The Laplace limit

The previous post discussed how to solve Kepler’s equation M = E − e sin(E) using a sine series. You could also solve Kepler’s equation using a power series, which Lagrange did in 1771. Both approaches express E as a function of e and M, but from different perspectives. Bessel though of his solution as a sum of sines in M, with coefficients that depend on e. Lagrange thought of his solution as a power series in e whose coefficients involve sines in M. You can rearrange the terms of either solution into the other. The most interesting thing about the power series solution, in my opinion, is that it only converges for e less than roughly 2/3 while the sine series solution is valid for all e < 1. In astronomical terms, this means the power series solution works for the orbit of some planets but not others! In our solar system, the planets all have eccentricity well below 2/3, but not all minor planets do. For example, the orbit of Eris has eccentricity 0.4407 but the orbit of Sedna has…