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Web edition: September 9, 2011
Print edition: September 24, 2011; Vol.180 #7 (p. 28)
Barely two centuries after Columbus found that the world wasn’t flat, scientists set out to establish whether it was really round. The question required comparing the distance between degrees of latitude in Europe — which had been measured — with that distance in the Arctic or at the equator.
In his latest book, Ferreiro gives life to three French scientists and their 1736 geodesic mission to the equator. Astronomer Louis Godin, mathematician Pierre Bouguer and geographer Charles Marie de La Condamine offer a snapshot of another time.
The scientists traveled to a plateau near Quito in South America. There they surveyed a 215-mile line and used it to calculate giant imaginary triangles using geometry, trigonometry, a device called a quadrant, star sightings and land distance measurements. Triangulating nearby peaks of the Andes with points on the ground, they found that one degree of latitude was shorter at the equator than in Europe. In other words, the Earth bulges at the equator.
At times the story can bog down in detail, but surprises make up for it. The mission was the first large international scientific expedition attempted and led to the discoveries of rubber and cinchona bark — the source of quinine — and the naming of Ecuador.
Not until 1743, after the calculations were complete, did the scientists realize they had also determined Earth’s size. La Condamine found out belatedly; he was on the first scientific expedition down the Amazon. Another time indeed.
Basic Books, 2011, 337 p., $28
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You say "One degree of latitude was shorter at the equator than in Europe." That's backwards. The equator is about 24,000 miles long, so one degree = 24,000/360 = 67 miles long. Now, rather than doing the measurement on the latitude line through Paris, let's continue to one mile south of the North Pole, where the principle is the same but the math is simpler. The line of latitude one mile south of the Pole is 1 x pi = 3.14 miles long, so one degree is (3.14/360 miles) x 5280 feet/mile = 46 feet.
You also say "In other words, the Earth bulges at the equator." This statement usually means that a point on the equator is farther from the center of the Earth than a point on the equator of an Earth-sized sphere, assuming the same length latitude at some familiar place like Greenwich or Paris. A sphere "bulges" equally at all points. But it's possible for the Earth to be an ellipsoid whose equator (longest line of latitude) is closer to its center than the equator of a sphere with the same latitude line through Paris but still has longer degrees than latitudes farther north. To show that the Earth is oblate (i.e.; bulges at the equator) requires careful measurements showing that the length of a degree at the equator, arrived at through astronomical measurements, is bigger than would be the case if the Earth was spherical, based on knowing the length of a degree in Paris. These guys were quite capable of such measurements.
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