Question 105470
We are concerened with the norm of the vector (x-x0, y-y0, z-z0) equaling 1. This happens when

{{{sqrt((x-x_0)^2+(y-y_0)^2+(z-z_0)^2)=1}}}}
Square both sides
{{{(x-x_0)^2+(y-y_0)^2+(z-z_0)^2=1}}}
Which you will recgonize as a circle of radius one centered at (x0,y0,z0)