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Frank Reed's posting calls for some comment.

He refers to a 1904 paper which "discusses the idea of placing much greater 
emphasis on measuring short distance lunars of just a few degrees since it's 
far easier to "hold" the objects together in the field of view of the 
sextant --which is true."; but it's hardly an overwhelming reason for doing 
so.

Adding- "He notes that this requires some changes in the way the sights have 
to be analyzed". It certainly would. Indeed, the traditional process of 
reverse-interpolating between predicted lunar distances at three-hour 
intervals would have had to be rejigged in some way. It relies on the 
reasonably steady change of the Moon's motion, from one 3-hour period to the 
next. This is already starting to break down when a lunar distance 
approaches its lowest predicted limit, in the 20� to 30� region, and it's 
for such distances that a second-difference correction has to be applied, in 
the way that Kent Nordstrom has recently reminded us about. The smaller the 
lunar distance gets, the worse this non-linearity becomes. At "just a few 
degrees", it would become overwhelming. Of course, with modern computing 
power, that's a trivial matter. Not so in 1904, however.

And as long as a lunar distance was kept to an angle that was large compared 
with the difference in celestial latitudes between the Moon and the body, 
the user could always be sure that the direction of that spacing would 
more-or-less correspond with the Moon's direction, for any predicted star or 
planet. But otherwise, the measurement can become useless, as it was in the 
case we were examining.

To my comment- "I'm a bit sad that this example has now shown up in 
Navigator's Newsletter, because its editor, David Burch, asked my opinion 
about it, and I explained, back in late 2006, the drawbacks of that 
particular camera-shot." Frank made this curious response-

"Well, it illustrates the principle despite that drawback, and in any case, 
who's going to lose GMT in this century??"

It was supposed to show a lunar distance measurement, so how can it possibly 
be considered as illustrating the principle if it's measured in an absurd 
direction? It was an illustration of how NOT to do it! And, as it was trying 
to show how to measure longitude that way, Frank's  comment about the 
likelihood of losing GMT is quite irrelevant.

And he added- "Nevertheless, this photograph with the Moon "passing" Jupiter 
can be used to generate a very nice "lunar LOP" (at known GMT) even if its 
geometry is poor for a "traditional" lunar distance observation.". That, 
too, is hardly relevant to my posting, since the observation was being 
portrayed as being of a lunar distance. Perhaps Frank will explain how this 
photo is to be used to generate that LOP.

And to my concluding comments "There are other drawbacks in making celestial 
observations by camera rather than by instrument, but that will do to be 
going on with."

he replied- "Which difficulties would you consider serious? It strikes me 
that a digital camera might make a much better angle-measuring instrument 
than a sextant
(though with a very small angular range). Of course, it has to be calibrated 
properly, but I would say that's much easier than calibrating a sextant. "

Absurd. Quite absurd. I will deal with some of the many problems of using a 
camera for such measurements in a later posting.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 


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