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单词 Kepler’s laws
释义
Kepler’s laws

Physics
  • Three laws of planetary motion formulated by Johannes Kepler in about 1610 on the basis of observations made by Tycho Brahe (1546–1601). They state that: (1) the orbits of the planets are elliptical with the sun at one focus of the ellipse; (2) each planet revolves around the sun so that an imaginary line (the radius vector) connecting the planet to the sun sweeps out equal areas in equal times; (3) the ratio of the square of each planet’s sidereal period to the cube of its distance from the sun is a constant for all the planets.


Astronomy
  • Three laws governing the orbital motions of the planets, discovered by J. Kepler. The first law states that the orbit of a planet is an ellipse with the Sun at one focus of the ellipse. The second law states that the radius vector joining planet to Sun sweeps out equal areas in equal times. The third law states that the square of the orbital period of each planet in years is proportional to the cube of the semimajor axis of the planet’s orbit. The first law gives the shape of the planet’s orbit; the second describes how the planet must continuously vary its speed as it follows its orbit, moving fastest at perihelion and slowest at aphelion. The third law gives the relationship between the planets’ average distances from the Sun and their periods of revolution.

    From his law of gravitation and three laws of motion, I. Newton generalized Kepler’s first law, verified the second law, and showed that the third law should be amended to the form

    4π2a3/T2=G(m+mp),

    where T and a are the period of revolution and semimajor axis of the orbit of a planet of mass mp about the Sun of mass m, and G is the gravitational constant.


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