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Three laws of planetary motion, published by Johannes Kepler using accurate observations by Tycho Brahe and shown by Isaac Newton to be a direct result of his theory of gravitation and his laws of motion:
1. Planets move in ellipses, with the Sun at one focus.
2. The line connecting the planets to the Sun sweeps equal areas in equal times.
3. The square of a planet's orbital period is proportional to the cube of its mean distance from the Sun.
Comments:
1st law: This corrected the simpler model of Copernicus, which assumed circles. More accurately, the focus is at the center of gravity of the Sun and orbiting body (discounting other planets) and non-periodic motions along parabolas or hyperbolas are also possible.
2nd law: The second law expresses the way a planet speeds up when approaching the Sun and the way it slows down when drawing away.
3rd law: The third law gives the exact relation by which planets move faster on orbits which are closer to the Sun, e.g. Venus moves faster than Earth (see retrograde motion). For a more precise formulation, "mean distance" should be replaced by semimajor axis.
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