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I'm a bit confused, I hope you can help.

The question is -

In the course of their orbits, the distance between the Earth and Venus changes. The radii of the orbits of the two planets are 1.5 x 10e8 km and 1.1 x 10e8 km. If a radio pulse is transmitted from Earth towards Venus, calculate the greatest and least times that would be measured before the return pulse was detected.

This is what I think I'm supposed to do:

a) Calculate the maximum radius of the Earth's orbit and Venus' orbit. Also, calculate the minimum radius of each planet's orbit.

I'm not sure how to do this. It's not mentioned in the question but as far as I know both those values of r (radius) given in the question are mean radii for each planets orbit around the Sun. I'm guessing I need a max and min value of r for each planetary orbit but I don't know how to work it out. I think it may involve using Kepler's Third Law but I have been unsuccessful with my attempts so far.

b) With my max and min values of r for each planet I can work out the max and min distance between Earth and Venus, then I can calculate the time a radio pulse travels between the two using time = distance/speed, speed being the speed of light 3 x 10e8 m/s.

I appreciate your help.

Thanks

Darren

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# Homework Help: Planetary motion, orbits

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