Asaph Hall finds two moons in six nights
Mars came within 56 million kilometres in 1877. Hall had nearly given up the search when, six nights apart, he caught Deimos and then Phobos.
Mars reached a perihelic opposition on 5 September 1877, closing to about 56 million kilometres — the sort of approach that comes round roughly every fifteen years. Asaph Hall, then 47 and in charge of the 26-inch Alvan Clark refractor at the US Naval Observatory in Washington, spent the weeks either side of it hunting the glare right next to the planet for anything that moved with it. The instrument had been the largest refracting telescope in the world when it was installed in 1873. The site was not: the observatory sat in Foggy Bottom, on low ground beside the Potomac, and the river mist routinely shut him down.
He had decided to stop. His wife, Chloe Angeline Stickney Hall — a mathematician who did much of the reduction work on his observations — told him to go back out. On the night of 11 August, Washington time, he caught a faint point of light close to the disc. Cloud and river fog then took Mars away from him for five nights. He recovered the object on 16 August, and the next night, while waiting for it to come round again, he found a second one, closer in and moving much faster. Modern catalogues date the two discoveries to 12 and 18 August, because the astronomical day in 1877 still began at noon, not midnight; the six-night gap is the same either way.
The names came from Henry Madan, science master at Eton, who took them 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.
What Hall had found took another century to understand. Phobos circles Mars in 7 hours 39 minutes, faster than the planet turns, so from the Martian 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; nobody knew their shapes until Mariner 9 photographed them in 1971. Their origin is still argued over — captured asteroids, or debris thrown up by a giant impact on Mars. Phobos is losing altitude at roughly 1.8 metres per century and will eventually be torn apart.
The timing is the joke of the era. The same close approach that gave Hall two genuine, permanent objects gave Giovanni Schiaparelli, working in Milan a few weeks later, the canali. One of those discoveries survived contact with better telescopes.