The Astronomia nova: “One Sustained Argument”
In his classic The Sleepwalkers, Arthur Koestler said this about the Astronomia nova:
Kepler was incapable of exposing his ideas methodically, text-book fashion; he had to describe them in the order they came to him, including all the errors, detours, and the traps into which he had fallen. The New Astronomy is written in an unacademic, bubbling baroque style, personal, intimate, and often exasperating. But it is a unique revelation of the ways in which the creative mind works.
Scolarship has decisively rebutted Koestler’s description. Gingerich first suggested this, after examining some of Kepler’s unpublished manuscripts:
Most commentators have assumed, because of Kepler’s sequential and at times autobiographical style, that Kepler has spared no detail in the chronicle of his researches. Examination of the manuscript material … shows, on the contrary, that the book evolved through several stages and represents a much more coherent plan of organization than a mere serial recital of his investigations would allow.
Stephenson’s Kepler’s Physical Astronomy stated this more forcefully:
This profoundly original work has been portrayed as a straightforward account of converging approximations, and it has been portrayed as an account of gropings in the dark. Because of the book’s almost confessional style, recounting failures and false trails along with successes, it has in most cases been accepted as a straightforward record of Kepler’s work. It is none of these things. The book was written and (I shall argue) rewritten carefully, to persuade a very select audience of trained astronomers that all the planetary theory they knew was wrong, and that Kepler’s new theory was right. The whole of the Astronomia nova is one sustained argument, and I shall make what I believe is the first attempt to trace that argument in detail.
Donahue says this in the introduction to his translation:
That is, although Kepler often seems to have been chronicling his researches, the Astronomia nova is actually a carefully constructed argument that skillfully interweaves elements of history and (it should be added) of fiction. Taken as history, it is often demonstrably false, but Kepler never intended it as history. His introduction to the “Summaries of the Individual Chapters” makes his intentions abundantly clear. Caveat lector!
Finally, Voelkel dug deeper into how and why Kepler composed the book as he did. Legal obstacles intervened. And Kepler’s correspondence with David Fabricius showed him what aspects of the “new astronomy” would prove most difficult for his fellow astronomers to swallow.
Even so, the Astronomia nova roughly follows the path that Kepler took. In broad strokes:
- True Sun:
- Use this instead of the mean sun (aka center of Earth’s orbit).
- Imitation of the Ancients:
- Aka the vicarious hypothesis. A model for an eccentric circular orbit for Mars, with equant. Good for longitudes, but not distances or latitudes.
- Earth’s Orbit:
- An eccentric circle with an equant. Also, bisection of eccentricity: the center of the orbit is midway between the sun and the equant.
- Speed Laws:
- The inverse distance law, giving way to the area law.
- The Ellipse:
- First realization: the orbit is an oval. Then many false starts, crowned finally with success.
Voelkel reveals the main deviations between this outline and the actual history. (1) Kepler started investigating Earth’s orbit before his work on the vicarious hypothesis. (2) In general, the phases were more entangled. (3) Far from “including all the errors, detours, and the traps” (as Koestler said), Kepler exercised selection with a purpose. As he put it himself: “What success came of that labor [i.e., his investigations], it would be boring and pointless to recount. I shall describe only so much of that labor of four years as will pertain to our methodical enquiry.”
Diagonal Argument 