Solar system · Planet
Planet
Mars
The red planet, rusty with iron oxide. Every two years or so Earth overtakes it, and it blazes for a few weeks.
Look for
Watch it for a few weeks around opposition and it moves backwards against the stars — the “retrograde loop” that puzzled astronomers until Copernicus.
History
Kepler used Tycho Brahe’s measurements of Mars to discover in 1609 that planets move in ellipses, not circles.