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On Thu, 15 Jan 2004 17:14:38 +0000, George Huxtable wrote:

>>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..
>
That is true, there will be retrograde motion or just slow forward
movement at latitudes other than the one I was using for the example.
This specific-for-the-day "hang" latitude would also be slightly north
or south each day, but would remain longest at the times of the year
that the moon is at it's extremities in altitude.  Sort of like the
solstices for the sun.  Lunstices?

>>>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.
>
I believe you could measure the _apparent_ altitude of the Moon from
the arc distances to two stars, three for a check.  I quite agree you
would need the GMT to get the true position of the moon from that.  I
also see that a secondary observation using the horizon would probably
be easiest and most accurate to fine the altitude of the celestial
body and the resulting latitude of the observation.  My problem is
with finding the GMT from a lunar if the apparent moon is nearly
stationary (except for a little altitude change) against the stars;
the time of observation is indefinite (or equally acceptable over a
wide range of times), but I believe the longitude can be found from
that apparent position.

>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.
>
Yes, and the land based observer has the additional benefit of being
able to determine South from his station without using GMT, especially
if he uses a theodolite or a support for the sextant and has reference
lights ... but that's not possible on a ship, with a sextant and a
chronometer that has to be restarted, I think.


--
Richard ...

   

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