American astronomer Asaph Hall discovered a moon of Mars known as Phobos on August 18, 1877. It was just six days after he discovered a moon of Mars known as Deimos on August 12, 1877. Hall’s historic observation from the U.S. Naval Observatory was a monumental triumph of observational astronomy that shattered years of scientific skepticism.
For decades, astronomers utilizing elite European telescopes had repeatedly searched the immediate vicinity of Mars, confidently declaring that the Red Planet was completely devoid of natural satellites. Operating the observatory’s massive 26-inch “Great Equatorial” refractor telescope, Hall battled intense summer humidity and near-blindness from the glare of the planet, systematically searching closer to the Martian atmosphere than any previous investigator had dared.
The discovery of the inner moon, which Hall beautifully named Phobos after the mythological Greek personification of fear, provided planetary science with invaluable data regarding gravitational orbital mechanics. Moving with an unprecedented velocity that allowed it to complete three full orbits around Mars in a single day, the tiny, potato-shaped moon challenged existing theories of satellite formation. Hall’s persistent, late-night vigilance turned a quiet Washington observatory into a global epicenter of scientific triumph, permanently cementing his place among the premier astronomers of the nineteenth century.
Image: Historic astronomical engraving of planetary observations via Wikimedia Commons, public domain
