The sextant is a double reflecting system that uses 2 mirrors to bring the celestial body targeted to the observer's horizon. It measures the angle of celestial bodies above the horizon from the observer's position. Normally, sextants are handheld and has a fixed telescope leveled at the observer's horizon.
On the sextant, it has a movable radial arm that slide against an arc scaled in degrees. The arm is adjusted till a known star's image is reflected from the index mirror--horizon mirror--down the telescope, until it lines up with the horizon. The radial arm's position on the scale gives the star's elevation.
Navigators at work!
An actual sextant .
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