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.
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."
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."
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.