Vulcan, in astronomy, hypothetical planet whose existence was proposed by Leverrier to explain part of the advance of the perihelion of Mercury, not all of which could be accounted for by gravitational effects of the other planets under the Newtonian theory of gravitation. The general theory of relativity explains the observed advance of the perihelion of Mercury as being caused by the curvature of space in the vicinity of the sun as a result of the sun's large mass.
Vulcan, in Roman religion and mythology, fire god. Chiefly a god of destructive fire, Vulcan seems to have originated as a god of volcanoes. His festival, the Volcanalia, was held on Aug. 23. He was later identified with the Greek Hephaestus.

Hypothetical planet within the orbit of Mercury. It was predicted in 1859 by Urbain-Jean-Joseph Le Verrier (1811–77) to account for an unexplained component of the precession of Mercury's orbit. Sightings were reported between 1859 and 1878, but these were not confirmed subsequently by observations made either during solar eclipses or when the planet was predicted to cross the Sun. The anomalies of Mercury's orbit were later explained by Albert Einstein's general theory of relativity.

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