Orbital Mechanics Question

I don't know if this is the right forum to post this, there doesn't sem to be anything more specific.

I am working on a Space Simulation, for which I need relatively realistic orbital mechanics.

I have used this link:
http://www.arachnoid.com/ruby/gravity/index.html

To implement a simple system, but it doesn't seem to be working at all, my Moon is moving in a straight line away from the Earth with no signs of slowing down or orbiting.

My code is pretty simple, I have global variables:

F64 grav_const = 6.6742e-11; 
F64 earth_mass = 5.972e24; 
F64 moon_mass = 7.34767309e22;

F64 dt = 50;

F64 velX, velY = 0;
F64 orbitposX = 0;
F64 orbitposY = 356700;//000;

and then in a loop called from script, I have:

ConsoleFunction( testorbit, void, 1, 1, "none")
{
	F64 distanceradius = 384400;//assume constant for now

//f = G M m
//   ------
//    r^2
	F64 force = (grav_const * earth_mass * moon_mass)/(distanceradius*distanceradius);

//a = f/m
	F64 acceleration = force/moon_mass; //moon is moving, so use it's mass

	velX = 0;
	velY = 0;
	velX = velX + acceleration * dt;
	velY = velY + acceleration * dt;

	orbitposX = orbitposX + velX * dt;
	orbitposY = orbitposY + velY * dt;

//send values to script function to move Moon object:

	char value2[256];
	dSprintf(value2,sizeof(value2),"%f",orbitposX);

	char value3[256];
	dSprintf(value3,sizeof(value3),"%f",orbitposY);

//Con::printf("OUTPUT: %f %f ", body_x, body_y);
Con::executef("moveorbits",value2,value3);


}

This is very little code, and so it seems simple enough to work. I have tried scaling down the position of the object in script to make it "fit" into my game world, but it didn't seem to change the results.

I would appreciate any help.