Astronomia Nova - Structure and Summary of The Astronomia Nova

Structure and Summary of The Astronomia Nova

In English, the full title of his work is the New Astronomy, Based upon Causes, or Celestial Physics, Treated by Means of Commentaries on the Motions of the Star Mars, from the Observations of Tycho Brahe, Gent. For over 650 pages, Kepler walks his readers, step by step, through his process of discovery so as to dispel any impression of "cultivating novelty," he says.

The Astronomia nova's introduction, specifically the discussion of scripture, was the most widely distributed of Kepler’s works in the seventeenth century. The intro outlines the four steps Kepler took during his research. The first is his claim that the sun itself and not any imaginary point near the sun (as in the Copernican system) is the point where all the planes of the eccentrics of the planets intersect, or the center of the orbits of the planets. The second step consists of Kepler placing the sun as the center and mover of the other planets. This step also contains Kepler’s reply to objections against placing the sun at the center of the universe, including objections based on scripture. In reply to scripture, he argues that it is not meant to claim physical dogma, and the content should be taken spiritually. In the third step, he posits that the sun is the source of the motion of all planets, using Brahe’s proof based on comets that planets do not rotate on orbs. The fourth step consists of describing the path of planets as not a circle, but an oval.

As the Astronomia nova proper starts, Kepler demonstrates that the Tychonic, Ptolemaic, and Copernican systems are indistinguishable on the basis of observations alone. The three models predict the same positions for the planets in the near term, although they diverge from historical observations, and fail in their ability to predict future planetary positions by a small, though absolutely measurable amount. Kepler here introduces his famous diagram of the movement of Mars in relation to Earth if Earth remained unmoving at the center of its orbit. The diagram shows that Mars’s orbit would be completely imperfect and never follow along the same path.

Kepler discusses all his work at great length throughout the book. He addresses this length in the sixteenth chapter:

If thou art bored with this wearisome method of calculation, take pity on me, who had to go through with at least seventy repetitions of it, at a very great loss of time.

Kepler, in a very important step, also questions the assumption that the planets move around the center of their orbit at a uniform rate. He finds that computing critical measurements based upon the Sun's actual position in the sky, instead of the Sun's "mean" position injects a significant degree of uncertainty into the models, opening the path for further investigations. The idea that the planets do not move in a uniform rate, but with a speed proportional to their distance, was completely revolutionary, and would become his second law (discovered before his first). Kepler, in his calculations leading to his second law, made multiple math errors, which luckily cancelled each other out “as if by miracle.”

Given this second law, he puts forth in Chapter 33 that the sun is the engine that moves the planets. To describe the motion of the planets, he claims the sun emits a physical species, analogous to the light it also emits, which pushes the planets along. He also suggests a second force within every planet itself that pulls it toward then sun to keep it from spiraling off into space.

Kepler then attempts to finally find the true path of the planets, which he determines is an ellipse. His initial attempt to define the orbit of Mars, far before he arrived at the ellipse shape, was off by only eight minutes, yet this was enough for Kepler to require an entirely new system. Kepler tried a number of shapes before the ellipse, including an egg shape. What’s more, he discovered the mathematical definition for the ellipse as the orbit, then rejected it, then adopted the ellipse without knowing that it was the same:

”I laid aside, and fell back on ellipses, believing that this was quite a different hypothesis, whereas the two, as I shall prove In the next chapter, are one in the same…Ah, what a foolish bird I have been!”