Applet shows the planets Mercury,Venus and Earth revolving round the Sun. The distances and times of motion are scaled correctly. You should be able to notice that while orbits of Venus and Earth are nearly circular (The eccentricities of their orbits are 0.0068 and 0.0167 respectively), the orbit of the innermost planet Mercury has noticeable ellipticity. ( It has an eccentricity of 0.2056 ). The semi major axes of the orbits of the three planets are 0.3871, 0.7233 and 1 A.U. respectively. Time the periods of revolution of each planet by watching its motion and find the ratio of square of time period and the cube of semi-major axis for each planet. You would find the ratio to be a constant. It may be convenient to express the time of revolution in terms of a year and the semi major axis in terms of A.U., so that this constant is 1 for the planet Earth. Obviously the constant would be 1 for the remaining two planets also.
Planetary Data
Planet Period of revolution Period of rotation Semi major axis( A U ) Eccentricity around the sun (in Years) around own axis Mercury 0.241 58.6 days 0.387 0.206 Venus 0.615 243 days 0.723 0.007 Earth 1.00 23 h 56 m 4 s 1.00 0.017 Mars 1.88 24 h 37 m 23 s 1.524 0.093 Jupiter 11.86 9 h 50 m 5.203 0.048 Saturn 29.46 10 h 25 m 9.54 0.056 Uranus 84.01 710 h 50 m 19.18 0.04 Neptune 164.79 16 h 30.07 0.008 Pluto 248.43 6.4 days 39.44 0.249