Make your own planet, Janet !
Let's start with the most simple application: A 2D view of Eart and one planet orbiting the sun on cicle Orbits. In contradiction to the Earth, the planet Orbit radius can be modified and hence the period time due to Kepler's 3rd law.
On the right you recognize how the planet size changes according to the distance planet-Earth.
In the "Earth at Rest" view, the Earth rests in the Center and the planet Orbits the Earth in Loops.
The next applets are closer to reality, but also more complex.