Struve Geodetic Arc: The 2,820 Km Line That Produced The First Accurate Measurement of The Earth’s Size

From the northern coast of Norway to the southern coast of Ukraine runs a chain of survey triangulation points that together forms the Struve Geodetic Arc. It stretches from Hammerfest (Norway) on the shores of the Arctic Ocean to Nekrasivka (Ukraine) by the Black Sea, a distance of 2,820 km, snaking in and out of numerous territories, that today belong to ten different countries. The arc was established by the German-born Russian scientist Friedrich Georg Wilhelm von Struve, who undertook a thirty-nine-year-long survey, between 1816 and 1855, to determine the shape and size of the earth. The survey yielded the first accurate measurement of a meridian arc, which in turn allowed the first precise measurement of the earth’s diameter.

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The northernmost station of the Struve Geodetic Arc is located in Fuglenes, Norway. Photo credit: Francesco Bandarin/Wikimedia

The determination of the size and shape of the earth was one of the most important problems for natural philosophers since the ancient times. In the 2nd century BC, Greek astronomer and mathematician, Eratosthenes, developed a method to determine the radius of the earth by measuring a portion of a meridian arc and comparing that length to the corresponding angle subtended at the Earth’s center. If one knows the arc length and the central angle, the radius can be easily calculated.

Eratosthenes calculated the arc length by measuring the time it took for caravans to cross the desert. The angle was determined by measuring the difference between the height of the sun between the latitudes. Although the accuracy of his measurements were low, Eratosthenes was able to calculate the earth’s radius with an accuracy of 1%. For two thousand years, Eratosthenes’ “method of degree observation” remained the most promising method of determining the size of the earth. What improved was the methodology of measurement.

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Map of the Struve Geodetic Arc where red points identify the World Heritage Sites.

In the 16th century, a new measurement technique called “triangulation” was developed using which only short distances were needed to be measured accurately, while long distances could be determined using a chain of connected triangles. Triangulation enabled astronomers and cartographers to accurately measure long distances that stretched for hundreds and thousands of kilometers.

During the 18th century, many arcs were constructed around Europe, but Struve’s arc was the longest such arc when it was created, and remained so for over a century. It was also the most accurate, producing measurement that was off by only 4 mm for every 1 km. In other words, using Struve’s arc the radius of the earth was calculated to within 223 meters of its actual value.

The Struve Geodetic Arc originally consisted of 258 triangles connecting 259 triangulation points. At the time of its creation, the arc passed through two countries —Union of Sweden-Norway and the Russian Empire. But changes in the political boundaries have now scattered the points across ten nations —Norway, Sweden, Finland, Russia, Estonia, Latvia, Lithuania, Belarus, Ukraine and Moldavia. The points or stations are marked in various ways — some are holes drilled in rocks, some are crosses marked on the rock surface, others are cairns built out of rocks. Some stations also had commemorative monuments erected.

Out of the original 259, 34 stations were collectively inscribed under the UNESCO World Heritage Sites list.

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