//Initalize variables
float G, M;
/* where:-
G is the gravitational force constant
M is the mass of the planet
*/

float r, Theta, Omega, v;

void setup(){
  size(800,600);
  frameRate(60);
  
  G = 6.67*pow(10,-11);
  M = 18*pow(10,17);
  
  Theta = 0;
}

void draw(){
  background(0);
  
  r=(dist(width/2,height/2,mouseX,mouseY));
  
  pushMatrix();
  translate(width/2,height/2);
  
  //draw planet
  stroke(0);
  fill(0,255,0);
  ellipse(0,0,80,80);
  
  //draw moon
  v = sqrt((G*M)/r);
  Omega = v/sqrt(pow(r,2));
  Theta += Omega/frameRate;
  stroke(0);
  fill(0,255,0);
  ellipse(r*cos(Theta),r*sin(Theta),20,20);
  
  popMatrix();
}