soniakeys/meeus — reverse-engineered prompt

Build me a Go library for practical astronomy calculations based on selected algorithms from Jean Meeus, Astronomical Algorithms, second edition.

I want it to be something other Go projects can import, with the algorithms split into clear topic packages that match the book chapters, plus shared helper functions for common math and angle work. Please include good package documentation so someone can find the right calculation by chapter or topic, and add tests and simple examples for the main routines.

The library should support planet position calculations using the public VSOP87 B data files. Make that part read the files from a folder set by an environment variable, and make the rest of the tests able to run even if those data files are not present.

Keep the code clean, accurate, and idiomatic Go. Please look up current Go module and documentation conventions online if you need to.