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How to use a sextant
Know, if possible, your position above sea level.
If you're not using the sextant from aboard a ship at sea, you'll have to correct your sighting for your height above sea level. We'll discuss the correction later; for now, you just need to know your elevation above sea level.
Sight the horizon by looking through the horizon mirror. The horizon mirror is only partially silvered, allowing you to look through it and through the sighting scope beyond it.