Pyramide de Cassini, Historic geodesic monument in Villejuif, France.

The Cassini Pyramid is a tapering stone marker on Avenue de Paris in Villejuif, built from light grey rectangular blocks. A wide square base with a projecting cornice carries a stepped shaft that narrows to a flat top, with a narrow dark slot recessed into one face of the base. It stands on private ground at 157 bis, at an altitude of 105 metres.

The monument was built in 1742 as a marker for measuring the Paris meridian line, which was later used to create detailed maps of France. This measurement line was one of the largest scientific projects of that era.

The pyramid is a mire, a sighting marker on the Paris meridian measured in 1683, and it served as the starting point for Jacques Cassini's map of France. It was built in 1742 on the orders of Louis XV by Cassini, his son César and the astronomer Nicolas-Louis de Lacaille, alongside the twin pyramid of Juvisy. The name Cassini refers to this family of astronomers and cartographers.

The monument sits behind a black gate along the street and is reached by climbing uneven stairs, so watch your footing carefully. It is next to a modern residential street but can be easily found from Avenue de Paris.

The monument is one of very few remaining measurement markers used in creating the Cassini map, one of the first accurate surveys of French territory. That map was groundbreaking for European cartography and revealed that previous knowledge of France's size was incorrect.