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The first measurement of the deflection of the vertical in longitude -- The figure of the earth in the early 19th century

During the summer of 1837 Christian Ludwig Gerling, a former student of Carl Friedrich Gauß's, organized the world wide first determination of the deflection of the vertical in longitude. From a mobile observatory at the Frauenberg near Marburg (Hesse) he measured the astronomical longitude difference between C.F. Gauß's observatory at Göttingen and F.G.B. Nicolai's observatory at Mannheim within an error of 0.4". To achieve this precision he first used a series of light signals for synchronizing the observatory clocks and, second, he very carefully corrected for the varying reaction time of the observers. By comparing these astronomical results with the geodetic--determined longitude differences he had recently measured for the triangulation of Kurhessen, he was able to extract a combined value of the deflection of the vertical in longitude of Göttingen and Mannheim. His results closely agree with modern vertical deflection data.