Hall finds a second Martian moon, tucked in close to the planet
Mars was making its closest approach to Earth in years, and Asaph Hall had spent weeks at the US Naval Observatory searching for moons no one had ever confirmed. On August 18, 1877, working with the observatory's 26-inch refractor, the largest in the world, Hall pinned down the inner moon after nights of clouds and poor visibility, with the confirming observations spanning what contemporary sources record as both August 17 and August 18 under differing date conventions. He recorded it plainly in his notebook: the observations of the 17th and 18th put the discovery "beyond doubt." Admiral Rodgers announced it publicly soon after.
The discovery happened on American government equipment, at the US Naval Observatory, using the largest refracting telescope on Earth at the time, at a time when European observatories held most of the field's prestige. Hall's find gave the young Naval Observatory international standing, proof that an American telescope could out-discover Europe's.
QWhy do two different dates, August 17 and August 18, both show up for the same discovery?
Astronomers of Hall's era didn't start the calendar day at midnight. Until 1925, the professional convention began the astronomical day at noon, so a discovery made in the small hours of what civilians called August 18 was logged as occurring on August 17. Under that old system, contemporary sources timestamp the find precisely: 16:06 Washington mean time on August 17. Hall's own notebook entry, written in ordinary civil dates, says the confirming observations spanned the 17th and 18th. Both dates describe the same observation, recorded under two different clock conventions, one of which astronomy abandoned in 1925.
QWhat did Hall tell his assistant before he was sure of what he'd found?
On August 16, two days before confirming Phobos, Hall pulled aside his assistant George Anderson and told him he thought he might be onto something, but he asked Anderson to keep it quiet. Hall wasn't ready to stake his reputation on a smudge of light that bad weather might yet explain away. It's a small detail in an otherwise technical notebook: Hall confided in one colleague before he was ready to tell anyone else.
QHow long did Hall wait to tell anyone besides his wife what he suspected?
Hall had Deimos in hand on August 12, but he did not show his measurements to another astronomer until August 17, five days later, when he brought them to Professor Newcomb specifically to prove the object was tracking with Mars rather than sitting still as a fixed star. That gap between discovery and outside confirmation meant Hall spent nearly a week verifying his own numbers alone at the eyepiece before seeking a second opinion. Only after Newcomb reviewed the measures, and further observations on the 17th and 18th removed any doubt, did Admiral Rodgers step in to make the find public.
