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In astronomy, Kepler's laws of planetary motion are three scientific laws describing the motion of planets around the Sun. #The orbit of a planet is an ellipse with the Sun at one of the two foci. #A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.〔Bryant, Jeff; Pavlyk, Oleksandr. "(Kepler's Second Law )", ''Wolfram Demonstrations Project''. Retrieved December 27, 2009.〕 #The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit. Most planetary orbits are almost circles, and careful observation and calculation is required in order to establish that they are actually ellipses. Using calculations of the orbit of Mars, whose published values are somewhat suspect,〔http://www.nytimes.com/1990/01/23/science/after-400-years-a-challenge-to-kepler-he-fabricated-his-data-scholar-says.html?pagewanted=1〕 which indicated elliptical orbits, Johannes Kepler inferred that other heavenly bodies, including those farther away from the Sun, also have elliptical orbits. Kepler's work (published between 1609 and 1619) improved the heliocentric theory of Nicolaus Copernicus, explaining how the planets' speeds varied, and using elliptical orbits rather than circular orbits with epicycles.〔 Isaac Newton showed in 1687 that relationships like Kepler's would apply in the solar system to a good approximation, as consequences of his own laws of motion and law of universal gravitation. Kepler's laws are part of the foundation of modern astronomy and physics.〔See also G. E. Smith, ("Newton's Philosophiae Naturalis Principia Mathematica" ), especially the section (''Historical context ...'' ) in ''The Stanford Encyclopedia of Philosophy'' (Winter 2008 Edition), Edward N. Zalta (ed.).〕 ==Comparison to Copernicus== Kepler's laws improve the model of Copernicus. If the eccentricities of the planetary orbits are taken as zero, then Kepler basically agrees with Copernicus: #The planetary orbit is a circle #The Sun at the center of the orbit #The speed of the planet in the orbit is constant The eccentricities of the orbits of those planets known to Copernicus and Kepler are small, so the foregoing rules give good approximations of planetary motion; but Kepler's laws fit the observations better than Copernicus's. Kepler's corrections are not at all obvious: #The planetary orbit is ''not'' a circle, but an ''ellipse''. #The Sun is ''not'' at the center but at a ''focal point'' of the elliptical orbit. #Neither the linear speed nor the angular speed of the planet in the orbit is constant, but the ''area speed'' is constant. The eccentricity of the orbit of the Earth makes the time from the March equinox to the September equinox, around 186 days, unequal to the time from the September equinox to the March equinox, around 179 days. A diameter would cut the orbit into equal parts, but the plane through the sun parallel to the equator of the earth cuts the orbit into two parts with areas in a 186 to 179 ratio, so the eccentricity of the orbit of the Earth is approximately : which is close to the correct value (0.016710219) (see Earth's orbit). The calculation is correct when perihelion, the date the Earth is closest to the Sun, falls on a solstice. The current perihelion, near January 4, is fairly close to the solstice of December 21 or 22. 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「Kepler's laws of planetary motion」の詳細全文を読む スポンサード リンク
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