practice, and on the precise details of the particular observation in question. He may well have journeyed to Orgeres to satisfy himself that he did not need to trouble himself further with Dr Lescarbault, but Leverrier returned to Paris convinced that a planet had been found orbiting between Mercury and the Sun. At a public meeting of the Academy of Sciences on 2 January 1860, Leverrier informed his audience of Lescarbault's letter and their subsequent meeting, and suggested that the details of his observations ought to be admitted to science.

This was sufficient to send shock waves through the scientific community and beyond. In France, Leverrier's reputation was further enhanced, and Lescarbault was catapulted to unwanted fame. On Leverrier's advice, Napoleon III awarded Lescarbault la Legion d'houneur, though the recipient did not travel to receive it in person. The romantic picture of an unpretentious amateur being responsible

8 Lescarbault to Leverrier. Published in the C. R. Acad. Sci. 50 (1859). Quoted from Baum and

9 Sheehan (1997).

This bizarre meeting subsequently was described at length by the Abbé François Moigno, with the characters referred to as the lion and the lamb (see Baum and Sheehan (1997), pp. 150-4).

for an epoch-making discovery - just as Herschel had been 80 years previously -was appealing to people the world over, and Lescarbault became a rather unlikely hero. The planet he had discovered soon acquired a name - it was 'Vulcan'.10

From Lescarbault's numbers, Leverrier calculated an approximate orbit for Vulcan. He placed the planet at 0.147 AU from the Sun in a circular orbit inclined to the ecliptic by just over 12° and with a synodic period of about 20 days. There were at least three important problems with this. First, Vulcan should have been visible clearly through a telescope when at elongation (which Leverrier calculated to be 8° from the Sun), and during a solar eclipse it would have shone like a bright star. Second, it should pass in front of the Sun at least twice, and possibly four times, each year. Finally, its mass (which Leverrier put at one-seventeenth that of Mercury), was insufficient to cause the necessary perihelion advance. In order to maintain a belief in the existence of this planet, therefore, one had to assume that it was unusually dim and that there were many more such bodies as yet undetected.

Far-fetched as it was, this is precisely what Leverrier did assume. His psychological attachment to the existence of asteroids within the orbit of Mercury was so great that he had no choice. Based on the unquestionable truth of universal gravitation, he had 'proved' theoretically that such matter must be present, much as he had 'proved' the existence of Neptune. If this meant that some established ideas about celestial phenomena had to change, then so be it. A proof is a proof, after all.

Leverrier's commitment to intra-Mercurial planets, and to Vulcan in particular, encouraged others. Reports of sightings started to flood in, including many claims that Vulcan had been seen prior to Lescarbault's observation. But not everyone was prepared to accept one man's theoretical calculations as a reason for turning a blind eye to all manner of unexplained facts that came with the assumption of the existence of Vulcan. Foremost among the critics was another French astronomer, Emmanuel Liais, who was working in Brazil at the time of Lescarbault's observation. Liais was not one to be bullied by Leverrier's

10 In 1846, another French scientist, Jacques Babinet, had hypothesized the existence of gaseous masses close to the Sun as a way of explaining the large prominences that had been observed around the Sun during the solar eclipse of 1842. He had used the name 'Vulcan' for this intra-Mercurial matter. Interestingly, Babinet (in 1848) had sided with those like Peirce who believed that the Neptune discovered by Galle was not the planet predicted by theory, and had suggested that the differences between the actual orbit of Neptune and those that had been predicted by Adams and Leverrier were due to the presence of another planet outside the orbit ^ of Neptune. He chose Hyperion for its name.

Leverrier could estimate the required mass of a disturbing planet as a function of its orbital radius by setting e' = 0 in Eqn (12.1).

reputation, and was one of those who, after the realization that the actual orbit of Neptune was rather different from those of Adams' and Leverrier's hypothetical bodies, had claimed that Galle's successful discovery had been just good fortune. Liais had good grounds for being sceptical about Vulcan, for he had been observing the Sun at precisely the same time that Lescarbault reported having seen his circular black spot, and he had seen nothing.12

Gradually, Vulcan mania died down. New 'sightings' appeared sufficiently often to rekindle the fire briefly, but no corroborated evidence for the existence

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