Sorry, I actually have to backup Tom here...
"Ideally, we would like to bounce radio waves off of the Sun, measure how long it takes
them to get there and back, and use the speed of light to find the distance to the Sun.
Since the Sun is a ball of gas and doesn’t reflect radio waves well, this idea will not work.
However, we can bounce radio waves off of Venus, and then use Kepler’s Third Law to
the Earth-Sun distance."
Straight out of the ol' HAM radio bible, and schools everywhere.