Next message: Frank Reed: "Re: Frank's method of observing"
Bruce S wrote:
" I also wanted to be able to measure long lunar distances accurately. An
inverting scope has wires in the field of view that help you keep the plane of
observation parallel to the plane of the instrument. On page viii of the
introduction to my Tables there's a small table showing how important this can be. "
Checking for parallelism of the telescope (which is what those passages in
old editions of Moore and Bowditch were discussing) does not require "wires in
the field of view". The references to the wires are simply descriptive. The
authors could as easily have said "right" and "left" but since
sextant scopes commonly had those wires back then, it was easier to reference
them. This check for parallelism can be done today just as easily as 200
years ago, but it's a little bit harder to describe verbally without those
wires...
You refer to this as a process designed to keep the plane of observation
parallel to the plane of the instrument. I think this is a misunderstanding. The
important factor is parallelism of the telescope (unless that is all you meant
by plane of observation!). If it's not parallel, then you will get different
angles, by a fraction of a minute of arc typically, depending on where in the
field of view you bring the two images together.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
