A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: John D. Howard
Date: 2017 Aug 25, 08:08 -0700
I re-read your message and decided to answer your question - Can one determine their Lat / Lon with only a sextant. The answer is No.
The sextant is only one tool used for position finding. What is needed in addition is an almanac of some kind and a way to tell time. Without a way to tell the time you cannot figure longutide because for all practical purposes longitude is time.
The almanac can be as simple as a one page table of declination of the sun and the equation of time. The way to tell time can be a watch, clock, radio, or any known event - like an eclipse.
I assume you want to learn cel-nav as a modern navigator would, and not as a shipwrecked person on Gillian's Island. Most people know about where there are in general terms, and that is good enought for an Assumed Position. The modern way of cel-nav is to pick a point ( lat / lon ), figure what they would have seen at the time of the sextant sight and then compare what they figured to what they did see and figure how big the difference is. The difference is called the intercept - you move from the assumed lat / lon the distance and direction of the intercept to a new point and say you should be somewhere along a line that includes the new point ( called a line of position )
The assumed position can be anywhere but is better if it is close to where you are. There was a thread on NavList a few years back about finding a position without a good AP. From a assumed point in the middle of the USA, I figured the person was on the west coast - then from an AP in California I figured North west. My third and last try I used the middle of Washington to get the persion's position in a neighborhood in Seatle.
I hope this helps and welcom aboard.