How can radar be used to find the distance from earth to sun?
The most basic principle of radar is something called the "Doppler effect." All radars send out a signal of a particular wavelength. When measuring distance the time it takes a signal to reach an object and be reflected back tells us the distance. The wave lengths compression or expansion tells us its motion via the Doppler effect. In the case of the sun, an radio signal directed to the sun would simply be sucked into the sun, there would be no bounce.