Struve meridian

• To prove that the Earth is not exactly spherical, the Paris Academy of Sciences organized two expeditions in 1735: one to the equator in Peru and the other to the Arctic Circle in Lapland.
• At the same time, a third degree measurement was carried out in France. The expeditions of the Paris Academy of Sciences determined that the length of a meridian arc corresponding to one degree is 110.9 km at the equator, 111.3 km at Paris, and 111.9 km at the Arctic Circle.

• Thus, it was proven through degree measurements that the Earth is flattened at the poles.
• In the 19th century, several degree measurements were carried out in Europe, India, and America. The most important of these were the Anglo-French degree measurement from northern Scotland to Algiers in 1783–1808 and the Russian Academy of Sciences' degree measurement from the Arctic Ocean to the Danube River in 1816–1855.

Friedrich Georg Wilhelm Struve (left) ja Carl Friedrich Tenner

Struve and Tenner

Friedrich Georg Wilhelm Struve (1793-1864) was a German astronomer and geodesist, academician of the St. Petersburg Academy of Sciences from 1832, director of the Tartu University Observatory from 1820 to 1839, professor of astronomy and geodesy at Tartu University from 1814 to 1838, He was one of the founders of the Pulkovo Observatory and its director from 1839 to 1862.

Carl Friedrich Tenner (1783–1859) was born in Auvere Manor in Vaivara Parish, Viru County. He began studying geodesy in St. Petersburg in 1802, and in 1809 he was tasked with creating a triangulation network from St. Petersburg across Narva towards Tallinn and from there on to Tartu. In 1816, Tenner and his colleagues began creating a triangulation network in what is now Lithuania. Based on an agreement concluded in 1828, the arc measurements of Struve and Tenner were combined, with Tenner responsible for the geodetic part and Struve for the astronomical part.

Measurement results

• The result of the meridian arc measurements was the determination of the length of the corresponding meridian arc (2880 km) as 25°20', which was the most extensive degree measurement in the 19th century.
• A comparison of the measurement data for different sections of the Tartu meridian showed that the length of the arc corresponding to one degree of latitude increases unevenly towards the pole.
• C. F. Gauss used the results of the meridian arc measurements to refine cartographic projections, and F. W. Bessel used them to determine the parameters of the terrestrial ellipsoid (Bessel 1841).
• The further (final) processing of Struve's meridian arc measurement data was carried out on Bessel's ellipsoid.
• The parameters of the Earth ellipsoid determined by Bessel were used in Estonia until 1946 and are still in use in some countries (Indonesia, Japan, Korea, etc.).

Tartu Observatory

• In Tartu, Struve used a point located at the center of the dome of the astronomical observatory for his measurements, which unfortunately has not been preserved.
• The projection of this point on the floor of the room under the dome was restored based on Struve's drawings and measurement data.
• This center was marked with a bronze center with a diameter of 12 mm, with a center hole at its spherical end.

Simuna-Võivere baseline endpoints

• The Simuna-Võivere baseline was located on the flat, evenly sloping fields of Avanduse and Võivere manors, where there were no surface features to interfere with measurements. The difference in height between the endpoints of the baseline was 6.3 m, and the length of the line was 4.5 km.
• One of the endpoints of the baseline in Simuna is marked with a 1.9 m high granite pillar bearing the date 1849. The meridian arc measurement marker was entered in the national register of cultural monuments (No. 5753) by Order No. 24 of the Minister of Culture on May 13, 1997.
• The other endpoint of the baseline in Võivere was thought to have been destroyed. It was found during instrumental searches carried out in 2001. (Note: Struve's result (at sea level) was 4512.279 m, while the length of the baseline calculated from GPS measurements carried out in 2001 was 4512.293 m; the difference between the results is 0.014 m or 14 mm, which proves that these are the points measured by Struve). The center mark is a foundation made of limestone and lime mortar, measuring 204×204 cm, on which a large granite stone with a drilled center hole is placed. The Võivere endpoint was marked with a glass pyramid in 2011.

Simuna (left) ja Võivere reference points