Before GPS, before LORAN, navigators (ocean) determined their latitude by the angle between the sun and the horizon at noon. They determined their longitude by the time when the sun was highest in the sky (or when the sun came above the horizon). But to do that they needed to know what time it was.
At first they had hour glasses to tell the time, and it was the job of somebody to always watch the hour glass and to turn it when the sand ran down. Eventually expert clock makers devised ever more accurate clocks to help navigators determine their longitude. The advent of the telescope made it possible to get an accurate time reference that did not drift. How might that work?
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Update and answer
See the comments below. Greg was onto the solution, but he didn’t carry it to conclusion. The answer is that the advent of telescopes enabled navigators to see the moons of Jupiter. There are four large ones that are visible to anybody with a good set of binoculars, and their motions provide a reliable time reference. Early navigators did make use of these observations to determine longitude with greater accuracy:
In 1612, having determined the orbital periods of Jupiter’s four brightest satellites (Io, Europa, Ganymede and Callisto), Galileo proposed that with sufficiently accurate knowledge of their orbits one could use their positions as a universal clock, which would make possible the determination of longitude. He worked on this problem from time to time during the remainder of his life.
To be successful, this method required the observation of the moons from the deck of a moving ship. To this end, Galileo proposed the celatone, a device in the form of a helmet with a telescope mounted so as to accommodate the motion of the observer on the ship. This was later replaced with the idea of a pair of nested hemispheric shells separated by a bath of oil. This would provide a platform that would allow the observer to remain stationary as the ship rolled beneath him, in the manner of a gimballed platform. To provide for the determination of time from the observed moons’ positions, a Jovilabe was offered — this was an analogue computer that calculated time from the positions and that got its name from its similarities to an astrolabe. The practical problems were severe and the method was never used at sea. However, it was used for longitude determination on land.