In 1840,
François Arago, the director of the
Paris Observatory, suggested to mathematician
Urbain Le Verrier that he work on the topic of
Mercury's orbit around the
Sun. The goal of the study was to construct a model based on Sir
Isaac Newton's laws of
motion and
gravitation. By 1843, Le Verrier published his provisional theory regarding Mercury's motion, with a detailed presentation published in 1845, which would be tested during a
transit of Mercury across the face of the Sun in 1848. Predictions from Le Verrier's theory failed to match the observations. Le Verrier postulated that the excess precession could be explained by the presence of some unidentified object or objects inside the orbit of Mercury. He calculated that it was either another Mercury-sized planet or, since it was unlikely that astronomers were failing to see such a large object, an unknown asteroid belt near the Sun. The fact that Le Verrier had predicted the existence of the planet
Neptune in 1846, using the same techniques, lent veracity to his claim.
Claimed discovery On 22 December 1859, Le Verrier received a letter from Lescarbault, saying that he had seen a transit of the hypothetical planet on 26 March of that year. Le Verrier took the train to the village of
Orgères-en-Beauce, some south-west of
Paris, to Lescarbault's home-made observatory. Le Verrier arrived unannounced and proceeded to interrogate the man. Lescarbault described in detail how, on 26 March 1859, he observed a small black dot on the face of the
Sun. After some time had passed, he realized that it was moving. He thought it looked similar to the transit of
Mercury which he had observed in 1845. He estimated the distance it had already traveled, made some measurements of its position and direction of motion and, using an old clock and a pendulum with which he took his patients' pulses, estimated the total duration of the transit (coming up with 1 hour, 17 minutes, and 9 seconds). at the meeting of the
Académie des Sciences in Paris. Lescarbault, for his part, was awarded the
Légion d'honneur and invited to appear before numerous learned societies. However, not everyone accepted the veracity of Lescarbault's "discovery". An eminent French astronomer,
Emmanuel Liais, who was working for the Brazilian government in
Rio de Janeiro in 1859, claimed to have been studying the surface of the Sun with a telescope twice as powerful as Lescarbault's, at the very moment that Lescarbault said he observed his mysterious transit. Liais, therefore, was "in a condition to deny, in the most positive manner, the passage of a planet over the sun at the time indicated". Based on Lescarbault's "transit", Le Verrier computed Vulcan's orbit: it supposedly revolved about the Sun in a nearly circular orbit at a distance of . The period of revolution was 19 days and 17 hours, and the orbit was inclined to the
ecliptic by 12 degrees and 10 minutes (an incredible degree of precision). As seen from the Earth, Vulcan's greatest
elongation from the Sun was 8 degrees. Shortly after 08:00 on 29 January 1860, F.A.R. Russell and three other people in London saw an alleged transit of an intra-Mercurial planet. Many years later, an American observer, Richard Covington, claimed to have seen a well-defined black spot progress across the Sun's disk around 1860 when he was stationed in
Washington Territory. No observations of Vulcan were made in 1861. Then, on the morning of 20 March 1862, between 08:00 and 09:00
Greenwich Time, another amateur astronomer, a Mr. Lummis of Manchester, England, saw a transit. His colleague, whom he alerted, also saw the event. Based on these two men's reports, two French astronomers,
Benjamin Valz and
Rodolphe Radau, independently calculated the object's supposed orbital period, with Valz deriving a figure of 17 days and 13 hours and Radau a figure of 19 days and 22 hours. Both Watson and Swift had observed two objects they believed were not known stars, but after Swift corrected an error in his coordinates, none of the coordinates matched each other, nor known stars. The idea that
four objects were observed during the eclipse generated controversy in scientific journals and mockery from Watson's rival
C. H. F. Peters. Peters noted that the margin of error in the pencil and cardboard recording device Watson had used was large enough to plausibly include a bright known star. A skeptic of the Vulcan hypothesis, Peters dismissed all the observations as mistaking known stars as planets. Finally, in 1908,
William Wallace Campbell, Director, and
Charles Dillon Perrine, Astronomer, of the
Lick Observatory, after comprehensive photographic observations at three solar eclipse expeditions in 1901, 1905, and 1908, stated: "In our opinion, the work of the three Crocker Expeditions ... brings the observational side of the intermercurial planet problemfamous for half a centurydefinitely to a close." == Hypothesis disproved ==