The Planet Mars:
A History of Observation and Discovery
Hall was the son of a failed clock maker. He and his wife, Angelina, were working as schoolteachers in Shalersville, Ohio, in the 1850s when Hall decided that he wanted to become an astronomer. Though he had not received much formal training (he had stayed only a year at the University of Michigan at Ann Arbor before leaving because of a lack of funds), he applied to become an assistant at the Harvard College Observatory. William Cranch Bond, the observatory's director, was, like Hall, the son of a clock maker. He too had begun his astronomical career without many advantages, and he duly hired the young man to assist himself and his son, George Phillips Bond.
The position was not a lucrative one, and Hall later recalled that when he first met G. P. Bond, who had been away from the observatory when Hall arrived, Bond "had a free talk with me, and found out that I had a wife, $25 in cash, and a salary of $3 a week. He told me very frankly that he thought I had better quit astronomy, for he felt sure that I would starve. I laughed at this, and told him my wife and I had made up our minds that we were used to sailing close to the wind, and felt sure we would pull through."
Hall left Harvard in 1863 for the U.S. Naval Observatory in Washington, D.C., and took charge of its great refractor, the first of the great refractors made by miniature painter--turned-optician Alvan Clark, in 1875. For two years it had been in the hands of Simon Newcomb, who was more interested in mathematical astronomy than in observing, and his assistant, Edward Singleton Holden. Hall later recalled finding "in a drawer in the Eq[uatorial] room a lot of photographs of the planet Mars in 1875. From the handwriting of dates and notes probably Holden directed the photographer, but whoever did the pointing of the telescope had . . . satellites under his eye."
Hall later retraced
the steps that led him to undertake his own search for Martian satellites:
In December, 1876, while observing the satellites of Saturn I noticed a white spot on the ball of the planet, and the observations of this spot gave me the means of determining the time of the rotation of Saturn, or the length of Saturn's day, with considerable accuracy. This was a simple matter, but the resulting time of rotation was nearly a quarter of an hour different from what is generally given in our text books on astronomy: and this discordance, since the error was multiplied by the number of rotations and the ephemeris soon became utterly wrong, set before me in a clearer light than ever before the careless manner in which books are made, showed the necessity of consulting original papers, and made me ready to doubt the assertion one reads so often in the books, "Mars has no moon."
On looking further into the matter, Hall learned that William Herschel had looked unsuccessfully for satellites in 1783, and that the director of the Copenhagen Observatory, H. L. d'Arrest, had done so in 1862 and 1864. (He did not mention Holden's photographic search with the great Washington refractor.) Of these searches, d'Arrest's had been the most thorough. He had been guided by rough calculations of the distance from the planet at which a satellite could exist before it was wrenched away by the Sun into its own planetary orbit, and had set this limit at a distance corresponding to 70' of arc from the planet at greatest elongation. Hall, on redoing the calculation, realized that the actual limit ought to be more like 30' of arc, and that Martian satellites were likely to be found even closer than that to the planet. He began to suspect, therefore, that d'Arrest, for all his thoroughness, had not paid sufficient attention to the inner space near the planet.
When Hall began his quest in early August, he naturally wanted to work alone, so as to receive full credit in the event of a discovery. By great good luck, Holden, his assistant, was invited by Henry Draper to Dobbs Ferry, New York, "at the very nick of time."5 Hall began by scrutinizing faint stars at some distance from Mars itself, but each one soon dropped behind the planet, proving it to be an ordinary field star.6 Next he pressed the search closer, "within the glare of light that surrounded [Mars]," using special observing techniques to reduce the glare, such as "sliding the eyepiece so as to keep the planet just outside the field of view, and then turning the eyepiece in order to pass completely around the planet." On the night of August 10, the first on which Hall attempted to examine the inner space near Mars, he found nothing, but the seeing on the banks of the Potomac was horrible that night, and the image of the planet appeared "very blazing and unsteady." He was on the verge of giving up, but Angelina encouraged him to have one more try, and the next night, at half past two, he found a suspicious object which he referred to in his notebook only as "a faint star near Mars." He scarcely had time to secure its position before the fog began rolling in from the Potomac. The next few nights were cloudy. On August 15, the sky cleared at eleven o'clock, but the atmosphere, Hall noted, was still "in a very bad condition." Not until August 16 did he again find the "star near Mars," which proved, in fact, to be the outer satellite. That night he showed the object to another assistant, George Anderson, but told Anderson to "keep quiet" about it. On August 17, while waiting for that satellite to reappear, he discovered the inner one. In closing his observing notes for the night, he remarked: "Both the above objects faint but distinctly seen both by G. Anderson and myself." Hall had by this time "spilled the beans" to Simon Newcomb, and on August 18, Hall and Anderson were joined in the dome by David Peck Todd, Newcomb, and William Harkness. Todd noted: "Seeing extremely bad: still I saw the companion without any difficulty. `Halo' around the planet very bright, and the satellite was visible in this halo." Only then did Hall announce the discovery of the two satellites. Newcomb tried to gain a share of the credit for himself, implying in an article that appeared in the New York Tribune two days after the discovery was announced that Hall had not fully appreciated what he had found until he---Newcomb---had worked out the period of revolution from the preliminary observations.
Meanwhile, in New York, Holden and Draper were also getting into the act. On August 28, Holden announced that they had used Draper's 28-inch (71-cm) reflector to discover a third satellite, and on returning to Washington, Holden claimed to have found yet a fourth. Hall was skeptical and wrote to Arthur Searle of Harvard: "I think it will turn out that the Draper-Holden moon and the recent Holden moon do not exist."7 He attempted to confirm these alleged discoveries with the Washington refractor without success, and later computations showed that Holden's moon did not even obey Kepler's laws of motion. "Its existence was therefore a mathematical impossibility," Hall wrote to Edward C. Pickering of Harvard Observatory, adding bitterly: "If I were to go through this experience again other people would verify their own moons."
Rumors of Holden's spurious moons would continue to circulate in the astronomical community for years, and Holden became known as the man "who had set all Washington astronomers laughing by detecting a . . . satellite of Mars with an impossible period and distance, and remaining deceived by it for months!" But Holden, in Hall's view, at least, had behaved more admirably than Newcomb. As late as 1904 Hall was still bitter about Newcomb's attempt to usurp credit for the discovery of the satellites, and wrote to S. C. Chandler, Jr.: "Newcomb was greatly excited over my discovery. Holden was away, and Draper made a blunder, and afterwards Holden behaved very well. Newcomb felt disappointed and sore, and something is to be allowed for human nature under such circumstances. He was always greedy for money and glory."
In response to a suggestion by Henry Madan of Eton, England, Hall named the satellites Phobos (Fear) and Deimos (Flight), after the attendants of Mars mentioned in the fifteenth book of Homer's Iliad: "He spake, and summoned Fear and Flight to yoke his steeds." Hall continued to watch the new satellites until the end of October, and his observations gave him the information he needed to work out the mass of Mars---the amount of matter it contains---from its effect on the moons' motions. It was 0.1076 times that of the Earth (a value very close to the currently accepted value of 0.1074). We will examine the satellites in greater detail in chapter 14.
Phobos and Deimos were seen not only by Hall but by viewers using much smaller instruments---indeed, Deimos was glimpsed by Hall, John Eastman, and Henry M. Paul with the U.S. Naval Observatory's own 9.6-inch (24-cm) refractor. This only goes to show that the discovery of the satellites of Mars owed quite as much to Hall's insight---his imagination and willingness to doubt conventional wisdom---as to the size of his glass. As he later wrote, "All that was needed was the right way of looking, and that was to get rid of the dazzling light of the planet."11 He was confident that with the right way of looking, the satellites could have been found "very easily" even with Harvard's 15-inch refractor in 1862.