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Re: Real accuracy of the method of lunar distances
From: Frank Reed CT
Date: 2004 Jan 8, 17:51 EST
From: Frank Reed CT
Date: 2004 Jan 8, 17:51 EST
George Huxtable wrote:
"I hope we are not talking at cross-purposes here."
Well, maybe. Let me ask you a question (to see if we are): if the Moon is passing nearly but not exactly overhead --let's say 80 degrees high-- and it's right on the meridian when you take the lunar, are you saying that you believe there's a significant reduction in the potential acuracy of the lunar because of the rate of change of the parallax correction?
George Huxtable wrote:
"Because
everything's in a straight line, the lunar distance is found by simple
subtraction rather than by a complex "clearing" process. And it's clear,
isn't it, that the effect of parallax on that distance is changing fastest
when (cos alt) changes fastest, as the Moon passes overhead, at the point
where (cos alt), and parallax are zero."
Yes, when it passes exactly through the zenith, I agree with that.
I earlier wrote:
"But not always. It increases the rate of change if the Moon happens to be on the other side. The Sun chases a waxing moon." [which is wrong]
George Huxtable replied:
"So with respect to the Sun, the Moon ALWAYS moves more slowly, by
about 1 part in 12, than it does with respect to the stars. No matter which
side of the Sun it's on."
Yes, that was a very silly error on my part. Thank you for the correction.
As a practical matter, the difference of about 8% is probably quite a bit less than the improvement in accuracy that results from using the Sun in a lunar (it's easier to bring two disks into contact than to place a twinkling star on the limb of the Moon).
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
"I hope we are not talking at cross-purposes here."
Well, maybe. Let me ask you a question (to see if we are): if the Moon is passing nearly but not exactly overhead --let's say 80 degrees high-- and it's right on the meridian when you take the lunar, are you saying that you believe there's a significant reduction in the potential acuracy of the lunar because of the rate of change of the parallax correction?
George Huxtable wrote:
"Because
everything's in a straight line, the lunar distance is found by simple
subtraction rather than by a complex "clearing" process. And it's clear,
isn't it, that the effect of parallax on that distance is changing fastest
when (cos alt) changes fastest, as the Moon passes overhead, at the point
where (cos alt), and parallax are zero."
Yes, when it passes exactly through the zenith, I agree with that.
I earlier wrote:
"But not always. It increases the rate of change if the Moon happens to be on the other side. The Sun chases a waxing moon." [which is wrong]
George Huxtable replied:
"So with respect to the Sun, the Moon ALWAYS moves more slowly, by
about 1 part in 12, than it does with respect to the stars. No matter which
side of the Sun it's on."
Yes, that was a very silly error on my part. Thank you for the correction.
As a practical matter, the difference of about 8% is probably quite a bit less than the improvement in accuracy that results from using the Sun in a lunar (it's easier to bring two disks into contact than to place a twinkling star on the limb of the Moon).
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
