Celestial navigation uses the Sun, Moon, stars, and sometimes visible planets as external reference points. With a sextant, the navigator aligns a celestial body with the horizon and reads the angle between them. A chronometer supplies the accurate time needed to turn that observation into latitude and longitude. Experienced sailors may estimate angles by eye or even use the angular width of a finger, but the instrument narrows the margin of error. Approximate positioning can still put a vessel on the rocks, so the smaller margin matters.
Chapter · 36:06-40:12
How a sextant turns celestial bodies into a fix
The sextant measures a celestial body's angle above the horizon, while accurate time helps convert that observation into position.