Feature idea: invert calculation for ET reference frame.

[jskrzypek]

It would be neat to also be able to invert the reference frame and figure out the (non-Earth) date of your next Earth birthday.

Of course, this would require adopting or establishing a convention for calendar dates on the celestial body of your choice, though perhaps such a convention already exists.

[emilyaviva]

I've thought a little bit about how to do that. The issue is timekeeping off Earth is already exceedingly difficult to keep track of. There's a good convention for Martian sols, but nowhere else really has a system that's anywhere near as developed. I can't even think what the difficulty of something like a Ganymedan calendar would have to look like (and if I remember correctly, Arthur C. Clarke more or less hand waved this away in 2061 and 3001).

I suppose one could do it with Mars, but to be quite honest the Wikipedia article https://en.wikipedia.org/wiki/Timekeeping_on_Mars makes me already want to heave great heavy sighs and bury my head in my hands.

[jskrzypek]

Hmm, I was looking into the proposed InterPlanetary InterNet (IPIN) stuff that now looks to be dormant. It looks like there are people who have worked on the problem in the past (cf. https://www.eecis.udel.edu/~mills/ipin.html), but I'm not sure if they actually have a proper working solution.

There is one relatively easy-to-implement possibility that is available to us since a) there aren't established/accepted conventions, and b) we aren't forced to couple the algorithms to practical or useful real-world implications. We could just pick a universally applicable (or pan-solarly applicable at least 😄) definition, and then take that as the base of the calculation for conversion between any two bodies. I'll add a proposal for such in a following comment.

[jskrzypek]

The basics terms seemed simple to define... or at least they did until I started to trip over how to define a day in the context of tidal locking. Astronomy is difficult.