Vulcan. Artwork of the hypothetical planet Vulcan, which was once believed to orbit in an asteroid belt closer to the Sun than Mercury. Vulcan was supposedly first sighted by amateur astronomer Lescarbault on 26 March 1859. French astronomer Le- verrier used this sighting and unexplained devia- tions in Mercury's orbit to calculate the mass and orbit of a hypothetical planet. Naming it Vulcan, he claimed that it had about 0.3% of the Earth's mass and orbited the Sun every 19 days 7 hours at an average distance of 21.4 million kilometres. Further observations were inconclusive, and in 1916 Albert Einstein's theory of relativity explained the deviations in Mercury's orbit.