How did Lewis and Clark use the sextant?

Answer 1

Lewis and Clark did not use a sextant, instead they used what was called a "Hadley's quadrant" or octant which was a predecessor of the sextant. The octant was developed around 1730 independently by John Hadley in England and Thomas Godfrey in Philadelphia. Two others who created octants during this period were Caleb Smith in England and Jean-Paul Fouchy in France.

On July 22, 1804, Lewis described the instrument, "A common octant of 14 inches radius, graduated to 20' which by means of the nonius was divisible to 1' half of this sum, or 30" was perceptible by means of a micrometer this instrument was prepared for both the fore and back observation; her error in the fore observation is 2 degrees+ & ____ and in the back observation 2 degrees II' 40.3" +."

A small sighting telescope was mounted on the frame along one side. One large index mirror was mounted at the point of rotation of the index arm with a smaller horizon mirror on the frame in the line of sight. The horizon mirror allows the observer to see the image in one half of the view and to see a distant object in the other half. A pivoting shade was mounted at the vertex of the instrument to allow one to observe a bright object. By moving the index arm, the index mirror can be made to reveal any object up to 90° from the direct line of sight. When both objects are in the same view, aligning them together allows the navigator to measure the angular distance between them.