How can radar be used to find the distance from earth to sun?

The fundamental idea behind radar is the "Doppler effect." All radars emit a signal with a specific wavelength; the length of time it takes for a signal to travel to an object and bounce back indicates its distance, and the Doppler effect's ability to compress or expand wave lengths indicates how fast an object is moving. When a radio signal is directed towards the sun, it would simply be sucked into the sun without any bounce.

By bouncing radar signals off the surface of Venus when it is at a known position in its orbit, scientists can use radar to measure the distance between Earth and the Sun. Then, using the known average distance between Venus and the Sun, they can calculate the distance from Earth to the Sun by measuring the time it takes for the radar signals to travel to and from Venus and knowing the speed of light.