Global Positioning System Reference
In-Depth Information
FIGURE 2.7.
Looking at stars to find out the
shape of the earth: The Cassini Room at the
Paris Observatory, with the Paris meridian
traced along the floor. Inset: Giovanni Cas-
sini, director from 1671 to 1712.
From Wiki-
pedia, courtesy of FredA.
marks the exact spot). The northern point for his survey was Sourdon,
about 110 km due north (another obelisk). The latitudes of these end
points were determined by astronomical observations, and the distance
between them was measured by triangulation. The result was an estimate
of the earth's radius of 6,329 km, which is less than half of one percent
below the currently accepted mean polar radius.
At this point, the Cassinis enter our story. Giovanni Cassini was an
Italian astronomer who moved to Paris at the time of Picard's survey to take
over the newly formed Paris Observatory (fig. 2.7). Over the next century,
