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Bill wrote:

> And:
> "I also have not resolved the question posted earlier: "My confusion is that
> 132d is the starting course of a great-circle  route. I am having trouble
> understanding why that would be my azimuth. (spherical trig is still a
> plug-and-chug function for me)."

Frank replied:

> Try drawing the two spherical triangles involved - one for the calculation of
> the great circle distance between two places on the Earth (assumed spherical
> for this), the other for the calculation of star's altitude. Does this help?

It does help, and I think it confirms my suspicions.  Frame of reference is
usually my failure. Let me restate and see if I have a clearer picture.

If I draw a great-circle on an Earth globe and place my eye in a position
over the line in a manner that I see it and the (inner) center of the globe,
I will see a straight line.

If I were able to see a tall flagpole at my destination from  my starting
point, I could consider it to be the azimuth relative to my position.  If
(ignoring leeway and currents for this problem) I steer directly towards the
flagpole I would be steering the great-circle course.

As a point (my craft) moves along the line segment from start to destination
its relationship to the man-made lat/long coordinate system  and true north
will change.  As it moves from one longitude to another its angle to the
parallels of latitude will change (like a straight line on a Gnomonic
projection chart).

If I move my eye toward the equator or pole I would see an arc that is part
of the great circle.

In the case of the the celestial sphere, if I could see the great circle it
would appear as a straight line between my position and the body, and body's
GP.  If I can see the body (which is over the GP like a tall flagpole), then
I would be observing the azimuth of the body.

Am I getting closer?

Thanks

Bill

   

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