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I will just pick up a few points from Richard Pisko's message of Jan 14th.


>Yes, and if the Earth were rotating twice as fast about its axis, it
>would seem lunars may have been of very little use in determining
>time, as it could appear to hang for hours against the stars for a
>given latitude and time of year ... although at a different position
>each night.

Well that's exactly what the present argument has been about. It was my
opinion too, for the last year, but has recently changed.

In the circumstances you describe, with a doubled Earth rotation speed and
the Moon overhead, it would indeed appear to stand still against the stars.
I don't think there's any dispute about that matter.

But the question presently in dispute is the same as your claim that in
those circumstances "lunars may have been very little use in determining
time": and that claim, I now think, is wrong. You don't determine Greenwich
time from the apparent lunar distance, you determine it from the true lunar
distance. The correction between those two, which can be accurately
calculated, means that an unchanging apparent Moon (with respect to the
stars) becomes a True Moon which is moving with normal speed against the
stars, 0.5 deg per hour.

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.

So as a result, the overall precision of a lunar does NOT depend on the
"parallactic retardation" of the apparant Moon. That's my new view, anyway.
Unshaken, as yet...

>On the other hand, an observer at (for example) one
>quarter of the Earth's circumference away at that same latitude, would
>see the moon hanging against a different part of the sky; so the
>longitudes might still be determined as accurately as they are now, if
>the latitude is known.

No, at a different latitude the retardation would be less, so the Moon
wouldn't be "hanging" stationary at all, observed from there..
>
>>It's also true that measuring the altitude of the Moon (which is usually
>>necessary in order to deduce the parallax) can provide a useful
>>position-line to use when the GMT has been obtained.
>>
>I think the accuracy of determining the altitude of the Moon from
>measuring its angular distance from any stars might ordinarily be
>poor, unless there is a star conveniently grazing the horizon right
>under it at the time of observation, but that is one reason I wanted
>to have a three star fix of the aparent position.   This would be
>enough to have a check on the position given by two stars.  I believe
>if the horizon is otherwise is visible a much better latitude may be
>determined and the altitude of the moon calculated ... but I very well
>may be misinterpreting your comment.

No, you wouldn't measure altitude of the Moon from its angle to a star. You
would need a daytime observation, to the Sun, or to a star at dusk, and
measure up from the horizon. You can't use angles to stars to determine the
Moon's altitude, unless you know the positions of those stars in the sky.
And for that, you need the Greenwich time. Which is where we came in.

As long as the horizon is visible, the altitude of the Moon can be
observed, it doesn't need to be calculated. Of course a land-observer with
an artificial horizon has an advantage here. Not needing to see the true
horizon he can measure altitudes any time he likes.

George.

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