One hundred and fifty two years ago today, on January 2nd 1860, Urbain Jean Joseph Le Verrier announced to a meeting of the Académie des Sciences in Paris his discovery of the planet Vulcan. Vulcan orbited the sun at a distance of 21 million kilometers (inside the orbit of Mercury which is around 58 million kilometers). The period of revolution was 19 days and 17 hours, and the orbit was inclined to the ecliptic by 12 degrees and 10 minutes. Urbain was already famous for discovering Neptune in 1846; Vulcan was his second planet.
Urbain had predicted the presence of Vulcan due to anomalies in the perihelion precession of Mercury. Classical mechanics fails to account for 43 arcseconds per century of the precession. Urbain had carefully accounted for discrepancies in Uranus’s orbit to discover Neptune; he was convinced the same thing was happening with the larger discrepancies in Mercury’s orbit.
For years documented sightings of Vulcan passing over the disk of the sun confirmed its existence. After Urbain died in 1877 the number of sightings decreased.
In 1915, 38 years after Urbain's death, Albert Einstein used his theory of relativity to more accurately predict the perihelion precession of Mercury.