How To Make A Model Of Solar System - Have Sun And Earth Revolving Around It

I'm attempting to make a model of the solar system. Up till now, I have a Sun and the Earth revolving around it. The problem I have is when I add inheritance to the program. The moon that I created does not orbit the Earth.

Here's my code for the classes I've created

public class Driver {
public static void main(String[] args){
SolarSystem model = new SolarSystem(700, 700);
/* Create new instances of each object in the solar system */
SunObject sun = new SunObject();
PlanetObject earth = new PlanetObject(140, 30, 15, "BLUE", 2);
MoonObject moon = new MoonObject(15, 30, 2, "WHITE", 1.5);

[Code] ....

Note that SolarSystem is a class given to me. That contains all the paint methods. I'm just creating new instances with the drawSolarObject()

And the drawSolarObjectAbout takes an extra two parameters:

centreOfRotationDistance - the distance part of the polar co-ordinate about which this object orbits.
centreOfRotationAngle - the angular part of the polar co-ordinate about which this object orbits

I have a PlanetObject class:

public class PlanetObject extends PointInSpace {
private double distance;
private double angle;
private double diameter;
private String colour;
private double speed;

[Code] ....

And a MoonObject class, which is EXACTLY the same as the PlanetObject class. The main problem I'm having is when I extend my PlanetObject class to a PointInSpace class below:

public class PointInSpace {
private double distance;
private double angle;
public double getPointDistance(){
return distance;

[Code] .....

What I'm trying to do is use the distance and angle in the PointInSpace class so that my Moon can orbit those two coordinates . I get it working when I method override when drawing the moon - using earth.getDistance() instead of earth.getPointDistance() etc. Have I inherited properly? Should I try a method overloading process instead?