Lunars: More musings on accuracy, Part II #252
Description
I was rereading Stark's "CLEARING the LUNAR DISTANCE". A couple thoughts I want to share:
He talks (quotes Bowditch) about "When a great angular distance is to be measured...the parallelism ... with respect to the plane of the instrument must be carefully examined...".
Stark then has a chart (attached) that shows the impact of this issue. And then combine this with the LD chart I made (attached) and you can see that innate and random measurement errors are the reason that lunars are so sloppy (though my observations last night are probably mostly poor observation/sextant skills).
I usually don't do long LDs, but last night was an exception. In taking the shots, I could see what Bowditch was talking about in the "parallelism of the scope to the plane of the sight." Thing get very squirrelly with large LD and stars!
But this is not nearly the case when you are taking a near LD or taking a sun lunar because it is much easier to center the bodies.
Taking star shots has anther factor that is, in my opinion, impossible to get better than 1' measurement accuracy:: the star image "shakes" a lot (and I was sitting in my stationary living room!). That shaking is not nearly so bad doing a sun shot--I think that is why, in the old lunar tables, they went up to 140° LD for sun shots, but for stars they only went to <90° LD (and >30°LD). Even with sun lunars, I think that LD measurement accuracy better than 0.5', can not be expected.
It is real cool that we can digitally calculate these numbers to six decimal places, but innate lunar observation errors make them orders of magnitude less accurate.
