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Let's go to Mars

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.

Two circular hemisphere charts side by side, each ruled with a grid of longitude lines numbered around the rim, filled with soft-edged dark blotches labelled with small italic letters.
The 1840 chart itself: two hemispheres, a coordinate grid, and features given letters instead of names — including the small dark 'a' that fixed the zero of longitude. · Image: W. Beer and J. H. Mädler

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 astronomer he employed to use it. From 1830 onward the pair observed Mars systematically, opposition after opposition.

Two centuries of Mars drawings disagreed so comprehensively that some observers had concluded the markings were atmospheric and unmappable. Beer and Mädler assumed permanence and tested it by prediction. They picked one small dark feature near the equator, labelled it "a", and made it their zero of longitude. They named nothing else; the whole chart is lettered.

Timing the return of feature "a" over years of observations pinned the sidereal rotation period at 24 hours 37 minutes 23.7 seconds. Their earlier figure, from the 1830 and 1832 oppositions, was off by almost 13 seconds; the 1837 value is 1.1 seconds from the modern one. The solar day, the sol every surface mission since has counted its life in, is 24 hours 39 minutes 35 seconds. The map, published in 1840 from ten years of work, is the first true map of another planet: two hemispheres, a coordinate grid, fixed features. The disc never showed more than about 24 arcseconds across, a golf ball at 400 metres.

Schiaparelli took feature "a" as the zero of longitude for his 1877 map; Camille Flammarion later named the surrounding region Sinus Meridiani, Meridian Bay. In 1972 the Mariner 9 cartography group, wanting a mark fixable to a fraction of a degree, chose a crater about 500 metres wide inside the crater Airy and named it Airy-0. Mars Global Surveyor photographed it on the ninth attempt, in January 2001. The prime meridian of Mars runs through that crater today.

They read the dark areas as water and the bright ones as land; the dark regions are wind-swept volcanic rock and the bright ones dust. Mars kept seas, bays and lakes in its official nomenclature for more than a century.