Although the first "spyglasses" were fabricated in Holland in the early 17th century, Galileo alone immediately realized their great astronomical potential. Using two lenses, a plane-convex objective and a plane-concave eyepiece, he perfected the telescope, improving it to a magnifying power of 20 and designing accessories that made it not only an observation instrument but also a device for measuring astronomical phenomena. With his instruments Galileo managed to tabulate the orbital periods of Jupiter's satellites, recording them on an extraordinary analogue computer, the Jovilabe. By using this instrument in combination with a clock featuring an innovative pendulum escapement, Galileo believed he could solve the crucial problem of determining longitude at sea.