Diagonal Argument

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Earth’s Orbit

Ptolemy’s solar model gives the sun an eccentric orbit, with uniform speed—no equant. Both Copernicus and Tycho adopted the uniform speed for the earth’s/sun’s orbit. As a consequence, they end up with twice the correct eccentricity. (See post 8.)

The Ptolemaic system further disguises the earth’s orbit as the epicycles of the outer planets and the deferent of Venus. (Mercury is sui generis.) The epicycles have neither equants nor eccentricities, which amounts to giving the earth uniform speed in a Sun-centered circular orbit. Venus’s deferent has both an eccentric circle and an equant.

Kepler of course would have none of this. No special treatment for the earth! He gave it an equant, with the center of the orbit situated midway between the equant and the true sun. So Kepler assigned the earth half its former eccentricity. This is known as the “bisection of eccentricity”.

How does this matter for Mars? Earth is our observing platform. If you are mistaken about your own celestial position, you will inevitably screw up how you interpret the data.

Kepler devised an ingenious triangulation scheme to determine the earth’s orbit. Mars has a period of 687 days. Observations separated by that interval thus involve Earth-Mars vectors from the variable position of the earth to a fixed position for Mars. Mentally reversing the direction of the vector gives us essential information about the earth’s orbit. I will look at how Kepler used this insight below. First, though, some more historical context.

Voelkel writes:

Kepler undertook to write the Astronomia nova in October or November 1602, long before the discovery of the ellipse and elucidation of the area law… At the time he resolved to write the book, he had written to Herwart von Hohenburg that his work on Mars had allowed him to solve the problem of the orbit of the sun (or earth). And in many ways, the Astronomia nova was as much about the earth’s orbit as it was about Mars’s orbit. [Voelkel (p.212)]

Kepler develops his theory of the earth in the first few chapters of Part III. Part II contains the vicarious hypothesis. In fact, this almost inverts the timeline. Not only the zeroth law, but the whirlpool force hypothesis pushed Kepler to investigate the earth’s orbit.

Presenting the vicarious hypothesis first served a rhetorical purpose. Kepler could say, “See? I tried to make the traditional scheme work. The failure of my Imitation of the Ancients, and the way in which it failed, indicate that there is something amiss with the existing model of Earth’s orbit.”

This innovation with the earth’s orbit met with considerable resistance. Tycho disliked it, as did Kepler’s pen pal Fabricius. Tycho felt, “If it ain’t broke, don’t fix it!” Likewise Fabricius, who wrote, “We ought not heedlessly to depart from Tycho’s well-developed solar hypothesis.” As Voelkel puts it:

…this unprecedented innovation, clashing as it did with Tycho’s solar theory and meeting Fabricius’s deep skepticism, needed an unusually sure foundation, especially because it was the ultimate justification for his physical astronomy. [Voelkel (p.236)]

The disapproval lasted—more so than for the elliptical orbit. The astronomer Giovanni Cassini, using a so-called heliometer (aka meridian line) at the Basilica of San Petronio, finally confirmed Kepler’s bisection in 1655. Heilbron (p.102–112) recounts the story. He writes:

Many astronomers objected to a solar equant. Thus began what Flamsteed, writing a generation after its resolution, called a “controversy … of no small moment”. It was this great and obscure controversy between adherents of Ptolemy’s traditional solar theory and proponents of Kepler’s “bisection of the eccentricity”, with its Copernican associations, that Cassini proposed to settle at San Petronio.

…to put the point in a few words (they are those of Astronomer Royal Flamsteed), “the Suns Excentricity is bisected as the Copernicans affirme.” This connection of ideas—the bisection of the eccentricity implies that the earth is a planet—became commonplace among Copernicans.

The Great Martian Catastrophe

In 1593 Tycho observed Mars. It was in the wrong position, by about 10 Moon diameters! The Ptolemaic and Copernican tables gave longitudes differing by about 5°, in opposite directions. The values remained off for several weeks. Gingerich called this “The Great Martian Catastrophe”.

Fixing the earth’s orbit reduced the size of the “great Martian catastrophe” tenfold. Specifically, it reduced the maximum longitudinal discrepancy from about 5° to about 0.5°, or 30′. Kepler’s area speed law plus the elliptical orbit reduced it further to about 2′.

Other simplification flowed from the modified earth’s orbit. Kepler called it “the Key to a Deeper Astronomy”.

The Two Methods

OK, now let’s look at how Kepler determined the position of Earth’s equant. The old theory had the earth move uniformly around the center of the orbit, which is displaced from the sun. Effectively, this makes the equant and the center identical. Kepler replaced this with the center midway between the sun and the equant. The figure below illustrates the first method, from Chapters 22 and 23. In Chapter 22 he shows that the equant is not the same as the center.

Earth’s Orbit: First Method

In the figure, S, C, and E are the sun, the center, and the equant; and O₁ and O₂ are two positions of the earth. Recall his triangulation trick: if the time interval between O₁ and O₂ is a multiple of 687 days, then Mars will occupy the same position M both times. The times of O₁ and O₂ are chosen so that the two angles θ are equal. Since motion is uniform about the equant, angles about the equant are measured by times. If O₁ and O₂ are separated by equal times from the aphelion and perihelion respectively, then the two angles will be equal.

Kepler also needed the line EM to be perpendicular to the line of apsides (the diameter in the figure). Again this is achieved by picking the right epochs. Altogether, this method requires observations at very particular times. We see the importance of Tycho’s treasure trove.

Now suppose that E and C were the same. Then the diagram would be symmetrical about the line EM, and angles α and β would be equal. (Mentally slide the line EM left until E coincides with C.) But Kepler found that α exceeded β by more than one degree. “Witness the great difference” he exclaimed.

In Chapter 23 he computes the relative positions of S, C, and E. The directions O₁M and O₂M are known from observation. That gives us the angles α and β. The times of the observations give him the angle θ (see above). So all relevant angles are known. I won’t go through the intricate trigonometry that computes the position of E in the line of apsides from this data. But intuitively, it is clear it uniquely determines the position: sliding E from perihelion to aphelion continuously increases α up from zero and decreases β down to zero, so there is exactly one spot where they have the right values.

Kepler found that the eccentricity (CE over the orbital radius) was 0.01834. Tycho’s value for the eccentricity was 0.03584. Half that is 0.01792, close enough to 0.01834. The bisection of eccentricity confirmed!

Chapter 24 sets out with the bold declaration, “Now that we have once made a hazard of this, we are buoyed by audacity and will begin to move more free on this battlefield.” Kepler proposes to take triplets of observations, separated by multiples of 687 days, and from them find the distances of the equant to three points of the orbit. Three points determine a circle. “If a fourth observation will be at hand, it will serve as a test.”

Earth’s Orbit: Second Method

The figure above reproduces the diagram from Chapter 24. Here κ is the fixed position of Mars. The variable Earth positions are θ,η,ε,ζ. The dashed circle is the earth’s new orbit, with the new center β bisecting the eccentricity.

The observations give the Earth-Mars directions θκ, ηκ, εκ, and ζκ, but not the distances. Earth-Equant directions are given by the times (because uniform motion). Mars-Equant directions are given by Tycho’s theory of Mars (or by the vicarious hypothesis). Consider an Earth-Equant-Mars triangle, such as △ηακ. We have all the directions, and thus all the angles. The law of sines tells us all ratios of sides. In particular, it gives us the distance αη in terms of ακ. We have this for all four Earth positions, which means we have the distances αθ, αη, αε, αζ in terms of some common unit. (For another exposition of this material, see Stephenson (pp.53–55), Voelkel (pp.105–106), or Donahue (pp.45–50).)

The distances differed, again disproving the identity of the equant with the center. The results once more agreed with the bisection of eccentricity. Kepler now had empirical proof of one of his deductions from his whirlpool force hypothesis.

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Here Comes the (True) Sun

I mentioned earlier “Kepler’s zeroth law”, a corollary to the motto:

Refer everything to the true sun, and not the mean sun. Nor should the earth receive any special treatment.

Kepler plucked several fruits from this tree. Here are two fruits of the zeroth law.

Apsides

Wrong Apsides

The solid circle is the true orbit for Mars. The dashed circle has the wrong apsidal line, passing through the mean sun instead of the true sun. X and Y are the positions of Mars in the two models at the same time. Note that the longitudes of X and Y differ little, whether viewed from the true sun or the mean sun. But the distances differ significantly.

Gingerich summarizes the situation nicely:

[T]he eccentricities assigned by Ptolemy to each of the planets are essentially vector sums of the earth’s and the planet’s eccentricities, with the result that in each case the line of apsides is slightly wrong…Copernicus did not bother to sort out the individual eccentricities, so each planetary eccentricity implicitly combines the earth’s. In addition, his apsidal lines are drawn through the mean sun… [Gingerich, Chap.19 “Kepler’s Place in Astronomy”]

So far as heliocentric longitudes are concerned, the wrong apsidal line won’t pose too serious a problem. See the figure above, which shows that solar distances are not so forgiving.

To understand this better, we need to look at acronychal risings. These are observations taken when the planet (here Mars) is in opposition to the sun. (See the figure below. ‘Acronychal’ means ‘night rising’.)

Key point: for an acronychal observation, the heliocentric and geocentric longitudes are the same. The astronomer can directly measure only geocentric longitudes. Except at opposition and conjunction, deducing the heliocentric longitude requires an assumption about the earth’s position. At conjunction, the brightness of the sun makes the planet invisible.

Kepler analysed the situation in Chapters 5 and 6. (These make slow reading; Stephenson (pp.31–39) gives a detailed discussion.)

This was a “must pass” test for the switch to the true sun. Tycho’s chief assistant Longomontanus (aka Christian Severinus) had worked out a model of Mars’s orbit that accounted for the acronychal data with an accuracy of 2 minutes, for a period of over 40 years. As Kepler explains in Chapter 7:

A hypothesis was invented which, it was proclaimed, represented all these oppositions within a distance of two minutes in longitude… It was only in the latitude at achronychal positions and also the parallax of the annual orb [i.e., observations out of opposition] that Christian got stuck… At the beginning there was great controversy between us as to whether it were possible to set up another sort of hypothesis which would express to a hair’s breadth so many positions of the planet, and whether it were possible for the former hypothesis to be false despite its having accomplished this so far over the entire circuit of the zodiac.

Chapter 6 showed the near agreement of the true sun and mean sun models for acronychal longitudes. But observations “from the side” (i.e., out of opposition) could amount to as much as 1° 20′, fourteen times the error at opposition and easily observable.

Latitudes

In post 13, I noted (following Swerdlow) that poor data bore most of the blame for Ptolemy’s complicated latitude theory. But having the orbital planes pass through the mean sun is just asking for trouble. The figure below illustrates the problem.

Latitudes and the Mean Sun

The diagram shows a cross-section of the three-dimensional geometry. O_aphelion and O_perihelion are Earth’s aphelion and perihelion, so the line through them is the ecliptic, and the mean sun is midway between O_aphelion and O_perihelion. The solid line at an angle is the orbital plane of Mars, with the inclination greatly exaggerated.

Shifting Mars’s orbital plane to pass through the mean sun, while keeping the inclination the same, gives the dashed line. Mars shifts from M to M; exactly where M lands depends on the longitude theory. Viewed from Earth’s aphelion its predicted latitude would be too large, from Earth’s perihelion too small, and from other points in the earth’s orbit also incorrect (except for two points). To fix this problem, one might naturally change the inclination, resulting in the dotted line. This dotted line represents a plane passing through the true position of Mars (M) and the mean sun. However, this fixes things only at one position M; at other points in Mars’s orbit, like M′, you’d need a different inclination. We see the cause of the rocking orbital plane.

This scheme still has a lot of leeway, since it takes three (non-collinear) points to determine a plane; an infinite number of planes pass through M and the mean sun. But it is impossible to avoid a varying inclination, simply because the mean sun does not lie in the actual orbital plane of Mars. Any plane passing through M and the mean sun will intersect Mars’s orbital plane in a line, which means it can correct matters for at most one other point of Mars’s orbit.

Copernicus’s latitude theory did not follow this scheme, but was essentially a transcription of the Ptolemy theory into heliocentric terms. It suffered from an additional drawback: the inclination depended on Earth’s position. This violates Kepler’s zeroth law.

Kepler showed if the orbital plane of Mars passes through the true sun, then you can use a fixed inclination. (The line of nodes does rotate very slowly. Kepler knew this.) He demonstrated this in three ways. I’ll outline only the first two.

Both involve finding the geocentric latitude for observations when this will equal the heliocentric latitude. When Mars is “at a limit”—that is, as far above or below the ecliptic as it gets—this will give the inclination.

Kepler noted that the highest accuracy is not required for these methods. Say M is Mars, and M′ is M projected onto the ecliptic. The distance MM′ doesn’t change that much for M near a limit.

Determining the Inclination, First Method

For the first method, suppose that the earth and the sun are equidistant from Mars, hence likewise from M′. Then MM′ appears the same whether viewed from the sun or the earth. So the geocentric and heliocentric latitudes are equal. (See Kepler’s diagram, reproduced above. Here A=Earth, B=Sun, E=Mars (my M), and C=projection of E to the ecliptic (my M′).)

How do we know when Sun-Mars-Earth form an isosceles triangle? Given the ratio of the Sun-Earth and Sun-Mars sides, (i.e., the orbital radii), basic trigonometry tells us the Sun-Earth-Mars angle. It turns out that you can use any observation where that angle is between (roughly) 60° and 72°.

Determining the Inclination, Second Method

For the second method, Kepler noted that when two planes cut one another, any two lines drawn in the respective planes to a point on the line of intersection, and perpendicular to that line, always include the same angle. (See the diagram above, also Kepler’s.) So if B is the earth, F is Mars, and D is its projection to the ecliptic, then ∠ DBF is the inclination. Therefore we need an observation when the earth lies in the line of nodes, Mars is at a limit, and the sun and Mars appear at right angles from the earth. For this method, we make no assumptions about the orbital radii. However, it requires very special observations.

Kepler consistently found an inclination close to 1.8° with all three methods and many observations. He concluded that not only is the inclination fixed, but it’s rather small. Tycho’s assistants had made a mess of things, finding not only that the orbit of Mars wobbled, but was also “fractured”, with a maximum latitude of 4.6° at one limit and 6.4° at the other.

I noted at the beginning of post 13 that Ptolemy “decoupled” longitudinal computations from latitudinal ones. Now, the speed of planet differs from the speed of its projection to the ecliptic. Moreover, the ratio of these two speeds is not constant. Ptolemy could safely ignore this issue because the effect is small; that’s true because because the inclinations are small. But with Tycho’s much greater accuracy, the matter could no longer be swept under the rug. (It’s known as the “reduction to the ecliptic”.) So in cleaning up the latitude theory, Kepler laid a crucial foundation stone for everything else.

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The Astronomia nova: “One Sustained Argument”

In his classic The Sleepwalkers, Arthur Koestler said this about the Astronomia nova: Continue reading →