If radio waves cannot be reflected from the Sun then how can radar be used to find the distance from the Earth to the Sun?

Answer 1

Yes, we don't use radar to measure the distance to the Sun directly.

However, we can use radar to measure the distance to objects like the

Moon and Venus, which will reflect radar.

We can measure the Venus to Earth distance when Venus is between us

and the Sun (more or less). Then we can simply use Kepler's 3rd Law to

derive the Earth to Sun distance.

Alternatively, trigonometry can be used, once the Earth Venus distance

is known.

We can use the occasions when the Earth-Venus-Sun angle is 90 degrees.

See "related links" below for details of this.

We can also use times when the Earth-Sun-Venus angle is 90 degrees.

It's not that radio waves wouldn't be reflected by the Sun; but the Sun

_GENERATES_ so much radio energy that any reflection would be entirely

swamped by the primary signal.

Think about it this way. You're in a dark theater, and people are wandering

around carrying mirrors. You have a bright flashlight. Will you be able to see the

reflection from the mirrors? Probably yes. Now somebody with one of the mirrors

turns on a spotlight and points it at you. Can you bounce your flashlight off of

his mirror ? Sure, but you can never see it. You're blinded by his spotlight, and

the wimpy reflection of your flashlight from his mirror is invisible in the glare.

Answer 2

Radar uses radar waves, not radio waves, to determine distance. Radar waves can be sent to the Sun and reflected back to Earth because they have shorter wavelengths than radio waves, allowing them to interact with the Sun's surface and return a signal for distance measurement.