calculating next position within an orbit (f5)

[kay_siegert]

i can´t figure out whats wrong
i´m trying to rotate an object about the origin(x=0, y=0).

to manage that i first tried to calculate the next x and the y coordinate for my object to move to.
NOTE: it´s only for one quadrant (where x and y are positive)!

onClipEvent(load){
        // init
	movingDegree=2;
	_x=50;
	_y=100;
}
onClipEvent(enterFrame){
        // calculating radius and angle referring to my origin
	a = Number(_x);
	b = Number(_y);
	r = Math.sqrt((a*a)+(b*b));
	Angle = Math.atan(b/a)*180/Math.PI;
        // works great so far

        // now i´m increasing the angle (2 degrees)
        // ... and calculate adjacent leg (nextX) and the opposite leg(nextY) that are the coordinates of my next point at the same time

	nextAngle = Number(Angle+movingDegree);
	nextX=Number(Math.cos(nextAngle)*r);
	nextY=Number(Math.sin(nextAngle)*r);
}

i get a completely different result when i calculate it with my pocket calculator.
i don´t get it.
the file is attached.
thankful for any help,
k.