association, open to everybody who is willing to collaborate. It promotes and circulates electronic versions of all kind of artistic and intellectual expressions. For more information see: http://www.liberliber.it/

Bibliography: Galileo – On the World Systems - translated by Maurice A. Finocchiaro- University of California Press