The Earth's Longitude and the Moon

Despite Harrison's clock, the method of calculating longitude using lunar distances continued well into the nineteenth century. Problems :-

Galileo had studied the motions of Jupiter's four moons, the now-named Galilean satellites, and had produced a set of tables with which he hoped to gain some of the prize money then on offer for a reliable way of calculating longitude at sea. In this hoppe, he was unsuccessful, but this method was used widely for mapping land areas. The method was refined by Giovanni Cassini in 1668, who soon afterwards became director of the new Paris Observatory. It was as an offshoot of these developments that Ole Roemer came up with an estimate of the speed of light.