I once actually knew how to do celestial navigation, but have no recollection of how precise the sextants were.
According to the Wikipedia writeup on sextants, a skilled navigator can determine position to within about a quarter mile.
https://en.wikipedia.org/wiki/Sextant
I did own a sextant and played with is for some time.
My interest was Lewis and Clark’s use of a sextant and artificial horizon. Close at best.
Note that Worsely successfully took them to Elephant Island (not all that big, 20 mi long and mostly in sight), then 800 miles to South Georgia Island.
Working from a small open boat on rough seas.
Amazing.