The Earth is on a tilted axis. The angle or radial tilt is 23.439281deg. This means the slice thru the center out to 23.439281deg and also extending out thru the center to -23.439281deg. How do you model this in spherical coordinates? I have a 3D Sphere: For phi = 0:360 For theta = 0:360 x = Rearth*cos(phi)*cos(theta); y = Rearth*cos(phi)*sin(theta); z = Rearth*sin(phi); plot(x,y,z) hold on; end; end; What I need is an equation for a slice out of the 3D sphere thru the center at the radial tilt angle, 23.439281deg (It does not have to be a computer program).