Earth and sun - How to make the sun revolve around the earth in the following code?

Question

I want to imitate the planetary system with Earth and Sun drawn as solid spheres on the same window. I have drawn the two: the earth as the bigger sphere and the sun as the smaller sphere.

Now I want the earth to revolve around the sun when I press key A and when I press key B the earth must rotate around its axes. How do I achieve this? Thanks.

Here is the code so far:

 #include <GL/glut.h>
 void init(void){
 glClearColor(0.0,0.0,0.0,0.0);
 glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 }
 void display(void){
 glClearColor(0.05,0.05,0.5,0.0);
 glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 glColor3f(0.03,0.05,0.09);
 glutSolidSphere(0.5,30,30);//Earth
 glMatrixMode(GL_MODELVIEW);
 glLoadIdentity();
 glTranslatef(.7,.7,.7);
 glColor3f(1.0,1.0,1.0);
 glutSolidSphere(0.2,50,50);//Sun
 glLoadIdentity();
 glFlush();

 }
int main(int argc, char **argv){
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE);
glutInitWindowPosition(10,10);
glutInitWindowSize(400,400);
init();
glutCreateWindow("Planets");
glutDisplayFunc(display);

glFlush();
glutMainLoop();

}

Answer 1

Lets approach this from outside the code. You are attempting to have user input make on object revolve around another. This means that we have two objects, the fixed object A, and the revolving object B. For A we draw it at one location and it never changes. For object B we start it at an initial position and update it's position every tick based on user input. Let's start with a simple path of movement and have B move on a circle centered at A. Recall that any point on a circle can be looked at as (r cos(x), 0, r sin(x)) where r is the radius and x is the angle, note that our y axis is 0 because we are rotating about it. So every time the user presses a key you can rotate by changing the current angle that object B is at.

Steps make it a bit easier.

Initialize position of objects A and B. Initialize angle of B to be 0.

User Input If (KeyDown = "A") angle x += 1 // degree, radian : your pick else if (KeyDown = "B") angle x -= 1

At Each Tick Draw B at (r cos(x), 0, r sin(x))