# Plane Intersecting a Sphere

#### Aristarchan

##### New member
I was thinking this morning of X,Y and Z axes, visualizing a plane intersecting a sphere at varying depths from it's surface. Would the movement of that plane in a direction perpendicular to it's two dimensions, so that a circle indicating where the sphere intersects with the plane changes diameter but stays centered on the same point in that plane, be always referred to as movement along a Z axis?

#### lev888

##### Senior Member
Not sure I understand the question. How exactly is the coordinate system defined? In any case, a plane moving in the direction of its normal will intersect any sphere in the way you describe.

#### pka

##### Elite Member
I was thinking this morning of X,Y and Z axes, visualizing a plane intersecting a sphere at varying depths from it's surface. Would the movement of that plane in a direction perpendicular to it's two dimensions, so that a circle indicating where the sphere intersects with the plane changes diameter but stays centered on the same point in that plane, be always referred to as movement along a Z axis?
Consider the sphere $$x^2+y^2+z^2=25$$ see here.
Now the points $$P_1: (5,0,0),~P_2,~(0,5,0)~\&~P_3,~(0,0,5)$$ are on that sphere.
Thus the plane determined by those points is $$x+y+z=5$$ SEE HERE