The first estimations of the Earth's circumference were made in Antiquity by measuring the distance and the difference in latitude between two places. Latitude was determined by measuring the maximum height of a particular celestial body above the horizon.
Determining longitude was not as simple. The position of a celestial body above the horizon can indicate the time at that particular place (for instance, midday when the Sun passes through the meridian). However, the ceaseless rotation of a celestial body prevents any direct measurement of its difference in position from one place to another. It depends on the difference in time between the two places, which is linked to longitude by 4 minutes for each degree. The history of longitude is thus the history of the search for a timekeeper - either astronomical or mechanical - that indicates the time of the place of departure.
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