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The schools of Thought in Astronomy |
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« ... the reader should be aware that there are two schools of thought among astronomers, one distinguished by its chief, Ptolemy, and by the assent of the large majority of the ancients, and the other attributed to more recent proponents. The former treats the individual planets separately and assigns causes to the motion of each in its own orb, while the latter relates the planets to one another and deduces from a single common cause those characteristics which are found to be common to their motions. Copernicus, with Aristarchus of remotest antiquity, ascribes to the translational motion of our home, the Earth, the cause of the planets appearing stationary and retrograde. Tycho Brahe, on the contrary, ascribes this cause to the Sun, in whose vicinity, he says, the eccentric circles of all five planets are "connected" by a kind of "knot" (not physical, as logic would suggest, but only quantitative). This knot, he says, revolves about the motionless Earth, along with the Solar body. Kepler … my aim,
in the present work, is chiefly to reform astronomical theory (especially
of the motion of Mars) in all three forms of hypotheses (Ptolemy, Copernicus
and Tycho), so that our computations, from the tables, correspond to the
celestial phenomena. This was not possible up to now. In fact, in August
of 1608, Mars was a little less than four degrees beyond the position given
by calculation from the Prutenic Tables. In August and September 1593,
this error was a little less than five degrees, while in my new calculation
the error is entirely suppressed ... »
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| Celestial phisics | |
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« ... Meanwhile, although I place this goal first and pursue it cheerfully, I also make an excursion into Artistotle’s "Metaphysics", or rather, I inquire into celestial physics and the natural causes of the motions. The eventual result is the formulation of very clear arguments showing that only the opinion of Copernicus concerning the world (with a few small changes) is true, that the other two are false … My first step in investigating the physical causes of the motions was to demonstrate that the planes of all the eccentrics intersect in no other place than the very centre of the solar body (Sun observed), contrary to what Copernicus and Brahe thought .... Whether it is the Earth or the Sun that is moved, it has certainly been demonstrated that the body that is moved, is moved in a non uniform manner; that is, slowly when it is farther from the body at rest, and more swiftly when it has approached this body .. old Ptolemy must be turned down. Who would believe that there are as many theories of the Sun (so closely resembling one another that they are in fact equal) as there are planets, while, for Brahe, a single Solar theory suffices for the same task. It is the most widely accepted axiom in the natural sciences that nature makes use of the fewest possible means (to be explained). That Copernicus is better able than Brahe to deal with celestial physics is proven in many ways. For Brahe, no less than for Ptolemy, besides that motion which is proper to it, each planet is still actually moved with the Sun’s motion, the two being mixed into one, the result being a spiral. That it results from this that there are no solid crystalline spheres, Brahe has demonstrated most firmly ... » |
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| 1) | Copernicus, on the other hand, entirely removed this non planetary motion assigning its cause to a deception arising from the circumstances of observation |
| 2) | if there are no celestial spheres, the conditions under which the intelligences and moving souls must operate are made very difficult, since they have to attend to so many things to introduce the planets within the permitted motions. They would, at least, have to attend at the same time to the principles, centres and periods of the motions. But if the Earth is moved, the most of this can be done with "physical rather than animated" faculties, namely magnetic ones. |
| 3) | for, if the Earth is moved, it has been demonstrated that the increases and decreases of its velocity are governed by its approaching towards and receding from the Sun. In fact the same happens with the rest of the planets: they are urged on or held back according to the approach toward or recession from the Sun. So far, the demonstration is geometrical. And now, from this very reliable demonstration, the conclusion is drawn, using a physical conjecture, that the source of the five planets’ motion is in the Sun itself. Therefore, it is likely that the Earth moves, since a likely cause is the cause of its motion. |
| 4) | on the contrary, that the Sun remains in place, in the centre of the world, is shown by its being the source of motion for at least five planets. |
| 5) | now let us consider the bodies of the Sun and the Earth, and decide which is better suited to being the source of motion for the other body. Does the Sun, which moves the rest of the planets, move the Earth or does the Earth move the Sun which moves the rest, which is many times greater? Are we forced to admit that the Sun is moved by the Earth? No, as this is preposterous: we must allow the Sun to be fixed and the Earth to move. |
| 6) | what shall be said of the motion’s periodic time of 365 days, intermediate in quantity between the periodic time of Mars of 687 days and that of Venus of 225 days? Does not the nature of things cry out, with a great voice, that the circuit in which these 365 days are used up, also occupies a place intermediate between those of Mars and Venus about the Sun, and thus, itself also encircles the Sun, and hence that this circuit is a circuit of the Earth about the Sun, and not of the Sun about the Earth? |
| 7) | for other metaphysical arguments that favour the Sun’s position in the centre of the world, derived from its dignity or its illumination, see my little book, just mentioned, on Copernicus; there is also something in Aristotle’s "De coelo", Book II, in the passage on the Pythagoreans, who used the name "fire" to signify the Sun. I also have touched this issue in the "Astronomiae pars optica", ch. 1, p. 7 and also ch. 6, especially p. 225. |
| 8) | but on the Earth’s being suited to a circular motion in some place, other than the centre of the world, you will find a metaphysical argument in ch. 9, p. 322 of this book ["Astronomia Nova"] |
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| The shape of the orbits | |
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« ... My exhausting task was not complete. I had a fourth step yet to make towards the physical hypotheses. By most laborious proofs and by computations on a very large number of observations, I discovered that the course of a planet in the heavens is not a circle, but an oval path, perfectly elliptical. Geometry gave assent to this and taught that such a path will result if we assign a law ["librandi corpus"] to the motion of the planets, so that the velocity of their motion is proportional [inverted] with their distance along a straight line extended towards the Sun. Not only this, but also the correct eccentric equations, agreeing with the observations, resulted from such a law. Finally, the pediment was added to this structure
and proven geometrically: that it is in the order of things for such a
law ["librationem"]
to be the "natural"
result of a magnetic faculty. Consequently, these motion capacities, belonging
to the planets individually, are shown with great probability to be nothing
but properties of the planetary bodies themselves, like the magnet’s property
of seeking the pole and catching up iron. As a result, every detail of
the celestial motions is purely "natural",
that is magnetic, with the sole exception of the whirling of the solar
body as it remains fixed in its space. For this, a "vital
faculty" [not
only "natural"],
seems required ... »
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| Johanne Keplero - "Astronomia Nova" - Introductio in hoc opus - 1609 | |
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| The Harmony of Kepler’s Cosmos | |
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