Today I know we have the technology to position anything accurately within a few feet. But how would sailors in old times measure distance and have maps of the sea? Why is there a nautical mile and different from the land mile?
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Speed of ship and stars. Drop a buoy in the water from the front and count the time it takes to get to aft. That gives you the knots or speed. Do that every so often and navigate by stars. Wasn’t accurate like gps but it worked well enough at the time. Also ocean currents weren’t really known of back then.
Knots traveled in a 1 minutes span. 1 knot is equal to 1 fathom. If you moved 20 fathoms in a minute you moved 180 ft. A fathom is 6 feet
Dead Reckoning: speed, heading, and distance from a known starting point.
We know what direction we’re going because we have a compass. We know how long we were pointed in that direction because we keep accurate time. We know how fast we’re going because we can drop a float and count the knots over a minute.
A nautical mile is a mathematical construction from a spherical Earth. A circle is divided into 360 “degrees”, and each degree is divided into 60 “minutes”. A nautical mile is equal to 1 minute of latitude, so there are exactly 5400 nautical miles between any point on the equator and the north pole. This allows a navigator to chart a course without doing any dumb conversions.
That’s a truly hard problem.
On short distance where you can see landmarks you could use triangulation. But on long distance where you don’t see the land you estimate your relative position by measuring how fast you go and in which direction of current are not too strong. But you will need to regularly correct it using absolute positioning strategies: using stars and time.
With the position of the sun or of the stars, you can relatively measure the latitude, usually using a sextant. It does not depends on time because the Earth “vertical axis” stays mostly parallel to itself in time (a little bit of precession but it’s negligible).
With the time you can know how much Earth rotated on itself, and therefore deduce where the sun or stars should be depending on your longitude.
Now, the trick is to keep track of time reliably on a moving ship. That’s actually the main driving factor for the invention of pocket watches and pendulum-less clocks.
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