Measuring the Length of the Meridian

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     Theoretically a degree of latitude is a constant, the same at the equator as at the pole. However, Isaac Newton believed that the earth was slightly flattened at the poles, an oblate spheroid, and that the length of a degree at the poles was longer than it was at the equator. On the other hand French mathematicians argued either for a perfect sphere or for a prolate spheroid, one which bulged at the poles. 
     The French Royal Academy of Sciences determined to settle the matter by sending expeditions to the Equator and to the Arctic Circle. If the length of a degree were longer at the Arctic Circle than at the Equator the spheroid would be oblate, flat at the poles; if it were shorter, prolate, and if the degrees were equal, then the earth would be spherical.
     In 1735 the French Royal Academy of Sciences sent out two geodetic expeditions to determine the length of a degree at the pole and at the equator. The expedition to the Arctic Circle was under the leadership of Pierre-Louis Moreau de Maupertuis went to the Arctic Circle. The one to the Equator had Charles Marie de La Condamine as its chronicler. Near Quito, in what is now Ecuador, a base line was established by triangulation, and the length of the degree of latitude was measured.

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           Although the expedition to the Equator left in 1735, a year earlier than the one to the Arctic Circle, it did not report is finding to the Royal Academy until 1744, nearly eight years after the northern expedition had made its report. In both cases the results were clear: the earth was not a perfect sphere. It was flattened at the poles.

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     The map in the upper left quarter of the sheet shows the map of the triangulation by which the length of the degree was determined. The other map shows the course of the headwaters of the Amazon River by the Jesuit priest, Samuel Fritz. Fritz established a number of mission stations among the Omagua from 1687 to his death in 1723. This map was the first of any real accuracy ever made of the river, though it, too, had mistakes. La Condamine acquired a copy of Fritz’ map and corrected many of the mistakes. The dates given for Fritz on the stamp are incorrect.

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       Pierre-Louis Moreau de Maupertuis (1698-1759) led the expedition of four to Lapland. At Stockholm they were joined by a Swedish astronomer. They arrived at Torneć, at the northern end of the Gulf of Bothnia in early summer. With the church spire as their starting point they plotted a series of triangles north to Kittis mountain. After determining the angle of a known star at Kittis and at the church in Torneć, they measured the distance between Kittis and Torneć to determine the length of a degree at the Arctic Circle.
     Although his measurements were not absolutely accurate, the length of a degree at the Arctic Circle was nearly half a kilometer longer than in France, proving that the shape of the earth was an oblate spheroid.

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The map on the Swedish booklet cover is based on Maupertuis’ map in his report to the French Academy.

     For further information on the work of Condamine see the article "Degree of Difficulty," by David Taylor in Mercator's World, May/June 1999, pp. 18-25.