Presentation #407.02 in the session History (iPosters).
After Urbain Le Verrier (1811-1877; Paris Observatory) announced that the precession of Mercury’s orbit must be caused by an inter-Mercurian planet not yet discovered, there were sporadic, unverified reports of such a planet in solar transit. However, there are plenty of other mechanisms through which this ‘data’ could be interpreted (e. g., sunspots). The best way to find the supposed new planet (to be named Vulcan) was during a total eclipse of the Sun.
The 1869 total solar eclipse seemed to be an excellent opportunity to locate Vulcan. Nearly every observer commented on the appearance of Mars to the left of the eclipsed Sun and Venus to the right. These bright orbs conveniently marked the plane of the ecliptic.
Indeed, Simon Newcomb (1835-1909; United States Naval Observatory) narrowed the pursuit still further. His calculations showed that Vulcan’s orbital radius must be only half or less that of Mercury itself. He knew just where to look. Newcomb traveled to Iowa in order to observe the total eclipse but later wrote in his private diary that he was “was disappointed in not finding [an] intra-mercurial planet . . .”
Elsewhere in Iowa, Benjamin Gould (1828-1896; formerly, Dudley Observatory) searched for Vulcan during the same total eclipse of the Sun. He memorized the star field that would be the total eclipse’s backdrop. Not a thing unexpected appeared in the eye piece of his telescope.
Fredrick Bardwell (United States Naval Observatory) hoped to find Vulcan using a special chart that he drew, showing the positions of stars and planets near the Sun in the sky for 7 August 1869. Nothing ‘extra’ showed up during the total solar eclipse as observed from Tennessee.
Alonzo Mossman (1835-1913; United States Coast Survey) was in Tennessee looked for Vulcan, too. Nothing to report.
George Searle (1839-1918; formerly, Harvard College Observatory) was on the hunt in Kentucky, as was James Haines (Cincinnati Observatory) from the Dakota Territory 1,200 kilometers away. Again, no.
That should have been it. Due diligence had been given Vulcan by well qualified, equipped, and spaced professional observers. Yet astronomers continued to search for the would-be planet during nearly every total solar eclipse until 1908!
Why? The 1869 total eclipse of the Sun demonstrated something else besides the absence of Vulcan. During that total eclipse, astronomers realized that the appearance of the solar corona changed from eclipse to eclipse. Thus, they could allow the ‘missing’ planet special pleading: Maybe during the ‘next eclipse,’ the corona would be dimmer, or appear smaller in radius, allowing the chimeric Vulcan finally to appear.