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Origin and history

The Northern Rim, also known as the Cassini pyramid, is a monument erected in the second quarter of the 18th century in the 18th arrondissement of Paris. It is located in the private garden of the Moulin de la Galette, bounded by the streets Girardon, Lepic and Junot Avenue. This 3-metre-high monument, composed of a parallelepipeda surmounted by a pyramid, served as a point of reference for the measurement of the meridian of Paris. Its axis is aligned to 2,931 toises and 2 feet from the Paris Observatory, as indicated by the inscription engraved on its base.

The origin of the sight dates back to the work of the Academy of Sciences to measure the arch of the meridian of Paris, initiated by Abbé Jean Picard in 1675 with a simple wooden post near the Blute-Fin mill. In 1736, Jacques Cassini, son of astronomer Cassini, replaced this post with the present monument, built by master mason Rondel on a plot belonging to a miller named Ménessier. The flower of lilies at the top, a royal symbol, was replaced by a ball during the Revolution, then by a lance in 1840.

The northern focus is part of a network of geodetic points around Paris, including the Montlhéry tower, the Brie-Comte-Robert bell tower, and the Montjoie tower. Ranked a historic monument in 1934, it reflects the scientific advances of the eighteenth century in astronomy and cartography. Its location also corresponds to medallion No. 129 of the Homage à Arago, a contemporary work by Jan Dibbets (1994).

The measurements made by Nicolas-Louis de La Caille in the 18th century revealed an angular gap of 12 seconds between the Observatoire-Mire axis and the meridian, about 0.33 metres. This difference, though small, illustrates the precision of the instruments of the time. The longitude of the target, determined by IGN, is 2° 20' 11.5" E, almost identical to that of the Meridian of the Observatory (2° 20' 11.4909" E), confirming its key role in geodetic surveys.

Today, the North's sight remains an unknown but essential vestige of French scientific history. Although located in a private property, its classification in 1934 made it a protected heritage, linked to major figures such as Cassini, Picard and Delambre. His study has helped to refine knowledge of the shape of the Earth and to establish cartographic references still in use today.

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