or much of the early era of global exploration, longitude had been difficult to calculate because either an elaborate set of astronomical tables and measurements were required or an accurate time-keeping device was necessary, and neither was available until the 18th century. As early as 1610 Galileo had devised a way to determine longitude by observation of the passage of the 4 moons of Jupiter.
By the early 18th century, while still using the transit of Jupiter's moons, navigators had learned to make approximate calculations of longitude by observing the changing angular distance between the Moon and a prominent star such as Antares. But numerous careful observations and extensive calculations were required for this method to work and neither of these was really feasible on board ship or in the field.
Finally, in 1735, partially in response to an offer of £20,000 by the British government to anyone who could solve the problem of determining longitude at sea, an English clockmaker named John Harrison devised a working spring-based chronometer that showed promise of a solution. Numerous sea trials and several improved versions of Harrison's original model were required before, in 1773, Harrison had a working chronometer, durable enough for extensive field observation. It was a version of this chronometer that Lewis and Clark carried with them.
--John Logan Allen