John and others,
I just checked a slide I took of the Giant Dial at Jaipur, the Sumrat
Yantra (large instrument). My note says that the smallest division is 4
seconds of time (1' of arc) and that the radius of the semicircular scale
is 45 feet. That works out for a distance between divisions of 1/6 inch or
about one-half barley corn. As I remember I did not think it possible to
read the fuzzy shadow that closely. Perhaps a shadow sharpener was used.
The gnomon reaches a height of 90 feet above the base and has a stairway
along the center of it of 100 steps.
With a shadow sharpener you would not need a 6" diameter cable. I have
seen the pinhole image (shadow sharpener) of a telephone wire (~1/4 ") at a
height of perhaps 30 feet.
You will always have some sag to the cable. It might be better to place a
tennis ball on the cable as a nodus. Its height could be determined., and
it would make an excellent image in the shadow sharpener. Its height might
vary with temperature, though.
Good luck!
Bill
