Tycho Brahe measured the distances from Earth to Mars using the parallax method over many years. When Kepler plotted the numbers he realized Mars moves in an elliptical orbit. And he generalized that all planets do.
Click or tap the Start Button under the black window. You might need to wait a little for the JavaScript to load and run.
If it's taking too long, you can run the program over on GlowScript:
Earth Mars Distance on GlowScript