The only object in the sky that remains somewhat immobile, is the North Star. The Babylonians divided the circle in 360 degrees with the idea that one degree was the sun's right ascension, i.e. the advance in relation to the starry background in one day.
From that, navigators used simple tools like a piece of wood hold at arm length to measure the distance between the North Star and the horizon. That was the latitude.
When navigating to a distant place, they had noted the latitude of that place and sailed until meeting it, never knowing how long it would take.
This is the way people navigated until the end of the 18th century, when John Harrison invented the marine chronometer.
Columbus, for example, had no way to know his longitude, i.e. progression in the east-west direction other than by so-called dead-reckoning, i.e. counting the distance covered per day.
Finding the longitude is finding a time angle with a place of reference. That one has been Rome, Paris, Amsterdam, etc. during the past centuries for the past 120 years, it is Greenwich in England. This is called the prime meridian and if you know the time at Greenwich accurately, comparing it to your local time, you get the Greenwich Hour Angle (GHA) and you convert it to a angle to get your longitude.
From the 17th century, an astronomical method to find the longitude was used: Tables of the occlusion of Jupiter's moons was established for a place of reference, e.g. Greenwich, then somewhere else on the earth, one could observe them working at a timepiece: the moons passage were the clock of the sky. Alas this method could only be used when safely seated on land and not at sea, since it required a very strong telescope.
As a side story, it is when calculating such a Jupiter's moon table that the Danish astronomer Rømer discovered that the time was slightly different from summer to winter; thus concluding that light needed a longer time to reach the earth and from that, made the first and relatively accurate calculation of the speed of light.
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