Longitude was more difficult to determine than latitude for several reasons.
First, longitude is associated with time - the word "meridian" comes from the Latin word for noon, when the sun reaches its highest point in the sky. Measuring longitude at sea requires knowing the exact local time and the time at the departure point, and although this sounds simple today, it was impossible until the 18th century, when Englishman John Harrison invented the first dependable timepiece.
Establishing the prime meridian was another problem. Unlike the equator, zero-degree longitude is not determined by natural laws. And ever since Ptolemy ran it through the Canary Islands in AD 150, mapmakers have chosen many locations for it, including Rome, Jerusalem, Copenhagen, St. Petersburg, Pisa, and, of course, Paris.
Arago came into the picture in 1806, after the meridian's route had been traced through France. Still a student, he was given the job of measuring its extension to the Balearic Islands off the coast of Spain, no easy task at the time. Captured by pirates and imprisoned by the Bey of Algiers, he was given up for dead.
But he returned to France in 1809 and was rewarded with membership in the prestigious Academy of Sciences at the age of 23. A gifted mathematician and scientist, he also had a successful political career before ending his days as director of the Paris Observatory.
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