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On this day in Lewis & Clark history...

At the mouth of the Ohio River, Lewis records equal altitudes, a method where a celestial body such as a star is measured at two different times. From that, a line of position can be calculated.

From the journals...

Took equal altitudes
Altitude Art. Horzn. & Sectns. 41° 26' 37" Sextant Error 8' 45"—

Meriwether Lewis


Old triangular navigation device