There are three ways to get a celestial endorsement.
- Buy both Bowditch, and take about 4 or five years and actually learn via trial and error to figure out how to pass the USCG exam.
- Do the seasources question database. Learn the exam. (,but not necessarily KNOW the material) probably a month or two of cramming.
Or 3. Attend one of the many schools that will teach you a basic understanding of fundamentals, and cram you on the 10 (or so) topics needed to show proficiency in to the USCG standards. About 10 days.
It’s up to you how hard you want to bang your head against the wall for something you will in all likelyhood never use again.
Regarding the topics of the masters level celestial navigation portion:
Initial great circle course,
Compass correction (either or deviation, CE, or gyro error)
Finding compass error by azimouth amplitude,
Calculating Local apparant noon by DR and sight.
Eta and etd for arrival notofocation.
Vessel documentation and crew list, and masters responsibility for crew articles, and foreign voyage responsibilities.
And one or two more that I can’t recall.
So, yes you can do this on your own. But unless you have a stellar master or mate to bounce questions off of you will likely be pounding your head against the wall for a bit.