Hi!

Here's the code you need:
I'm not the best in astronomy so you have to adjust the angles/speed/directions you need by your selfe.

Code:

---

// planetarium
// ------------------------------------------
string 	str_mdl_moon	= <moon.mdl>;
define 	_moondist, skill1;

function trabant{
var lv_angle;

	while(you){
				
		// rotate
		lv_angle=(lv_angle-time*2)%360;
		
		// around "you" (angles)
		vec_for_angle(temp,vector(lv_angle,0,0));

                // with this distance (from "you")
		vec_normalize(temp,you._moondist);
		vec_add(temp,you.x);
		vec_set(my.x,temp.x);		
		
		// and turn to earth
		vec_set(temp,you.x); 
		vec_sub(temp,my.x);
		vec_to_angle(my.pan,temp);
		
		wait(1);
	}	
}

//uses: _moondist
action planet {
	
	// check values
	if(my._moondist==0){
		my._moondist=1000;
	}
	
	// create a trabant with distance
	you=ent_create(str_mdl_moon,nullvector,trabant);	
	
	// rotate
	while(1){
		my.pan=(my.pan+time*2)%360;		
		wait(1);
	}	
}

---

Just assign the action planet to your earth model and be shure to hava a moon model (moon.mdl) or rename the string at the top of the script.
Adjust the property (skill1 of the planet in WED) for your purpose...

have a nice moon, mercuryus