In 1669, King Louis XIV of France summoned the celebrated Italian astronomer, Giovanni Domenico Cassini, from Bologna to Paris. The King’s ambition was to create a new map of his kingdom, one based on scientific precision instead of guesswork. For this grand project, Cassini was made director of the new Royal Observatory. At its heart, he established the Paris Meridian, a line running north-to-south through the building itself. The era of maps based on travelers' tales was over. This new meridian served as the zero-longitude reference for all French mapping for more than two centuries.
The Cassini Maps of France
Four generations of one family measured France with triangles and produced the first accurate map of a whole country.
Chapter 1
A King's Invitation
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Frequently Asked Questions
Why did the Cassini map show France as smaller?
Before the Cassini survey, maps were based on less accurate information, like estimates from sailors on travel times. The Cassini project used scientific triangulation and astronomical observations, which revealed that France's western coastline, in particular, was much further east than previously drawn.
What is triangulation?
Triangulation is a geometric method for mapping. Surveyors measure a single line on the ground, then measure the angles from each end of that line to a distant point, forming a triangle. With these measurements, they can calculate the distances to that point without physically traveling there.
What happened to the Cassini family's map?
In 1793, during the French Revolution, the new government nationalized the map project. They seized the engraved copper plates from the Cassini family, viewing the map as a common good. The map was completed under state control and became a foundational tool for French administration.
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