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Vulcan is a small hypothetical planet that was proposed to exist in an orbit between Mercury and the Sun. Attempting to explain peculiarities of Mercury's orbit, the 19th-century French mathematician Urbain Le Verrier hypothesized that they were the result of another planet, which he named "Vulcan". A number of reputable investigators became involved in the search for Vulcan, but no such planet was ever found, and the peculiarities in Mercury's orbit have now been explained by Albert Einstein's theory of general relativity.〔 Searches of NASA's two STEREO spacecraft data have failed to detect any vulcanoids between Mercury and the Sun that might have accounted for claimed observations of Vulcan.〔 It is doubtful that there are any vulcanoids larger than in diameter.〔 Other than Mercury, asteroid with a semi-major axis of has the smallest known semi-major axis of any known object orbiting the Sun.〔 == Argument for Vulcan's existence == In 1840, François Arago, the director of the Paris Observatory, suggested to the French mathematician Urbain Le Verrier that he work on the topic of the planet Mercury's orbital motion around the Sun. The goal of this study was to construct a model based on Sir Isaac Newton's laws of motion and gravitation. By 1843, Le Verrier published his provisional theory on the subject, which would be tested during a transit of Mercury across the face of the Sun in 1843. As it turned out, predictions from Le Verrier's theory failed to match the observations. Le Verrier renewed his work and, in 1859, published a more thorough study of Mercury's motion. This was based on a series of meridian observations of the planet as well as 14 transits. The rigor of this study meant that any differences from observation would be caused by some unknown factor. Indeed, there still remained some discrepancy.〔 During Mercury's orbit, its perihelion advances by a small amount each orbit, technically called perihelion precession. The phenomenon is predicted by classical mechanics, but the observed value differed from the predicted value by the small amount of 43 arcseconds per century. Le Verrier postulated that the excess precession could be explained by the presence of a small planet inside the orbit of Mercury, and he proposed the name "Vulcan" for this object. In Roman mythology, Vulcan was the god of beneficial and hindering fire, including the fire of volcanoes, making it an apt name for a planet so close to the Sun. Le Verrier's recent success in discovering the planet Neptune using the same techniques lent veracity to his claim, and astronomers around the world attempted to observe a new planet there, but nothing was ever found. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vulcan (hypothetical planet)」の詳細全文を読む スポンサード リンク
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