system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a sextant work ?
gpt
Sextants are designed to accurately measure the apparent angle between two visible objects.  The key component is a semi-silvered mirror that allows half of the light through and half the light to be reflected.  The reflected part is bounced off a second mirror that you can rotate.  The idea is that you rotate the mirror until two objects are aligned.  The angle scale on the bottom allows you to read off the angle between the two objects.  Even though the mirror rotates through about 60&deg;, the scale reads double the angle, i.e., up to about 120&deg;, because the angle of reflection is double the angle of the mirror.

For navigation the directly viewed object is generally the horizon and the reflected object is the sun, moon, planet or star.  For the sun and moon there are dark filters that are used to avoid damaging your eyesight.  Those objects are obviously also large, so generally you measure the angle to the bottom of their discs; trying to guess where the centre is is not precise enough.  Sextants usually have a small, low-power telescope to view through but this is not essential.