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On Jan 15, 2004, at 5:12 PM, George Huxtable wrote:

> In response to my statement-
>
>>> To get to the real lunar distance you MUST measure the apparent lunar
>>> distance (whether it's changing with time or not), and then apply a
>>> correction which, being precisely known, does not degrade the
>>> accuracy
>>> of
>>> that measurement. Because the resulting true Moon is always moving
>>> about
>>> the same speed across the sky, it can always be used to measure time
>>> with
>>> about the same accuracy.
>
> Fred Hebard responded-
>
>> This is enough to make my head spin!
>>
>> Now that it's phrased this way, parallactic retardation _seems_ to be
>> an effect that could affect accuracy: if the apparent lunar distance
>> were not changing at all for some period, then clearly the times at
>> the
>> beginning and end of that period could not be differentiated based
>> upon
>> the apparent distance, which would be the same.

> In the special circumstances we have chosen, that falling-back around
> noon
> happens to be about equal, and opposite, to the motion of the true Moon
> across the star background, so that with respect to the stars, the
> apparent
> Moon has come to a stop. So what? We are not trying to discover the
> time
> when the apparent Moon passes a certain value. We are trying to
> discover
> when the true Moon passes a certain value. And we know, rather
> exactly, the
> difference between the positions of the apparent Moon and the true
> Moon,
!
> by
> observing the Moon's altitude,
!
> and then doing some calculation.  That gives
> the parallax correction, the changing value of which caused the
> apparent
> Moon to go slow in the first place. So, we add that known parallax
> correction back in to the apparent position of the Moon. That step is
> heavily disguised as part of the "clearing" process, but it is very
> real,
> and very important. And that correction brings us back to the true
> Moon's
> position, which will be changing steadily at 0.5 degrees per hour,
> unaffected by parallax.


It would seem then, that the altitude measurement (not especially its
accuracy, but the measurement itself) is a key component of the overall
determination of the cleared distance.  The cleared distance is a
function of _both_ the altitude and the observed distance.  In the
hypothetical case here, of completely stopped motion due to the earth
spinning twice as fast, it would be the magic ingredient that allows
one to map two identical observed distances to two different cleared
distances.

I suppose right around meridian passage of the moon might be a bad time
to take a lunar, especially where it's altitude is about 90 degrees?

   

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