Please take a look at this diagram:

torus1.jpg
It is a diagram of a torus (doughnut, tyre, bangle etc). It can also be shown as in this diagram:

torus3.jpg

which can be rendered as this:
torus4.jpg

Going back to the first diagram,
Ro is the distance from the centre of the the large object (the Origin of these axes) to the point midway between the inner and outer edges of the ring
R1 is the radius of the cross section between the inner and outer rings.
theta is an angle measured anti-clockwise from the X-axis to Ro.
phi is an angle made by the radius measured from origin the X/Z axis of the cross section ( consider the case of theta = 0) to place that cross section.

If theta is know, is it possible to determine phi through geometry?
If it is, please post the steps of your calculation.

Old Man Emu