Struggling to Understand GL.Translate & GL.Rotate

/*
 * Solar.c
 *
 * Program to demonstrate how to use a local
 * coordinate method to position parts of a 
 * model in relation to other model parts.
 *
 * Draws a simple solar system, with a sun, planet and moon.
 * Based on sample code from the OpenGL programming guide
 *		by Woo, Neider, Davis.  Addison-Wesley.
 *
 * Author: Samuel R. Buss
 *
 * Software accompanying the book
 *		3D Computer Graphics: A Mathematical Introduction with OpenGL,
 *		by S. Buss, Cambridge University Press, 2003.
 *
 * Software is "as-is" and carries no warranty.  It may be used without
 *   restriction, but if you modify it, please change the filenames to
 *   prevent confusion between different versions.
 * Bug reports: Sam Buss, sbuss@ucsd.edu.
 * Web page: http://math.ucsd.edu/~sbuss/MathCG
 *
 * USAGE:
 *    Press "r" key to toggle (off and on) running the animation
 *    Press "s" key to single-step animation
 *    The up arrow key and down array key control the
 *			time step used in the animation rate.  Each key
 *			press multiplies or divides the times by a factor 
 *			of two (2).
 *	  Press ESCAPE to exit.
 *
 */
 
 
#include "Solar.h"   
#include <stdlib.h> 
 
#include <GL/glut.h>	// OpenGL Graphics Utility Library
 
static GLenum spinMode = GL_TRUE;
static GLenum singleStep = GL_FALSE;
 
// These three variables control the animation's state and speed.
static float HourOfDay = 0.0;
static float DayOfYear = 0.0;
static float AnimateIncrement = 24.0;  // Time step for animation (hours)
 
// glutKeyboardFunc is called below to set this function to handle
//		all normal key presses.  
static void KeyPressFunc( unsigned char Key, int x, int y )
{
	switch ( Key ) {
	case 'R':
	case 'r':
		Key_r();
		break;
	case 's':
	case 'S':
		Key_s();
		break;
	case 27:	// Escape key
		exit(1);
	}
}
 
// glutSpecialFunc is called below to set this function to handle
//		all special key presses.  See glut.h for the names of
//		special keys.
static void SpecialKeyFunc( int Key, int x, int y )
{
	switch ( Key ) {
	case GLUT_KEY_UP:		
		Key_up();
		break;
	case GLUT_KEY_DOWN:
		Key_down();
		break;
	}
}
 
 
static void Key_r(void)
{
	if ( singleStep ) {			// If ending single step mode
		singleStep = GL_FALSE;
		spinMode = GL_TRUE;		// Restart animation
	}
	else {
		spinMode = !spinMode;	// Toggle animation on and off.
	}
}
 
static void Key_s(void)
{
	singleStep = GL_TRUE;
	spinMode = GL_TRUE;
}
 
static void Key_up(void)
{
    AnimateIncrement *= 2.0;			// Double the animation time step
}
 
static void Key_down(void)
{
    AnimateIncrement /= 2.0;			// Halve the animation time step
 
}
 
/*
 * Animate() handles the animation and the redrawing of the
 *		graphics window contents.
 */
static void Animate(void)
{
	// Clear the redering window
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
    if (spinMode) {
		// Update the animation state
        HourOfDay += AnimateIncrement;
        DayOfYear += AnimateIncrement/24.0;
 
        HourOfDay = HourOfDay - ((int)(HourOfDay/24))*24;
        DayOfYear = DayOfYear - ((int)(DayOfYear/365))*365;
		}
 
	// Clear the current matrix (Modelview)
    glLoadIdentity();
 
	// Back off eight units to be able to view from the origin.
    glTranslatef ( 0.0, 0.0, -8.0 );
 
	// Rotate the plane of the elliptic
	// (rotate the model's plane about the x axis by fifteen degrees)
	glRotatef( 15.0, 1.0, 0.0, 0.0 );
 
    // Draw the sun	-- as a yellow, wireframe sphere
	glColor3f( 1.0, 1.0, 0.0 );			
    glutWireSphere( 1.0, 15, 15 );
 
    // Draw the Earth
	// First position it around the sun
	//		Use DayOfYear to determine its position
    glRotatef( 360.0*DayOfYear/365.0, 0.0, 1.0, 0.0 );
    glTranslatef( 4.0, 0.0, 0.0 );
    glPushMatrix();						// Save matrix state
	// Second, rotate the earth on its axis.
	//		Use HourOfDay to determine its rotation.
	glRotatef( 360.0*HourOfDay/24.0, 0.0, 1.0, 0.0 );
	// Third, draw the earth as a wireframe sphere.
    glColor3f( 0.2, 0.2, 1.0 );
    glutWireSphere( 0.4, 10, 10);
    glPopMatrix();						// Restore matrix state
 
	// Draw the moon.
	//	Use DayOfYear to control its rotation around the earth
   	glRotatef( 360.0*12.0*DayOfYear/365.0, 0.0, 1.0, 0.0 );
    glTranslatef( 0.7, 0.0, 0.0 );
    glColor3f( 0.3, 0.7, 0.3 );
    glutWireSphere( 0.1, 5, 5 );
 
	// Flush the pipeline, and swap the buffers
    glFlush();
    glutSwapBuffers();
 
	if ( singleStep ) {
		spinMode = GL_FALSE;
	}
 
	glutPostRedisplay();		// Request a re-draw for animation purposes
 
}
 
// Initialize OpenGL's rendering modes
void OpenGLInit(void)
{
    glShadeModel( GL_FLAT );
    glClearColor( 0.0, 0.0, 0.0, 0.0 );
    glClearDepth( 1.0 );
    glEnable( GL_DEPTH_TEST );
}
 
// ResizeWindow is called when the window is resized
static void ResizeWindow(int w, int h)
{
    float aspectRatio;
	h = (h == 0) ? 1 : h;
	w = (w == 0) ? 1 : w;
	glViewport( 0, 0, w, h );	// View port uses whole window
	aspectRatio = (float)w/(float)h;
 
	// Set up the projection view matrix (not very well!)
    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    gluPerspective( 60.0, aspectRatio, 1.0, 30.0 );
 
	// Select the Modelview matrix
    glMatrixMode( GL_MODELVIEW );
}
 
 
// Main routine
// Set up OpenGL, hook up callbacks, and start the main loop
int main( int argc, char** argv )
{
	// Need to double buffer for animation
	glutInit(&argc,argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH );
 
	// Create and position the graphics window
    glutInitWindowPosition( 0, 0 );
    glutInitWindowSize( 600, 360 );
    glutCreateWindow( "Solar System Demo" );
 
	// Initialize OpenGL.
    OpenGLInit();
 
	// Set up callback functions for key presses
	glutKeyboardFunc( KeyPressFunc );
	glutSpecialFunc( SpecialKeyFunc );
 
	// Set up the callback function for resizing windows
    glutReshapeFunc( ResizeWindow );
 
	// Callback for graphics image redrawing
    glutDisplayFunc( Animate );
 
	// Start the main loop.  glutMainLoop never returns.
	glutMainLoop(  );
 
    return(0);			// Compiler requires this to be here. (Never reached)
}