how does a sextant work?
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It’s like a protractor…**FOR THE STARS!**
Seriously though, that’s what it is. It helps you sight two different far away objects (typically 2 stars, or the horizon and a particular star), and then has little gradations so you can measure how many degrees apart they are from where you’re standing.
The main use of a sextant is to figure out your latitude (that is, how far you are from the equator). In the northern hemisphere, there is a star that’s almost exactly at the north pole. The angle between that star and the horizon gives you how far you are from the equator. In the southern hemisphere, there is unfortunately no convenient star, but you can figure it out by taking the intersection of a couple of lines made by a couple constellations. A similar method is to measure the height of the sun at noon (90 minus this number is your latitude), though obviously it’s somewhat dangerous to look directly at the sun without a filter.
Sextants may also be used more generally to measure distance between any pair of celestial objects – planets, stars, the moon, etc., which together with complex tables can be used to figure out other stuff like the time and date, or your approximate longitude.
They can also be used to figure out your distance from shore if you find the angular distance between two objects (like a pair of lighthouses, or a lighthouse and a church spire) whose straight-line distance you know.
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.
The key parts are a hinged, rotating arm and a half-mirror or semi-silvered mirror. That mirror lets you see both straight through and also see a reflected view. The reflected view is of another mirror attached to the rotating arm. The arm leads to a curved scale of angles, like a protractor, and its mirror is angled so that when the scale reads zero, the mirrored view is exactly the same as the straight-through view.
For navigation you’re generally measuring angles above the horizon. You start with the scale at zero and point up at the object you want to measure. Then you gradually point the sextant lower while rotating the arm so as to keep the object in the mirrored view. When you see the horizon in the straight-through view, you very carefully align it exactly with the object in the mirrored view. Now the scale shows the angle between the horizon and the object.
Often sextants will have a low-powered telescope to look through but sometimes it’s a simple tube; it’s hard to use if the field of view is too narrow. Semi-silvered mirrors show the straight-through and mirrored images superimposed on each other whereas a half mirror might shown the straight-through view on the left and the mirrored view on the right. There are also dark filters that need to be flipped into place when the object is very bright, especially for the sun.
The angle scale reads exactly double the measure of a normal protractor because of the way light bounces off mirrors. A sextant rotates around 60° (a sixth of a revolution, hence sex-tant) but the scale goes to 120°
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