How do you use a sextant to measure celestial altitudes of heavenly bodies (Sun, Moon, Star)??

Principle of operation of a Sextant

In 1976, a man who had never used a compass sailed 2500 miles across the Pacific. His name was Mau Piailug, a master navigator from Satawal in Micronesia. He guided the Hokulea, a traditional double-hulled canoe, from Hawaii to Tahiti to prove that ancient Polynesians intentionally settled the Pacific. Mau had no compass, sextant, or charts; he relied on ancestral knowledge of stars, ocean swells, and winds. His expertise demonstrated that ancient wayfinding was viable, sparking a renaissance of