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On 6 Jan, I wrote-

Responding to Jan Kalivoda, I said, inter alia-

>One effect remains, however. Let's say, for argument, that the effect of
>changing >parallax halves the apparent rate of change of lunar distance
>(which may not be far >from the truth, in a bad example). Then measuring
>the apparent lunar distance to a >certain accuracy implies that the true
>lunar distance is only being measured to half >that accuracy, because only
>half of the change in apparent distance results from >changes in the true
>distance, the other half coming from changing parallax.
>
>==========
>
>Looking back at that once again, I am no longer completely convinced by my
>own >argument. Please don't take it too seriously at present, while I
>think again.

=================

Since then, I have pondered further about that argument, and the more I
pondered, the less convinced I have become. So, with apologies, I wish to
withdraw that paragraph, please, if nobody minds.

The effect of changing parallax is (nearly) always to reduce the
rate-of-change of the apparent lunar distance, and that effect is greatest
when the Moon passes the meridian.  The effect can be very great at high
Moon altitudes, such as to (roughly) halve the Moon's apparent motion in
the sky. This will show up readily if apparent lunar distance is plotted
against time, when its slope can often be seen to be considerably less than
the expected value of about 0.5 degrees per hour. This is what we have
named "parallactic retardation" and it's a real effect. Arthur Pearson on
his website www.ld-DEADLINK-com shows a number of graphs which illustrate
the reality of parallactic retardation on the apparent lunar distance.

But from that, I have, in the past, presumed that when the rate of change
of the Moon's apparent motion is (for example) halved by the effects of
parallax, then the accuracy of any GMT deduced from a lunar-distance
measurement will be halved also. However, that conclusion doesn't follow: I
was wrong.

As I see the matter now, a more correct view is this-

The "clearing" process corrects the apparent lunar distance for the effects
of parallax (etc) to give the true lunar distance, which after correction
will always be changing at about 0.5 degrees per hour. And it's from the
true lunar distance that the GMT is deduced. So as long as the parallax
correction is made precisely in the clearing process (and that's important,
because it can be so large, compared to the accuracy required) then the
accuracy of the resulting GMT will not be diminished by the Moon's
parallax, even in circumstances when that parallax is changing rapidly.

==============================

I have put forward that erroneous view, just a year ago, on 11 Jan 03, in a
posting to this list "About Lunars, Part 4a" (which itself was a correction
to an earlier posting, "About Lunars, Part 4").

In Part 4a, I wrote, about parallactic retardation-

>This effect has some serious implications for lunar observations when the
>Moon is high in the sky. Not that one will get an erroneous answer, the
>corrections for parallax see to that. But one will get a less precise
>answer. If there's an error of 1 arc-minute in the lunar distance, then
>with a high Moon, the resulting error in GMT becomes not 2 minutes of time
>(as we presumed before), but 4. And so the resulting error in longitude
>doubles, from 30 to 60 arc-minutes.

and-

>One recommendation could clearly be made as follows- If lunars were always
>limited to times when the Moon altitude was less than 30 degrees, the
>adverse effects of Moon parallax would be less that half the values
>referred to above, because the rate-of-change of parallax varies with
>sin(alt). And yet, Moon altitudes should be kept above 10 deg, to minimise
>uncertainties in refraction, so it doesn't leave a big allowable range.

I now think that those "serious implications" do not exist, and that
"recommendation" now makes no sense. There is no reason to limit lunars to
altitudes below 30 degrees.

That posting "About Lunars, Part 4a" is archived at-

www.irbs/com/lists/navigation/0301/0066.html

Somehow, it needs an amended version, Part 4b, which I must think about.

Thanks to Jan Kalivoda, whose penetrating questions triggered my pondering
process. I apologise if I have led listmembers up the garden path about
this matter.

George.



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