Mathematicians and astronomers were just coming into their own, in the late 1800s. The sciences were taken seriously when it came to looking at our solar system. So when something abnormal occurred, it was these scientists who latched onto what data they could gather, and formulated an explanation.

Urbain Jean Joseph Le Verrier was a well-established mathematician, with credits that included predicting the position of Neptune, from calculations related to the unusual motions of Uranus. He had been working on a recently discovered abnormality in the movements of Mercury, and announced during an 1860 lecture, that he suspected an intra-Mercury planet, or an asteroid belt within Mercury's orbit, to be the root cause.

But the only way to observe such a body, would be during its transit across the face of the sun, or during a total eclipse. Fellow scientists contributed their observations of some two dozen spots on the Sun that appeared to have orbits of 26 and 38 days.

Le Verrier's conclusions however, had been drawn largely on the evidence of an amateur astronomer, who in 1859 had supplied him with reports of sighting a black dot on the sun, as well as data pertaining to its transit time, orbital inclination, and length of observation. From this, Le Verrier calculated the length of the body's orbit, the inclination, and mean distance from the sun.

When all the math was done, the "body" was estimated to have only one-seventeenth of Mercury's mass, and a diameter that was much smaller, which meant it was not big enough to affect the stability of Mercury's orbit. Le Verrier then theorized that is was the largest single component of an intra-Mercury asteroid belt, and he named it Vulcan.

Despite subsequent sightings of spots on the sun, most notably after the total eclipse of 1878, no further evidence of Vulcan was ever seen. Albert Einstein's 1916 publishing of his Theory of Relativity, gave a much more stable explanation for Mercury's unusual movements.