NavList:

   

Reply

On Mon, 12 Jan 2004 23:23:49 +0000, George Huxtable wrote:

First, thanks for reading the very long analogy attempt.

>>Locating the Part I or "parallax scroll" position with a fixed
>>precision and accuracy of observation would give the angular position
>>of the platform to a varying degree of accuracy, greatest when nearest
>>the street lamp, and least at the far left or the far right.  Greatest
>>when the vector arrow length (arc distance divided by time) of the
>>parallax is longest.  Translating this to Celestial navigation, it
>>would seem the accuracy of position finding would be greatest when the
>>observed moon crosses the meridian.
>
>Richard may be trying to derive the angular position of the Earth
>(carousel) from the perturbation by parallax of the apparent position of
>the Moon.
>
I think that I actually *am* trying to do that, eventually.  Not just
from the perturbation of the moon's apparent position by the effect of
parallax given by the earth's rotation; I need to calculate that
amount in order to find the true position of the moon against the star
background first (as seen by an observer sitting at the center of the
carousel).  For that, the diameter of the carousel (latitude of the
observer) would also be needed.  Then the time of the observation can
be determined ... there should be only one instant per month when the
moon is in exactly that place ... if I can work the math.

But first I want to get a feeling for which would be the best general
relative position of the observer, the lamp, and the center of
carousel in order to make meaningful observations.  Locating the true
position of the lamp against the background, with the smallest "error
bars" would be my goal.  In other words, would taking the lunar when
it crosses the meridian be a bad idea taking everything into account.

That observation angular position (lamp-center-observer) of the
carousel depends on time; and, because the lamp is orbiting around the
carousel also, determining the true lamp position against the distant
background will give that time.

Finally, I hope to be able to check the chronometer for time within
"probable error" resulting from the degree of precision of the
observations, *and* the longitude to within similar error bands.


>If I have understood it correctly, that isn't the way a lunar
>works. However, I should add that because it is such a big correction,
>under certain circumstances, parallax can play a large part in the
>end-result.
>
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.  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.

>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.

>If these comments betray my own misunderstanding of Richard's analogy, I
>hope he will put me right.
>
I think, from the way I understand your comments, you were just a
little ahead of what I had said.  My goal was to see if I had
understood the real world apparent motions of the the Moon against the
stars as seen from different locations on the earth.  Differences in
latitude would be analagous to sitting closer to (or further from) the
center, and on a higher or lower chair.

>I thought it might be helpful to get these matters straight before
>considering that picture in further detail.
>
I hope we are even closer to mutal understanding now, but I think we
were not far off before.


--
Richard ...

   

Reply