Question #a0a4c
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When we have a spacecraft at another planet, we know the time it takes a radio signal to travel between Earth and the spacecraft. This give you the distance between a given planet and Earth. Then you can use Kepler law to determine the others.
We can also send a powerful radar signal toward a planet and time how long it takes for the echo to return. We know these signals travel at the speed of light, so measuring how long they take makes it easy to calculate the distance very accurately.
As an aside it may be interesting for you to know that one of the first people to make a good measurement of the distance to a planet was the great astronomer Gian Domenico Cassini. In 1672, Cassini used a technique called parallax to measure the distance to Mars.
