Kepler’s Third Law
Kepler’s Third Law relates the period of revolution of a planet, denoted by T, to the semimajor axis of the orbit of that planet, denoted by a. The square of T is directly proportional to the cube of a.
We know that the rate of change the area swept out by a radius is constant so
Our derivation consists of finding the appropriate expressions for A and DA. Recall that
and
We have the final form for A but will have to work a while more on DA. In doing this, we will find an expression for H that is more meaningful to the geometry of an ellipse.
Consider the orbit equation derived in a previous section:
In the ellipse below, we have picture the point in the orbit of the planet when \nu=\frac{\pi}{2}. This is when the segment SP is perpendicular to the major axis of the ellipse.
Let p be the length of the segment SP. So p=\frac{H^2}{\mu} from the equation for r with \nu=\frac{\pi}{2}.
We may compute p another way from the ellipse below:
By the Pythagorean Theorem
and solving this for p gives us
so
Therefore,
and so
Therefore,
so that finally we have Kepler’s Third Law
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