I''ve taken and plotted sights on trips to Bermuda and Transatlantic. There''s a lot of art and judgement involved. Any sextant is going to have some error in it. The cheapest crummiest plastic one you can buy is likely more exact than what Columbus used - and he kept returning to the same spots on both ends of his trips. Until I was good enough to really notice and calculate the difference, I''d learn on a plastic one. They''re lighter and easier to handle than the metal ones, so your results might even be more accurate! If I had an extra $100 and a usable horizon, (I sail Long Island Sound, mostly, so there really isn''t much useable horizon) I''d go for one of the nicer plastic ones that look official but don''t cost $1,000. For $1000 you can get three
GPS receivers and plot the different positions of the bow, stern, and amidships of your boat without having to calculate anything.