Longitude is based on the problems of determining the east/west position on a sphere.
It was called "the problem of the Longitude." To locate a position on the surface of a sphere requires two numbers. On a rotating sphere, one of these numbers can be the distance in degrees from the points which do not move, the poles. This is "latitude," and can be found by taking the distance of the sun at noon, and consulting a table. An experienced mariner of the 18th century could calculate north/south position in a matter of minutes with a good deal of accuracy.
However, there is no easy way to determine the second number, that is east/west position. The best method, which was in use by the mid 1700's was "lunar distance." This method relies on the fact that the moon's apparent position moves by roughly one half of one degree per hour. This means that a navigator can measure the distance between the moon and another celestial object on or close to ecliptic, the line which traces the sun's motion across the sky. After correcting for errors, such as the difficulty of knowing where the center of the moon is, the navigator can correct for the parallax across the distance of the earth, and look up what time the moon would have that apparent distance at some known point. Because England was the center of pushing navigational technology at this time, it became more and more the common reference point, but it would not be officially so until the late 19th century, during the age of standardization.
In the present navigation occurs with satellites, which circle the earth, and receivers, these Global Position Satellites are so common that ordinary people take for granted knowing where they are to a degree of accuracy that would have astonished mariners from older times.
This piece, entitled "Longitude," allows the viewer to look at the spinning globe from space, or take a ride on a satellite.