Saturn's tropical year?
A quick skim of a few papers on Earth's tropical year, e.g. M.J.White, suggests to me that existing formulas are best-fit to data, not analytic.  How Saturn's ellipticity of orbit &amp; axial precession(s) play into this is beyond my learning, but I'm getting the feeling that "it just is," and that any number of cofactors such as planetary layers, densities, and so on might affect what's being observed.