Longitude (book) consults the home port clock, every hour's discrepancy between them translates into another fifteen degrees of longitude. One degree of longitude equals four minutes of time the world over, although in terms of distance, one degree shrinks from 60.15 nautical miles or 111 km [Earth's circumference being: 21,653.521 nautical miles, or 24,901.55 statute miles at the Equator], to virtually nothing at the poles. Precise knowledge of the hour in two different places at once - a longitude prerequisite so easily accessible today from any pair of cheap wristwatches - was utterly unattainable up to and including the era of pendulum
Longitude rewards Longitude Act only addressed the determination of longitude at sea. Determining longitude reasonably accurately on land was, from the 17th century onwards, possible using the Galilean moons of Jupiter as an astronomical 'clock'. The moons were easily observable on land, but numerous attempts to reliably observe them from the deck of a ship resulted in failure. For details on other efforts towards determining the longitude, see History of longitude. The need for better navigational accuracy for increasingly longer oceanic voyages had been an issue explored by many European nations for centuries before the passing of the Longitude Act in England