Credit: Cambridge University Library
‘Whereas it is well known by all that are acquainted with the Art of Navigation, That nothing is so much wanted and desired at Sea, as the Discovery of the Longitude, for the Safety and Quickness of Voyages, the Preservation of Ships and the Lives of Men : And whereas in the Judgment of Able Mathematicians and Navigators, several Methods have already been Discovered, true in Theory, though very Difficult in Practice, some of which (there is reason to expect) may be capable of Improvement, some already Discovered may be proposed to the Publick, and others may be Invented hereafter : And whereas such a Discovery would be of particular Advantage to the Trade of Great Britain, and very much for the Honour of this Kingdom;…’
One idea was to use the positions of Jupiter's moons as an astronomical clock. Their positions had been observed and recorded for many years. In 1714 telescopes were not very good and at sea it was impossible to hold them steady. In cloudy weather they could not be seen anyway.