In 1669 Giovanni Cassini arrived in Paris and was presented to Louis XIV, who among other things desired to have the most prestigious collection of learned scientists residing at the newly established Royal Academy of Sciences.

Personally, I know the name Cassini as the astronomer who gave a name to the gap in Saturn’s rings – the Cassini Division. But in 1669 it was Cassini’s published tables of the eclipses of Jupiter’s moons that brought him to the King’s attention, but not for the reasons you might think. Although Cassini agreed to join the Royal Academy in the hope of getting his hands on better telescopes, the King’s minister of finance, Jean Baptiste Colbert, want Cassini to work on his pet project: the accurate mapping of France.

Why would a cartographer require the services of an astronomer? The connection is this: In order to draw accurate maps, you need to know whereabouts on the surface of the earth you are, both degrees of Latitude and Longitude. The problem of latitude had been solved some time earlier by taking note of the noonday’s sun’s angle from the perpendicular. (Remember Eratosthenes and the noon sun shining down the well in Syene?)

The solution to the problem of Longitude, however, still eluded scientists without an accurate timekeeping device. Once you declare a specific line passing through an arbitrary location – say, Paris – joining the two poles of the earth as 0 degrees longitude, you know exactly how many degrees East or West you are from that line, providing you know two things: first, choose a specific local time of day (sunrise, sunset, noon, whatever) and b) what the exact time is *in Paris* at that same moment. Every hour ahead (or behind) in time you are represents 15 longitudinal degrees East (or West).

Many scientists believed that the answer to the longitude problem lay in the heavens, and the regular periodic eclipsing of Jupiter’s moons seemed promising. The four moons visible in the telescopes of the day appeared to have a relative motion that was as predictable as a pendulum clock.

So all you’d need, they proposed, was an accurate table of eclipses and the times at which the moons should reappear (in Paris), a table that used the predictable orbital motion to extrapolate these times into the future, and one could, with careful observation using a telescope and a pendulum clock running on local time, determine the differential in time between the two locations, and therefore the difference in Longitude.

Slowly, Cassini worked on revising his tables of Jovian eclipses, training cartographers in the use of the tables, and sent them out across the country. Cassini also corresponded with other astronomers in other cities across Europe, requesting that they make their own observations and return the data to him. He cleared the third floor of his observatory and created a polar-projection map (centered on the north pole) of Europe that steadily made every other map of Europe totally obsolete.

As Cassini aged, he employed his son to assist him in the observations and triangulation measurements. In total, four generations of the Cassini family devoted themselves to creating an accurate topographical map of France, the first country in the world to be so mapped.

Why the “Cassini Division” did not end up being a cartography term, I don’t know.

Book of the month is still The Mapmakers, by John Noble Wilford.