The first real map of Mars, and a zero meridian
Ten years of watching through a 95 mm refractor in a Berlin garden produced the first true map of another planet — and a prime meridian Mars still uses.
Wilhelm Beer was a Berlin banker, brother of the composer Meyerbeer, with a private observatory in the Tiergarten and a 95-millimetre Fraunhofer refractor. Johann Heinrich Mädler was the professional astronomer he employed to use it. From 1830 onward the pair did for Mars what nobody had troubled to do before: they observed it systematically, opposition after opposition, and treated what they saw as permanent geography rather than as passing cloud.
That was the gamble, and it was not obviously right. Two centuries of Mars drawings disagreed with one another so comprehensively that some observers had concluded the markings were atmospheric and therefore unmappable. Beer and Mädler assumed permanence and then tested the assumption by prediction. They picked one small dark feature near the equator, labelled it plainly "a", and made it their zero of longitude, measuring everything else against it. They named nothing else either — the whole chart is lettered, not named, a deliberate refusal to say what any of it was.
The discipline paid in precision. By timing the return of feature "a" across years of observations, they pinned the rotation period at 24 hours 37 minutes 22.7 seconds. Their earlier figure, from the 1830 and 1832 oppositions, had been a second longer; the 1840 one lands within a tenth of a second of the value in use today, which is more than the instrument had any right to give. The map itself, published in 1840 and combining ten years of work, is the first true map of another planet: two hemispheres, a coordinate grid, features in fixed positions. It came out of a disc that never showed them more than about 24 arcseconds across, about the size of a golf ball seen from 400 metres.
Feature "a" outlived everyone involved. Schiaparelli adopted it as his zero of longitude for the 1877 map that made him famous; Camille Flammarion later named the surrounding region Sinus Meridiani, Meridian Bay. In 1972 the Mariner 9 cartography group, wanting something they could pin down to a fraction of a degree, chose a crater about 500 metres wide sitting inside the larger crater Airy and named it Airy-0. Mars Global Surveyor took nine attempts to photograph a target that small, finally succeeding in January 2001. The prime meridian of Mars runs through the middle of that crater today because two Germans in a Berlin garden liked a particular smudge.
They were still wrong about what they were looking at. Beer and Mädler read the dark areas as water and the bright ones as land, the reverse of the truth in the sense that matters — the dark regions are wind-swept volcanic rock, the bright ones dust. The vocabulary stuck anyway. Mars kept seas, bays and lakes in its official nomenclature for more than a century after everyone had accepted there was no water to put in them.