While looking for how to shoot a nadir image for a spherical panorama, I found this relevant question which is a superset of what I am looking for.
Only thing is that answers are long-winded, convoluted and do not really address the question except for one which says in one part use a fisheye lens which I do not want (my question would require using a rectilinear lens) and in another part:
There is of course a way to shoot nadir and zenith with a tripod to eliminate it from the picture but believe me it is extremely complicated and involves moving the whole tripod several times.
...which is too vague but what I am looking for. So, can this be cleaned up? If not, then I will need to ask another similarly sounding question. I guess my concern is that the question I wish to ask included in the one I found but the answer is not.