Johannes Kepler (1571-1630)
The most radical thing Kepler
did was to remove the assumption of circular motions, a motion
viewed by the Greeks and Copernicus as sacred. This simple
generalization allowed Kepler to make a model which was consistent
the the data acquired by Tycho.
Kepler's Laws of Planetary Motion
Law I: Law of Ellipses
Each planet moves about the Sun in an orbit that is an
ellipse
with the Sun at one focus of the ellipse. The eccentricity is defined as
the ratio of the distance of the focus (from the center of the ellipse) to
the semi-major axis (c/a in the notation of the figure). For a circle,
the eccentricity is then 0. For the planets in our Solar System, the
eccentricities range from 0.007 (Venus) to
0.248 (Pluto).
The orbits are nearly circular, however. The distortions are quite small and
visually hard to see, in general:
Law II: Law of Equal Areas
A straight line joining a planet and the Sun sweeps out equal areas
in equal amounts of time. The consequence of this is that a planet moves
fastest when it is closest to the Sun.
Law III: Harmonic Law
The square of the sidereal revolution period of a planet
is proportional to the cube of the size of its orbit
(actually,
semi-major axis of its
orbit). The planets closer to the Sun have the shortest sidereal
orbital periods. For the Solar System,
P2 (years) = a3 (Astronomical Units).
The above is a log-log plot, log P vs. log a |
In tabular form,
Note that the last two entries in the above table are discrepant.
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Application
The asteroid belt sits between the orbits of Mars and Jupiter. Compare
the orbital period for an object that has orbit of size 4 A.U. to the
orbital period of the Earth. Using Kepler's 3rd Law we have that the
square of the orbital period measured in years is
P2 = a3 = 43 = 64
So the orbital period of the asteroid measured in years
is the square root of 64, that is, the orbital period of the asteroid
is 8 years.
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