Mars gives up the size of the solar system
Observing the same Mars opposition from Paris and Cayenne gave the first sound distance to the Sun: about 140 million km, seven per cent short of the true value.
On 8 February 1672 a ship left La Rochelle carrying Jean Richer, a quadrant, a pendulum clock and orders from the Académie des Sciences. His destination was Cayenne, in what is now French Guiana, five degrees north of the equator and some 7,000 kilometres from Paris. The target was Mars. That September the planet would reach a perihelic opposition, its closest approach in fifteen years, and two observers far enough apart would see it shifted by a hair against the background stars. Measure the shift, know the distance.
Richer observed through August, September and October 1672. Giovanni Domenico Cassini observed the same nights from the new Paris Observatory. The displacement they were chasing amounted to a few arcseconds — roughly the width of a coin seen from a couple of kilometres — and extracting it from seventeenth-century instruments, shipped across the Atlantic and back, was closer to an act of faith than a routine measurement. It worked. The pair got an Earth–Mars distance, and Kepler's third law did the rest: the relative spacing of the planets had been known for decades, so one absolute distance set the scale of everything.
Cassini's figure for the Earth–Sun distance came out around 140 million kilometres. The accepted value today is 149,597,870.7 km. He was seven per cent low on a quantity that no one before him had been able to measure at all — every earlier estimate, Ptolemy's included, had been too small by a factor of about twenty. John Flamsteed in England reached a comparable answer the same year by a different route, using the Earth's own rotation to shift his position between evening and morning.
The expedition also produced a result nobody had gone looking for. Richer's pendulum clock, painstakingly adjusted in Paris, ran slow at Cayenne; to keep true seconds the bob had to be shortened. Gravity was weaker near the equator. Newton later took this as evidence that the Earth is not a sphere but bulges at its waist — a voyage to measure Mars that came home having measured the shape of the Earth instead.
What the campaign did not do was show anyone anything new about Mars itself. No feature, no atmosphere, no clue about the surface. Mars was simply the most convenient ruler available, close enough at opposition to show a measurable parallax and far enough to anchor the rest of the system. For roughly a century after, that remained Mars's main job in astronomy: not a world to be understood, but a yardstick. Richer got little credit for carrying it; he returned in 1673 and died in 1696 largely forgotten.