You're right, radio waves cannot be reflected from the Sun in the same way they are reflected from solid objects like planets or satellites. This is because the Sun's atmosphere (the corona) is extremely thin and tenuous, meaning there's not enough material to effectively reflect the radio waves back.

Instead of using radar to directly measure the distance to the Sun, we use a different technique called radio ranging. Here's how it works:

1. Sending Signals: We transmit powerful radio signals from Earth towards the Sun.

2. Solar Wind Interaction: These radio signals interact with the charged particles in the solar wind, which streams out from the Sun.

3. Shifting Frequency: The interaction causes the frequency of the radio waves to change slightly (a phenomenon known as Doppler shift).

4. Measuring the Shift: We carefully measure the change in frequency of the radio waves.

5. Calculating Distance: The amount of frequency shift is directly related to the speed of the solar wind, which in turn is related to the distance between Earth and the Sun.

Therefore, we don't use radar reflection to find the distance to the Sun; we use the Doppler shift of radio waves interacting with the solar wind. This method allows us to accurately measure the distance between Earth and the Sun.