system
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human
ELI5: How did people start measuring distance at sea where it's water all around?

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?
gpt
This was a massive problem for ancient mariners - with no landmarks you were totally lost the moment you lost sight of shore.

Eventually two breakthroughs enabled open sea travel.

The first was the discovery of magnetic stones, and the concurrent discovery that the Earth itself was magnetic.  This lets you build a compass and find north no matter where you are.

The second is the development of celestial navigation.  There’s no *land*marks at sea, but on a clear night the sky fills with hundreds of them.  If you know what day it is and approximately how big the Earth is, you can chart what stars should be visible from what points on the globe and in what direction.

Following the sun, moon, and stars with the aid of star charts and a compass was the complex but accurate method pre-industrial sailors used to reach their approximate destination across the open ocean.

Once accurate maritime clocks were developed the system became extremely accurate.  If you know exactly what minute it is and know exactly when stars *should* rise and set at a certain location you can determine how close you are with high accuracy.

Of course, this all requires clear skies.  Many ships lost course in stormy or overcast conditions and ended up way off target.