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Let's go to Mars

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.

A crowded seventeenth-century painting. A king in a plumed black hat and red stockings sits just right of centre, a man in black robes wearing a cross badge stands at the centre beside him, and rows of wigged scholars and courtiers fill the space behind. A painted terrestrial globe fills the lower left and a celestial globe with constellation figures the lower right; an armillary sphere and animal skeletons stand against a red hanging at the upper left, a clock hangs by a green marble column, books and rolled papers cover a draped table, a large map hangs at the right with a youth on a ladder beside it, and a brass frame with a graduated arc rests on the floor. Through a tall opening between drawn curtains at the back stands a two-storey classical building with arched windows and a balustraded roof, its left end unfinished.
Henri Testelin's imagined scene of Colbert presenting the members of the Académie royale des sciences to Louis XIV, the half-built Paris Observatory in the curtained opening behind. Cassini is among the academicians shown; the Académie sent Richer to Cayenne in 1672, and Cassini observed the same Mars opposition from Paris. · Image: Henri Testelin

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. Mars would reach a perihelic opposition that September, its closest approach in fifteen years. Seen from two points that far apart, the planet shifts against the background stars by an angle that gives its distance.

Richer observed through August, September and October 1672; Giovanni Domenico Cassini observed the same nights from the new Paris Observatory. The displacement was about fourteen arcseconds, roughly the width of a coin seen from three hundred metres. From it they derived an Earth–Mars distance. Kepler's third law supplied the rest: the relative spacing of the planets was already known, so one absolute distance set the scale of the whole system.

Cassini's figure for the Earth–Sun distance came out around 140 million kilometres, seven per cent below the accepted value of 149,597,870.7 km. Ptolemy's estimate had been too small by a factor of about twenty. John Flamsteed reached a comparable answer in England the same year by a different route, using the Earth's rotation to shift his position between evening and morning.

Richer's pendulum clock, adjusted in Paris, ran slow at Cayenne, and the bob had to be shortened to keep true seconds. Gravity was weaker near the equator. Newton later took this as evidence that the Earth is not a sphere but bulges at its waist.

The campaign showed nothing new about Mars itself: no feature, no atmosphere, no clue about the surface. Mars served as a ruler, close enough at opposition to show a measurable parallax and far enough to anchor the rest of the system. That remained its main use in astronomy for roughly a century. Richer returned in 1673 and died in 1696 largely forgotten.