Asaph Hall finds two moons in six nights

Mars reached a perihelic opposition on 5 September 1877, closing to about 56 million kilometres — an approach recurring every fifteen to seventeen years. Asaph Hall, then 47, ran the 26-inch Alvan Clark refractor at the US Naval Observatory in Washington, the world's largest refractor when installed in 1873. He spent the weeks either side searching the glare beside the planet for anything moving with it. The observatory sat in Foggy Bottom, low beside the Potomac, and river mist routinely shut him down.

He had decided to stop. His wife, Chloe Angeline Stickney Hall, a mathematician who did much of his reduction work, told him to go back out. On the night of 11 August, Washington time, he caught a faint point of light beside the disc. Cloud hid Mars for three nights; bad seeing cost a fourth. He recovered it on 16 August and found a second one the next night, closer in and much faster. Modern catalogues date the discoveries to 12 and 18 August: the astronomical day in 1877 began at noon, not midnight. The six-night gap is the same either way.

Henry Madan, science master at Eton, supplied the names from Book XV of the Iliad, where Ares is attended by Deimos and Phobos — Dread and Fear. The largest crater on Phobos, nine kilometres across on a moon barely twenty-two across, is named Stickney.

Phobos circles Mars in 7 hours 39 minutes, faster than the planet turns, so from the surface it rises in the west and sets in the east twice a day. Deimos takes about thirty hours. Both are small, dark, irregular lumps. Mariner 7 photographed Phobos in 1969, and Mariner 9 resolved both in 1971. Their origin is still argued over: captured asteroids, or debris from a giant impact on Mars. Phobos is losing altitude at roughly 1.8 metres per century and will eventually be torn apart.

The same close approach gave Giovanni Schiaparelli, in Milan a few weeks later, the canali.