At first sight such a discovery, however interesting, would seem to have not the slightest bearing upon the sailor's craft, or upon the commercial progress of one nation or another. But Galileo quickly saw in it the promise of great practical usefulness. The question of the determination of the place of a ship when in the open ocean really resolved itself into this: How could the navigator ascertain at any time what was the true time, say at the port from which he sailed? As already pointed out, it was possible, by observing the height of the sun at noon, or of the pole-star at night, to infer the latitude of the ship. The longitude was the point of difficulty. Now, the longitude may be expressed as the difference between the local time of the place of observation and the local time at the place chosen as the standard meridian. The sailor could, indeed, obtain his own local time by observations of the height of the sun. The sun reached its greatest height at local noon, and a number of observations before and after noon would enable him to determine this with sufficient nicety. But how was he to determine when he, perhaps, was half-way across the Atlantic, what was the local time at Genoa, Cadiz, Lisbon, Bristol, or Amsterdam, or whatever was the port from which he sailed? Galileo thought out a way by which the satellites of Jupiter could give him this information. For as they circle round their primary, they pass in turn into its shadow, and are eclipsed by it. It needed, then, only that the satellites should be so carefully watched, that their motions, and, consequently, the times of their eclipses could be foretold. It would follow, then, that if the mariner had in his almanac the local time of the standard city at which a given satellite would enter into eclipse, and he were able to note from the deck of his vessel the disappearance of the tiny point, he would ascertain the difference between the local times of the two places, or, in other words, the difference of their longitudes. The plan was simplicity itself, but there were difficulties in carrying it out, the greatest being the impossibility of satisfactorily making telescopic observations from the moving deck of a ship at sea. Nor were the observations sufficiently sharp to be of much help. The entry of a satellite into the shadow of Jupiter is in most cases a somewhat slow process, and the moment of complete disappearance would vary according to the size of the telescope, the keenness of the observer's sight, and the transparency of the air. As the power and commerce of Spain declined, two other nations entered into the contest for the sovereignty of the seas, and with that sovereignty predominance in the New World of America--France and England. The problem of the longitude at sea, or, as already pointed out, what amounts to the same thing, the problem how to determine when at sea the local time at some standard place, became, in consequence, of greater necessity to them.