EDIT: vec2.normalize: ... E NON DIVIDE PER ZERO !

planet/planet.c

@@ -65,7 +65,7 @@ void show_system(struct gfx_context_t* ctxt, system_t* system) {
 	// draw planets
 	for (uint32_t i = 0; i < system->nb_planets; ++i) {
 		const coordinates xy = vec2_to_coordinates_centered(
-				vec2_normalize(system->planets[i].pos),
+				vec2_mul(vec2_norm(system->planets[i].pos) / (PER_MARS * 1.25), vec2_normalize(system->planets[i].pos)),
 				ctxt->width,
 				ctxt->height);
 		draw_full_circle(ctxt, xy.column, xy.row, 20, COLOR_BLUE);
@@ -95,7 +95,6 @@ void update_system(system_t* system, double delta_t) {
 		next_pos = vec2_add(next_pos, vec2_mul(delta_t * delta_t, a));
 		system->planets[i].prec_pos = system->planets[i].pos;
 		system->planets[i].pos = next_pos;
-		printf("%lf  %lf  %lf\n", system->planets[i].pos.x, system->planets[i].pos.y, vec2_norm(a));
 	}
 }
 

vec2/vec2.c

@@ -67,6 +67,7 @@ double vec2_norm(vec2 v) {
 /// @return The new normalized vector.
 vec2 vec2_normalize(vec2 v) {
 	double norm = vec2_norm(v);
+	if (0 == norm) { return vec2_create_zero(); }
 	return (vec2) {.x = v.x / norm, .y = v.y / norm};
 }