In theory, a sextant can be very accurate. In reality, they are off by degrees. They work on the assumption the earth is round. It isnt, hence the need for a datum used by GPS to account for anomalies in the earth as a perfect sphere.<br /><br />They are accurate enough to get you near where you want to be. After that you figure out where the port is. In Christophers day, they sometimes had to sail up and down the coast to find the port, but the sextant got them to the general area of the coast.
He set off from the Med, not far from my own boating grounds, so he did well. Local fishermen are periodically in the news here because they've gone out twenty miles or so and got lost - found a few days later after a search - thought their mobile phones would work well enough so they didn't bother with buying a vhf!<br /><br />My turn next.
<br /><br />Seriously, you have to start slow and get some experience. I have a good chart plotter, it's one of the best investments I ever made for the boat.<br /><br />I remember hearing a boater calling the Coast Guard one morning because he had gotten lost and ran out of gas. He had no idea where he was. He was able to tell them the heading on his compass but that's not any help at all unless you can identify what it's pointing to. Then when they connected him with a tow service he declined because he didn't have enough money. I guess someone eventually found him and towed him in. 
