It may work if you put the all the files (in both folders - all the .nb all the .m) as you see them in github in your HOME folder of your pc (example: c:\Users\yourname\celestial.m). That is my understanding of the source (which i quickly overviewed). If you are privy to Mathematica settings you could have a new folder found in you Mathematica path to hold them instead of HOME - if you find placing so many files in your home folder a nuisance.

You should read the ".nb" to look for any instructions on what can be done with the package and how, whereas the ".m" are meant to serve and not be read (but can be read).

You could ask the Contributor/author however GITHUB IS NON-FREE - may ask for money when you try to initiate a message or other features of github. Also it is an old project perhaps abandoned. It was made with mm 11.0 not mm 12.n, so there may be a hangup (hopefully not). If there is, that kind of question (versional fixes) is appropriate for this forum/communities. Try contacting the author first.