Scientific Document
Lewis’s Astronomical Observations and Calculations
Lewis's records of astronomical observations taken throughout the expedition to determine geographic coordinates. Though many calculations contained errors (later corrected by mathematician Ferdinand Hassler), these observations formed the scientific basis for mapping the expedition's route and establishing the geography of the American West.
Lewis was trained in celestial navigation by Andrew Ellicott and Robert Patterson before the expedition. Using a sextant, chronometer, and artificial horizon, he recorded observations of the sun, moon, and stars at numerous points along the route to calculate latitude and longitude. The observations were essential for Clark’s mapmaking. After the expedition, the raw data was sent to mathematician Ferdinand Hassler for computation, but publication was delayed for years. Some original observation notebooks have been lost.
