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Let's go to Mars

Huygens draws a spot on Mars and clocks the day

Huygens sketched a dark triangle on Mars, watched it come back night after night, and concluded a Martian day runs about 24 hours. He was 37 minutes out.

A circle drawn in thin dark ink on cream, aged paper, its top and bottom arcs running off the edges of the frame, with a broad V-shaped patch of dense pen hatching filling the middle.
Christiaan Huygens's drawing of 28 November 1659: a dark wedge on the disc, the marking now called Syrtis Major and the first identifiable feature made out on the surface of Mars. This is the engraving Percival Lowell printed in his book Mars (1895), reproduced there from Camille Flammarion's La Planète Mars. · Image: Christiaan Huygens; engraving from Camille Flammarion, La Planète Mars, as reproduced in Percival Lowell, Mars (1895)

Galileo turned a telescope on Mars in 1610 and could report only that the disc did not look perfectly round; he was catching the gibbous phase, not a surface. The disc is never wider than about 25 arcseconds even at its closest, and the first generation of lenses smeared that into a coloured blob. On the evening of 28 November 1659, Christiaan Huygens sat at a long refractor with a lens he had ground himself and drew a dark, roughly triangular blotch spanning the middle of the planet.

It was the first identifiable feature made out on the surface of another planet. Huygens gave it no name. Later observers called it the Hourglass Sea; today it is Syrtis Major Planum, not a sea but a low volcanic shield, dark because wind keeps it scoured clear of the bright dust that covers most of Mars.

He watched it. Over the following nights the blotch crawled across the disc, vanished around the limb, and came back to nearly the same place at nearly the same hour. On 1 December 1659 he noted in his journal that the rotation of Mars seemed to take 24 terrestrial hours, like the Earth's. The modern sidereal figure is 24 hours 37 minutes 22.7 seconds. He was thirty-seven minutes out, working from a handful of nights, a home-made lens and a clock. Giovanni Cassini tightened the figure to 24 hours 40 minutes from Bologna in 1666, and it has barely moved since.

The same markings returning meant they belonged to the ground rather than to weather, which gave Mars a geography: coordinates, a north and a south, places that could be found again.

In Cosmotheoros, published in 1698 after his death, Huygens argued that the other planets were inhabited by rational beings equipped with hands and ships and a taste for astronomy, on the reasoning that a Creator would not waste the real estate.