by David Baron
Transits of Mercury occur infrequently, just thirteen or fourteen times a century, while those of Venus are even rarer. Because of the alignment of orbits, Mercury can cross the face of the sun only in November or May.
On the first Saturday in May of 1878, the New-York Tribune alerted readers that just such an event would occur in a couple of days:
Next Monday the astronomers of Europe and this country will bend their energies and point their telescopes toward the planet most nearly bathed in solar fires. Perhaps, however, after this investigation, such a phrase will no longer describe Mercury. For the main object to be attained by Monday’s research, is to determine whether or no there is some other planet or planets nearer the sun.
For two decades, this question had cleaved the astronomical community: Does another planet circle the sun within the orbit of Mercury? Some scientists were convinced that such an intra-Mercurial planet did exist. They gave it a name—Vulcan, for the Roman god of fire—and listed it in encyclopedias and textbooks as a legitimate member of the solar system’s family. Such pronouncements sparked public imagination about life on a world where, given the planet’s hurried orbit, the years raced by. “The Fourth of July must return with maddening rapidity, and the Vulcanites must be scourged with Centennial Exhibitions at least four times as often as the inhabitants of any part of our slower and more considerate planet,” mused The New York Times.
Another camp of astronomers, though, “declined to believe in the existence of this planet,” the newspaper explained. The skeptics scoffed at the believers as if they were “pretended discoverers of the sea-serpent. In fact, the alleged planet Vulcan was looked upon very much in the light of an astronomical sea-serpent.”
No astronomer was more skeptical than C. H. F. Peters, the world’s most prolific asteroid hunter. “Professor Peters of Hamilton college, the indefatigable discoverer of new stars . . . pronounces [Vulcan] a myth,” one newspaper reported. “While setting out his own views . . . he takes pains to say that Professor Newcomb of Washington, at one time swallowed this intra-Mercurial planet whole, so to speak, but it is a bait he has himself always scorned.”
Newcomb, in reality, had long expressed doubts about Vulcan’s existence, but he kept an open mind. James Craig Watson, on the other hand, had firmly taken the bait—and the proverbial hook, as well. “Prof. Watson has long been a believer in the existence of a planet inside the orbit of [M]ercury, as those attending his lectures in past years are fully aware,” the University of Michigan’s student newspaper explained.
The U.S. Naval Observatory hoped that the 1878 transit of Mercury, carefully observed, would resolve this dispute. Instead, the day’s events would only inflame it.
THE IDEA THAT VULCAN existed had arisen from earlier observations of Mercury, a planet whose orbit is swift and eccentric, tracing a path more oval than round. Mercury’s closest approach to the sun, a point called its perihelion, shifts incrementally forward with each passage, so that as the planet wheels around the sun, its orbit makes its own gradual orbit.
That Mercury’s perihelion would advance slowly could be explained by the gravitational attraction of the other known planets, but the magnitude of this effect did not match scientists’ calculations. “Mercury has, or has been thought to have, defied the ordinary rules for a planet’s behavior,” wrote a Chicago newspaper. “[T]he point of perihelion in the orbit of Mercury has moved from west to east more rapidly than is allowed by the figures of the mathematicians.”
The mathematician who had studied this mystery most carefully was the late director of the Paris Observatory, Urbain Le Verrier (sometimes Anglicized “Leverrier”), the same man who had snubbed Maria Mitchell on her visit to France. A brilliant yet imperious scientist—“Even members of the [French] Academy could not suppress their detestation of him,” Simon Newcomb recalled—Le Verrier had established his genius in 1846 when he discovered the planet Neptune, not with a telescope but by sheer calculation. (As a colleague put it, he found the planet au bout de sa plume, at the tip of his pen.) Le Verrier had accomplished this feat by studying the motion of another world—Uranus. Previously the outermost known planet, Uranus was found to veer perplexingly from its predicted orbit, and Le Verrier concluded that the strange behavior could be explained by the gravitational attraction of an unknown planet residing even farther from the sun. He then calculated the size and location of this hypothetical object based on its effects. When astronomers in Berlin aimed their telescope at the appointed corner of the sky, they found Neptune. It was a triumph of Newtonian mechanics and French skill.
Later, Le Verrier turned his attention to Mercury. As he had done with Uranus, he calculated what kind of object would explain the perturbations in Mercury’s orbit, and he prophesied the existence of Vulcan (or, perhaps, multiple Vulcans). In 1859, he called on astronomers to search for such an intra-Mercurial planet, but finding a world at the extreme inner reaches of the solar system presented problems. Sitting so close to the solar disk, Vulcan must rise and set with the sun, and therefore it would not be visible at night—perhaps not even at dawn or dusk. Like Mercury and Venus, however, Vulcan should be seen, on occasion, in transit.
Over the years, some astronomers claimed to see just that—an unknown planet sailing across the solar face—but these supposed transits of Vulcan were always suspect, reported by amateur astronomers using inferior equipment. In several cases, the observers had likely seen sunspots that they mistook for the hypothetical planet. In no cases were the observations corroborated.
“[D]uring the last ten or fifteen years the sun has been studied so assiduously by professional astronomers that they necessarily would have fallen in with a transit if a planet at a distance from the sun less than Mercury’s existed,” C. H. F. Peters concluded. “We have to consider, therefore, the non-existence of such a planet or group of planets as a question set at rest.” As Peters saw it, Vulcan had been called into existence for one reason—to solve a small mathematical discrepancy in Mercury’s orbit—and a more logical solution to the mystery lay not in inventing a new planet but in questioning whether there was a problem that needed solving at all. Given Mercury’s proximity to the sun, the planet was difficult to observe (so difficult, in fact, that Copernicus was said never to have seen it), and its orbit was therefore hard to pin down. Perhaps historical calculations of Mercury’s orbit had been inaccurate. Perhaps new, more exact observations would show that Mercury’s motion was perfectly explainable without the need to conjure a new planet.
Such was the importance of the upcoming transit of Mercury—it now offered an opportunity to measure the planet’s orbit with unprecedented precision. As Simon Newcomb explained, the event should answer “whether the result of LE VERRIER, that the motion of the perihelion of Mercury is much greater than that due to the action of the known planets, is really correct.” If new observations showed Le Verrier to be wrong, then Vulcan would vanish as it had first appeared—in a mathematical puff of smoke.
EARLY ON MONDAY, MAY 6, the sun lifted into a clear sky over the fields and treetops of eastern Michigan. James Craig Watson had spent weeks preparing for this day. Behind the Detroit Observatory he had erected a photoheliograph—essentially a forty-foot-long horizontal telescope that, by means of a rotating mirror, projected a stationary image of the sun onto a photographic plate. With this, he and two assistants planned to take dozens of pictures of Mercury’s slow traverse, which would last seven and one half hours. First, however, they had to identify the precise time when the event began.
During any transit or eclipse, astronomers are careful to note several key junctures, called contacts. First contact would be the moment when Mercury’s outer edge—its limb—first appeared to touch the limb of the sun. (Maria Mitchell described first contact as “the exact instant when an unseen spherical body appears to touch a seen spherical body.”) At that time the planet, viewed through a telescope, would look like a tiny notch on the sun’s external edge. Second contact would o
ccur a few minutes later when the entire sphere of Mercury could finally be seen, like a bead of black ink about to drip from the sun’s edge into the fiery interior. Third and fourth contacts would come at the end of the transit when, respectively, Mercury reached the other limb of the sun and then exited the solar disk altogether.
If Le Verrier’s computation of Mercury’s orbit was correct, first contact should occur just shy of 9:38 A.M. Ann Arbor time. Watson opened the observatory dome and swung the large equatorial telescope into position, aiming it at the sun’s eastern limb. He focused his gaze through an eyepiece that magnified the heavens by a factor of 400 (the light having already passed, presumably, through a dark filter to protect his vision). He watched for that tiny dent—evidence that Mercury had begun its ingress. Accurate timing was essential. The seconds ticked by.
Across the country, other eyes and telescopes were similarly fixed on the sun. In Poughkeepsie, Maria Mitchell used the transit as a teaching opportunity. She assembled her astronomy class at the Vassar observatory to view and photograph Mercury. Farther upstate, at Hamilton College, C. H. F. Peters entertained a broader public. He cast an image of the sun onto a paper screen and narrated the unfolding drama to assembled townsfolk, students, and faculty. When a small black dot appeared on the sun’s disk, Peters pointed. “There, professor,” he said to Edward North, who taught Greek, “you see now what Copernicus never saw.” In New Jersey, Thomas Edison—newly passionate about astronomy—had borrowed a telescope to view the event from his Menlo Park laboratory.
FAC-SIMILE OF A PHOTOGRAPH OF THE TRANSIT.
Edison’s friend George Barker joined a team of scientists at a private observatory owned by Henry Draper, a New York University professor whose telescopes inhabited a twin-domed building north of the city, on the east bank of the Hudson River. Barker, like James Craig Watson, strove to time precisely when Mercury first appeared on the sun, but the observing conditions were far from ideal. Although the skies were clear, instability in the atmosphere caused telescopic images to jiggle as if viewed through vapor rising off a boiling pot. Mercury danced so erratically that Barker was unable to pinpoint the moment of first or second contact. He expressed his frustration on this otherwise-beautiful spring day with a Latin-inspired pun. “Sick transit,” he said, “glorious Monday.”
In Ann Arbor, however, Watson enjoyed an unhindered view of Mercury. “[T]he planet appeared as a sharply defined black disk on the sun,” he wrote, and the precisely timed contacts seemed to leave little doubt as to the day’s conclusion. “The fact that the planet made its first contact with the sun within nine seconds of the computed time shows that the calculations, upon which Leverrier based his theory, were accurate,” wrote The Detroit Free Press. “Therefore the unexplained motion of the perihelion of Mercury is not due to inaccurate observations and calculations, but to an unknown planet.” The newspaper then added: “Prof. Watson thinks that the result of Monday’s observation cannot fail to convert even Dr. Peters to Leverrier’s theory.”
DR. DRAPER’S OBSERVATORY AT HASTINGS-ON-THE-HUDSON.
Such was not the case—“Dr. Peters reiterates his disbelief that there is such a planet as Vulcan,” read a widely printed article—but many other scientists moved toward Watson’s camp. “As the truth of Leverrier’s discovery of an apparently unexplained motion of the perihelion of Mercury is now established beyond all doubt,” the U.S. Naval Observatory announced, “it is important to renew the search for an intra-Mercurial planet or planets.”
One of the few opportunities to search for Vulcan, a planet so close to the sun that it could never been seen at night, would occur in just a few months’ time—when the moon hid the sun from view, during the brief midday darkness of the solar eclipse.
“DEAR SIR: I HAVE just heard that we have a sum of money appropriated for the Solar Eclipse of July 29th,” read the timely letter from the superintendent of the U.S. Naval Observatory, Rear Admiral John Rodgers. Congress had finally approved that munificent sum of $8,000 to send expeditions to the West, and the government now sought volunteers. Rodgers’s form letter, copied with Edison’s electric pen, went to astronomers at America’s top universities and observatories. “Please inform me whether you are inclined to assist in making the desired observations, and which ones you would prefer to make. If you accept, then Railroad tickets to and from your destination, and five dollars per day, will be allowed for expenses.” The replies, marked “accepts” or “regrets,” came in like responses to a wedding invitation.
“It is a great temptation for me to go West for the Eclipse, but I ought not to go,” answered C. H. F. Peters, who was tied up with university obligations and was clearly uninterested in standing in the moon’s shadow to look for a planet he was convinced did not exist. Furthermore, newspapers reported growing tension in Idaho’s Snake River country between white settlers and the Shoshone-Bannocks, the result of broken treaty promises and inadequate government food rations that left the tribes hungry. “So, you go to Montana,” Peters wrote to a friend at the Naval Observatory. “Take care of not being scalped by the Indians!” The Bannock War soon erupted, and the Naval Observatory scrapped its planned expeditions to western Montana.
Despite the unrest, James Craig Watson eagerly agreed to journey to the frontier. “I accept your polite invitation to take part in the Observations of the total eclipse,” he wrote to Admiral Rogers, making it clear that Vulcan was his quarry. “I think I had better search for the intra mercurial planet during the totality, as so many observers will pay attention to the Corona.”
Other responses flowed in as well, from Harvard, Yale, Princeton, Johns Hopkins, the University of Rochester, and observatories elsewhere. Noticeably missing from the list of invitees, however, was the observatory that Maria Mitchell oversaw at Vassar, that brazenly upstart female college in Poughkeepsie.
CHAPTER 8
“GOOD WOMAN THAT SHE ARE”
SATURDAY, JUNE 22, 1878—
Poughkeepsie, New York
RAISED ON AN ISLAND, MARIA MITCHELL WAS PERHAPS USED to being an outsider, but she never accepted her exclusion from science’s inner sanctum, a sex-based segregation that continued even as her fame grew. This discrimination was often promulgated by sympathetic men who professed respect yet were blind to their own condescension.
In April 1878, Mitchell had visited the Smithsonian Institution, although not, of course, for the meeting of the all-male National Academy of Sciences. Rather, she had met the prior week with leaders of the Woman’s Congress to plan for that group’s annual convention, to be held in Providence in October. Before going to Washington, Mitchell had reached out to her supporter Joseph Henry, head of the Smithsonian, seeking a space for her humble gathering of prominent women. “Our meeting will be for a few hours only,” she wrote. “Even, for so small a meeting, it would help us in the eyes of many who do not investigate, if we were under the roof of a building devoted to the diffusion of knowledge.”
Henry responded awkwardly. He consented to the use of a Smithsonian office, but on the stipulation that Mitchell “not . . . publish an account of your meetings in the newspapers.” His stated excuse was practical; he did not want to encourage a flood of requests from other organizations seeking meeting space. The subtext was implied. He likely did not want to be seen supporting a group that so flagrantly promoted women’s rights and abilities.
Mitchell’s sex—and that of her students—had been used, in a more bald-faced way, to block their participation in a celebrated scientific undertaking just a few years earlier. Astronomers long knew that December 1874 would bring an exceedingly rare occurrence, a transit of Venus, the first in more than a century. By carefully observing the event from different parts of the globe, scientists hoped to calculate a fundamental unit of the solar system—the distance from the earth to the sun. The transit would not be visible from North America or Western Europe, however, so the United States made plans to send astronomers to Asia, the South Pacific, and the Indian
Ocean. (Russia, England, France, and Germany made similar plans.) Among those chosen to lead expeditions were the inveterate planet hunters James Craig Watson, who would travel to China by commercial steamship, and C. H. F. Peters, whose destination was New Zealand via Navy vessel. The expeditions sought young scientists as assistants, and in early 1874 Maria Mitchell inquired if her students might participate. Mitchell’s old boss and friend Charles Henry Davis, founding superintendent of the Nautical Almanac Office and now head of the U.S. Naval Observatory, replied, “[I]t would be absolutely out of the question to make a cultivated woman decently comfortable on board of a man-of-war.” He also ruled out sending women by commercial transport. As he asserted with Victorian bombast, “I should hardly think it worth while to expose a woman to the fatigue, hardships and dangers of so long a winter’s journey, unless her services were actually indispensable.” The short answer was no. In the end, one woman was permitted to accompany the expedition to China, but not in an official capacity, that being James Craig Watson’s dutiful wife.
Now, in 1878, Mitchell found herself yet again excluded. The U.S. Naval Observatory, having received its appropriation from Congress, rapidly scaled up plans for eclipse expeditions and eased the way for astronomers to participate. It bought them train tickets, loaned them telescopes and other instruments, and arranged to ship the equipment—weighing an estimated 10,000 to 15,000 pounds—ahead of time in a specially guarded freight car. (It was a Pennsylvania Railroad fast mail car, “fitted up in a style suited to the requirements of a post office,” with boxes and bins for sorting letters.) The U.S. government even extended courtesies to foreign astronomers, ensuring they could bring their scientific equipment into the country duty free and encouraging railroads to sell train tickets at a discount to scientists from abroad.