The summer of 1877 was a period of intense astronomical anticipation. Mars, the enigmatic red planet, was approaching a particularly favorable opposition, drawing closer to Earth than it had in years. This celestial dance offered a golden opportunity for astronomers, a chance to peer deeper into the mysteries of our planetary neighbor. Telescopes across the globe were being pointed skyward, but it was at the United States Naval Observatory in Washington, D.C., that a discovery of profound significance was about to unfold, forever etching the name Asaph Hall into the annals of astronomy.
The Man and the Machine
Asaph Hall III was not a man to shy away from a challenge. Born in 1829 in Goshen, Connecticut, he had a somewhat unconventional path into astronomy. Largely self-taught after a basic schooling, his early life was marked by carpentry work to support his family. However, a passion for mathematics and the stars led him to Harvard College Observatory as an assistant, and eventually, in 1862, to the U.S. Naval Observatory. By 1877, Hall was a seasoned astronomer, known for his meticulous observational skills and quiet determination.
His principal tool for this Martian quest was the jewel of the Naval Observatory: the 26-inch Alvan Clark & Sons Great Equatorial Refractor. Installed in 1873, it was then the largest refracting telescope in the world. Its immense light-gathering power and resolving capabilities provided an unprecedented view of the cosmos, making it the perfect instrument to scrutinize Mars for any faint companions.
Whispers of Martian Moons
The idea of Mars having moons was not new. Johannes Kepler, in the 17th century, had speculated that Mars might possess two moons, reasoning partly by analogy with Earth’s one moon and Jupiter’s then-known four Galilean moons. Literary works, notably Jonathan Swift’s “Gulliver’s Travels” (1726) and Voltaire’s “Micromégas” (1750), had even fancifully described two Martian moons, remarkably close to their actual orbital characteristics. However, these were imaginative conjectures, not scientific observations. Numerous astronomers had searched for Martian satellites before Hall, including luminaries like William Herschel, but all had come up empty-handed. The prevailing belief was that if Mars had moons, they must be incredibly small or faint, or perhaps nonexistent.
The Arduous Search
Despite the history of failures, Hall was encouraged to undertake a systematic search for Martian moons, particularly by his wife, Angeline Stickney Hall, a mathematician and an intellectual herself. As Mars neared its closest point to Earth in August 1877, Hall began his painstaking observations. The work was grueling. He had to contend with Washington’s often humid summer nights, the glare from the bright Martian disk which could easily overwhelm any tiny, nearby objects, and the sheer fatigue of long hours at the eyepiece.
His initial efforts in early August were discouraging. He scanned the immediate vicinity of Mars night after night, finding nothing. According to some accounts, Hall was on the verge of abandoning the search. It was reportedly Angeline who urged him to persevere, to give it one more try. This encouragement proved pivotal.
Asaph Hall’s persistence, bolstered by his wife Angeline Stickney Hall, led to a monumental breakthrough in planetary science. Using the powerful 26-inch refractor at the U.S. Naval Observatory, he systematically scanned the Martian vicinity during its 1877 opposition. This dedication overcame previous failed attempts by other astronomers and revealed two previously unknown natural satellites.
First Glimmer: Deimos
On the night of August 10th, Hall noted a faint object near Mars, but fog rolled in, cutting his observation short. He was uncertain. Was it a star? A defect in the lens? The next night, August 11th (often cited as August 12th Universal Time), the seeing conditions were better. There it was again – a tiny speck of light. He carefully recorded its position. Over the next few nights, he re-observed it, confirming its motion relative to Mars, consistent with an orbiting body. This was no mere background star; it was a moon. This first discovery would later be named Deimos.
Hall initially kept his findings quiet, wanting to be absolutely sure and to gather more data. The astronomical community was small, and news, especially of this magnitude, traveled fast, but verification was paramount.
A Second Surprise: Phobos
As Hall continued to track his first discovery, he also widened his search pattern slightly. Just a few nights later, on August 17th (often cited as August 18th UT), his diligence was rewarded yet again. He spotted another faint object, this one even closer to Mars and moving more rapidly than the first. This second object was brighter and more readily apparent. He had found a second moon, which would come to be known as Phobos.
The discovery of two moons, where none were thought to exist, was astonishing. Phobos, being closer to Mars, orbits the planet in just over 7.5 hours, faster than Mars rotates. This means that to an observer on Mars, Phobos would appear to rise in the west and set in the east, multiple times a Martian day.
Naming the Celestial Twins
With the existence of two Martian moons confirmed, the next task was to name them. Hall, perhaps reflecting his classical education or taking a suggestion from Henry Madan, a science master at Eton College in England, turned to Greek mythology. In Homer’s Iliad, Ares (the Greek counterpart to the Roman god Mars, the god of war) is attended by his two sons, Phobos (meaning fear or panic) and Deimos (meaning dread or terror). These names seemed fitting companions for the warlike planet.
It’s widely acknowledged that Angeline Stickney Hall played a significant role not just in encouraging the search but possibly in the intellectual environment that led to appropriate naming. Her mathematical abilities and support were invaluable to Asaph Hall throughout his career.
The Nature of Phobos and Deimos
Subsequent observations, and much later, spacecraft missions, revealed more about these celestial bodies. Phobos and Deimos are not the majestic spherical moons like Earth’s. Instead, they are small, dark, and irregularly shaped, resembling asteroids. Phobos, the larger of the two, measures about 27 x 22 x 18 kilometers, while Deimos is about 15 x 12 x 10 kilometers. They are among the smallest moons in the solar system.
Their surfaces are heavily cratered, testament to a long history of impacts. Both moons are tidally locked with Mars, meaning they always present the same face towards the planet, much like Earth’s Moon. Their dark, carbonaceous composition and irregular shapes strongly suggest they are captured asteroids, perhaps from the asteroid belt, snared by Mars’ gravity long ago. However, some aspects of their near-equatorial, circular orbits pose challenges to the simple capture theory, and alternative origin hypotheses, such as formation from debris around Mars, are still debated.
A Legacy Cast in Starlight
The discovery of Phobos and Deimos was a major scientific event of the late 19th century. It electrified both the scientific community and the public. It demonstrated the power of new, larger telescopes and the importance of persistent, careful observation. For Asaph Hall, it brought international fame and numerous accolades, including the Gold Medal of the Royal Astronomical Society in 1879.
More than just adding two new objects to the solar system’s inventory, Hall’s discovery provided crucial new data points for understanding planet formation, orbital mechanics, and the diverse nature of planetary satellites. It fueled further interest in Mars and spurred astronomers to look for other faint moons around other planets. The story of Asaph Hall and the Martian moons remains a classic tale of scientific dedication, the power of observation, and a little bit of timely encouragement, forever changing our view of the Red Planet and its place in the cosmos.
The legacy of that summer in 1877 continues to inspire. Every time we look at images of Mars with its tiny, tumbling companions, we are reminded of Asaph Hall’s keen eyesight, his formidable telescope, and his unwavering quest to uncover the secrets held by the night sky. These tiny moons, Fear and Dread, serve as constant reminders of a remarkable human achievement in the grand theater of celestial exploration.
