how does a sextant work?
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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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