In particular, at the moment I want to do Sunrise, Sunset, Moonrise, Moonset and Moon phase calculations
which all require a good definition of Pi to as many decimal places as possible.

I've already ported this:
http://www.sci.fi/~benefon/rscalc.c
Sunrise/set times code to the Sony Playstaton Portable (still using C)
which is not that hard because the PSP has an FPU, and float variables.
Here is the Moonrie/Moonset/Moon phase calc working:


That one was ported from Commodore 64 style BASIC which STILL use floats (as did the C64).

I've no idea how to reproduce the same programs with a pic and no FPU.
Also, I have similar ideas for GPS calculations where coordinates have many decimal places.
Cheers, Art.