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| The Copernican system does not contradict, but improves the Ptolemaic theory ... | |
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SALV. I conclude
this from observations that are very evident and hence necessarily binding.
The most palpable of these observations that exclude the Earth from this
center and place the Sun there, is the fact that all planets are found
to be sometimes closer to the Earth and sometimes farther; these differences
are so large that, for example, when Venus
is farthest it is six times farther from us than when it is closest [see
the animation: Ptolemy - Venus] and Mars
recedes almost eight times more in one position than in the other. So,
you can see here whether Aristotle was wrong by a small amount in thinking
that they are always equally distant from us.
SIMP. What, then, are the indications that their motions are around the Sun? SALV. For the three superior planets, Mars, Jupiter and Saturn, this is inferred from their being always found closet 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 [here the motivation of this event is of different nature... even if, once again we find Ptolemy, not exactly, but approximately by conveying Copernicus into a geocentric system]. 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. [...] » |
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| Some verities on the motion of Venus | |
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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 appearances 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 determined 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 sextile 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 appear 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 [...] » |
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| Only the telescope allows to observe certain phenomena | |
| « SALV.
SALV. [...]
There are then other very serious and very beautiful difficulties, which
are not easily solved by mediocre intellects, but which were understood
and explained by Copernicus: we will discuss them below, after answering
other objections which seem to undermine this position. Now, coming to
the clarifications and solutions of the three very serious objections advanced
above, I say that the first two not only do not contradict the Copernican
system, but favour it considerably and absolutely; for both Mars and Venus
do vary in apparent size in accordance with the required proportions and
Venus does appear sickle-shaped when between
us and the Sun, and in general changes in apparent shape exactly like the
Moon.
SAGR. But how could this be hidden from Copernicus and revealed to you? SALV. These thinks can be understood only with the sense of vision, which nature has not granted us in such a perfect state as to be able to discern such differences; indeed the very instrument for seeing contains impediments within itself. However, in our time God saw fit to allow the human mind to make a marvellous invention, which renders our vision more perfect by increasing its power by 4, 6, 10, 20, 30 and 40 times; as a result, countless objects that were invisible to us because of their distance or extremely small size are now rendered highly visible by means of the telescope.[…] » « [from a letter of Galileo...] Know, therefore, that about three months ago, I began to observe Venus with the instrument, and I saw her in a round shape and very small. Day by day, she increased in size, and maintained that round shape until finally, attaining a very great distance from the Sun, the roundness of her eastern part began to diminish and, in a few days she was reduced to a semicircle. She maintained this shape for many days, all the while, however, growing in size. At present she is becoming sickle-shaped, and as long as she is observed in the evening her little horns will continue to become thinner, until she vanishes. But when she then reappears in the morning, she will appear with very thin horns, again turned away from the Sun, and will grow to a semicircle at her greatest digression (from the Sun). She will then remain semicircular for several days, although diminishing in size, after which in a few days, she will progress to a full circle. Then for many months she will appear, both in the Morning and then in the Evening, completely circular, but very small in size. The quite obvious consequences that originate from the above, are very well known to you. […] » |
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| Copernicus and the telescope allow conclusions, not obvious otherwise | |
| « [...] I have observed Jupiter and Venus together several times, when they were 25 or 30 degrees away from the Sun, and the sky was very dark; when I observed them with the naked eye; Venus appeared at least 8 and perhaps even 10 times brighter than Jupiter; but when they were observed with a telescope the disc of Jupiter was seen to be at least four times larger than that of Venus and the brightness of Venus’s shine was incomparably greater than the extremely dim light of Jupiter; this derived only from the fact that Jupiter was extremely far from the Sun and from us, and Venus was close to us and the Sun. […] » | |
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| The motion of the Earth explains the retrograde motions of the "wandering stars" | |
| « [...] SALV. Sagredo, you will see them proceeding in such a way, that this supposition alone should be enough, to satisfy whoever is not so arrogant or rebellious to give his consent to all such learning. I, therefore, tell you nothing has changed in the Saturn’s motion of 30 years, nor in Jupiter’s of 12, nor in Mars’s of 2, nor in Venus’s of 9 months, nor in Mercury’s of about 80 days. Only the yearly motion of the Earth between Mars and Venus is the cause of the apparent inequalities in the motions of the mentioned 5 stars. To make things easy and fully understandable let us describe its figure. […] » | |
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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 Galilei – On the World Systems
- translated by Maurice A. Finocchiaro- University of California Press
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