how does a sextant work?
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The point of a sextant is to measure angles. Usually that angle is something like “how many degrees above the horizon is the sun” or “how many degrees above the horizon is that star” or, if you hold it sideways, “how many degrees apart are those two things.” It’s very similar to a telescope on a swivel mount with an angle measurement, except it’s got some extra mirrors and filters to make it practical to measure stuff like the angle of the sun without you having to stare at the sun.
It turns out that knowing how high above the horizon the sun is can tell you stuff like your latitude (if you know when it’s noon, and you can measure the exact angle of the sun above the horizon, some quick math can tell you your latitude). And if you know where stars are supposed to be and exactly what time it is, you can also work out your longitude.
That “exactly what time it is” bit is important. It’s easy to tell when it’s noon (keep taking reading, the highest reading was noon). That makes telling your latitude pretty easy. But working out the longitude with a sextant was really hard before they invented clocks that worked on ships, which is a big part of why ocean exploration took off in the 1500s.
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