Kepler’s introduction to Astronomia Nova is somewhat of a lengthy ramble: 20 pages skipping from topic to topic and back again. In addition, he lays out somewhat of a preview of the structure of the book itself. But since we’ll be digesting that chapter by chapter anyway, I won’t discuss most of it and will instead sift through for the important parts. In addition, my summation won’t strictly follow the format Kepler does as I will try to group ideas on a similar topic.
The first important topic Kepler hits on is the
schools of thought among astronomers, one distinguished by its chief, Ptolemy… and the other attributed to more recent proponents [Copernicus], although it is the most ancient.
What Kepler is referring to here is the geocentric model, championed by Ptolemy and most other astronomers, contrasted with the heliocentric model which is generally attributed to Copernicus although its roots are much more ancient with Aristarchus of Samos (230 BC) being the first recorded example. But beyond these two models, Kepler notes that his employer, Tycho Brahe, had developed his own model.
These three models are,
for practical purposes equivalent to a hair’s breadth, and produce the same results.
This is truly a remarkable statement, that three fundamentally different models produce results that are so similar, that observational data could not tell them apart at the time. But distinguishing between these three competing hypotheses is the entire goal of Astronomia Nova, refining them where he may, and rejecting them when he must.
The chief test by which he proposes to do this is the study of Mars which had previously been the most challenging. He notes:
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 of 1593 this error was a little less than five degrees, while in my new calculation the error is entirely removed.
The Prutenic tables were a set of predictions for the positions of planets (known as ephemeris). To be 4-5º off is quite large. The Sun and moon are each about 1/2º in angular diameter, so this means 8-10 times the width of the full moon off! This would be easily distinguishable to the naked eye, let alone Brahe’s instruments that could read differences in position around 500 times smaller.
Although Kepler doesn’t expound on the three models, understanding them is fundamental to reading this work as Kepler assumes that the reader understands them well. As such, I’ll be devoting some posts to that shortly and will edit them into this post as I do.
Another idea that Kepler mentions here is the causes of motion. Plato’s motions were driven by celestial spheres which carried the various objects in the sky. Various astronomers in Plato’s time had different ideas on what these spheres were with some (such as Anaximander and Anaximenes) suggesting they were solid but transparent crystalline spheres. Aristotle believed them to be an unknown fifth element (besides the alchemic ones of fire, earth, air, and water).
This of course, begs the question of what moves the spheres, but that is not addressed. Regardless, the idea of solid spheres was done away with by Brahe who observed a comet that would have passed through these spheres, shattering them.
Without a physical cause, philosophers started turning to “animate faculties.” This phrase is the same as the one used to describe the movement of animals, namely they had a spirit or soul that caused them to move. This train of thought was a large step backwards, giving life to the objects, a sort of primitive consciousness that moved them as was their nature. In addition, it complicates things by imbuing each celestial object with its own driver instead of a single, unified cause (in as much as one could say the numerous spheres and epicycles were unified).
But Kepler dismissed this asserting a new physical cause. Although Newton wasn’t around yet to have developed the idea of gravity to the extent necessary for orbital mechanics, Kepler still attributed the motions to a force, born of the sun. Even outside the Copernican, heliocentric model, this idea was catching on and was part of Brahe’s model as well. At least to an extent. Brahe still asserted that the Earth was at the center with the sun going around the Earth, but still felt the sun was the driver of motion of the planets. However, Kepler pushes for the Copernican model saying,
it is more likely that the source of all motion should remain in place rather than move.
Building on this, Kepler goes on to make another philosophical argument stating that it is absurd for the Earth to move the sun as even the Greek astronomers knew the sun to be substantially larger than the Earth. Hipparchus (190 – 120 BCE) worked out a geometrical method for attempting to measure the distance to the moon and sun, and although it didn’t quite work out, it still demonstrated qualitatively that the Sun was much larger than the Earth.
Unless we are to be forced to admit the absurd conclusion that the sun is moved by the Earth, we must allow the sun to be fixed and the Earth to move.
But what force might the sun emit to fix the planets? He suggests the mysterious force is similar to a magnetic force. While, again, not a hard and fast physical model, this was a brilliant prediction as the two forces behave very similarly. Consider these modern equations for each:
Newton’s Law of Universal Gravitation:
$$F = \frac{Gm_1 m_2}{r^2}$$
Coulomb’s Law:
$$F = \frac{Kq_1 q_2}{r^2}$$
With the exception of the constant and masses being replaced by charges, the two equations are fundamentally identical.
Kepler even understood that the gravitational forces should be mutually attractive meaning that while the sun would pull the planets, the planets must also pull the sun although proportionally to their masses. I haven’t worked through the math, but given that predictions of the distance and size of the sun at that time were orders of magnitude too low, I expect that, had Kepler thought this through further, it should have had a notable impact on the motion of the sun as it would have been perturbed by all the planets.
However, this raised another problem is Kepler’s mind: If the force is attractive, why then would not everything fly together in a large clump? Today we recognize that the sideways motion forces the planets to perpetually miss the body which they orbit in a manner that is self sustaining. But since the concept of inertia and gravity had not yet been developed to that point, Kepler proposed another idea: He suggested that the sun
itself sends into the space of the world an immaterial species of its body, analogous to the immaterial species of its light. This species, as a consequence of the rotation of the solar body, also rotates like a very rapid whirlpool throughout the whole breadth of the world, and carries the bodies of the planets along with itself in a gyre, its grasp stronger or weaker according to the greater density or rarity it acquires through the law governing its diffusion.
Bit of a mouthful there, but Kepler is proposing that the sun spins and while doing so, it emits some sort of particle that also has that sideways motion as it leaves the sun. As that cloud of particles spreads through the solar system, it drags the planets sideways, faster for the close in planets where the density of particles would be higher, slower further out where they have diffused.
Another fuzzy objection Kepler raises is that Ptolemy’s models often centered orbits where there was no object. Thus, Kepler again questioned what the driving force could be.
Let the physicists prove that natural things have a sympathy for that which is nothing.
Kepler then moves into preemptively tackling some of the objections he knows he will receive, spending several pages responding to those that may charge that a sun centered universe challenged the scriptures.
Chiefly, this argument is that scripture
speaks in accordance with human perception when the truth of things is at odds with the senses, whether or not humans are aware of this.
In other words, scripture simply describes how things appear, often as poetic metaphor. He likens this to how sailors would describe land as receding as they sailed away. Obviously the land does not move, but that is still how we use language. Similarly, Kepler proposes, so too do the scriptures.
Although Kepler tackles many passages that may have been raised in objection, one of the best examples of this is Psalm 104 which, in part, states:
He set the Earth on its foundations; it can never be moved.
Kepler suggests that this Psalm,
is a commentary upon the six days of creation in Genesis… For it is still true that the land, the work of God the architect, has not toppled as our buildings usually do, consumed by age and rot; that it has not slumped to one side; that the dwelling places of living things have not been set in disarray; that the mountains and coasts have stood firm, unmoved against the blast of wind and wave, as they were from the beginning.
For those that cannot accept that physical observation must triumph over literal interpretation, Kepler has harsh words.
But whoever is too stupid to understand astronomical science, or too weak to believe Copernicus without affecting his faith, I advise him that, having dismissed astronomical studies and having damned whatever philosophical opinions he pleases, he mind his own business and betake himself home to scratch in his own dirt patch, abandoning this wandering about the world.
…
As for the opinions of the pious on these matters of nature, I have just one thing to say: while in theology it is authority that carries the most weight, in philosophy it is reason.
Thus Kepler turns his reason towards the heavens.