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Wolfgang, you wrote:
"Then I did a quick "Sumner": I varied the input to see what happens to the 
result. On the assumption that there is no error in latitude (having bent my 
sextant only after taking the height of Polaris or a star on the meridian) 
and that my time is correct, I varied my assumed longitude and got a 
perplexing result: Using Frank's program and assuming I am about 150 miles 
out in the Atlantic (= assumed longitude 77deg 10 min W) I got the following 
result:
'Error in Lunar: 0 min
Approximate Error in Longitude: 0 deg 00.4 min'
which - following David Burch � I interpret as saying I am almost at my 
assumed position. As the "Photo Sextant Sight" supposedly was taken 3 
degrees further west something doesn't fit. What did I get wrong? It's not 
the input, I checked it several times." 

FIRST, as George has already pointed out, the "error in longitude" item in 
the calculator on my site is just 30x the error in the distance. This was 
designed to serve a specific purpose, but if you don't know this in advance, 
it can be misleading since it doesn't cover cases where the objects are out 
of line, like this specific case. 

NEXT, some links that, I believe, lead to the original photo and the 
analysis:
Here's the (very short) thread on the Starpath boards that seems to discuss 
the case you're describing:
http://www.starpath.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=31;t=0000 
17 

Here's a link (from that thread) to the original digital photo:
http://www.starpath.com/resources2/pics/03192006_Jup-Moon_0038.jpg
I'll call this the "19 March" case. 

There's a similar photo taken from the same location here:
http://www.starpath.com/resources2/pics/06152005_Nikon-evening-work-sm.jpg
I'll call this the "15 June" case (this is actually two images; I would 
consider only the upper one to be useful). 

NOW, what can we do with these images? First of all, this is indeed a viable 
method for measuring short angular distances, but there are some issues:
1) can we take images like these with a handheld digital camera? Are the 
exposures short enough? If not, then we probably need to wait until the next 
generation of digital imaging before we could try this at sea.
2) how do we calibrate the angular scale? The choice in the starpath 
analysis is to use the Moon's angular diameter. Unfortunately, that's not 
enough since the scale is likely to vary slightly across the field of view. 
In addition, the limb of the Moon is somewhat hard to define in photos like 
these. I would guess there's an error of about +/-2% resulting from that 
difficulty. Nonetheless, there are simple means to calibrate a digital 
camera for angular observations. You could photograph a good measuring tape 
for example. If it's 34.38 meters away from the camera (and arranged on an 
arc so that every point on the tape measure is the same distance from the 
camera), then every cm mark corresponds to 1 minute of arc in the image. 
This calibration is easy, and it's essentially permanent.
3) It's easy to over-expose the Moon. This might be handled by holding a 
shade in front of the camera so that the stars around the Moon can be seen 
more clearly. Then it would possible to measure three or four distances at 
once in a single image. 

Does it make sense to use such a modern system to determine GMT in an 
emergency? Myself, I can't imagine that this would EVER happen. There are 
abundant sources for time checks in an emergency. If I've lost track of the 
time, and I just happen to have a high-end digital camera handy, I think I 
can probably just turn on the camera and check the built-in clock! 

So is there anything else we can do these images? Well, it seems to me that 
this is a great case where we could use the lunar distance observations to 
generate "lunar LOPs" (at KNOWN Greenwich Time). These images are not 
properly calibrated, but it's relatively easy to see that we could expect an 
accuracy of 0.1 minutes of arc or maybe a little better. The "15 June" case 
has about 15 pixels per minute of arc. If we combine this with the "19 
March" case, we get two lines of position and thus a position fix (since we 
don't have a proper calibration, there's not enough info to do it with these 
photos, but the principle is fine, and you could at least try it as an 
experiment with these two photos). In ocean sailing races, it might be 
possible to get away with calling this a celestial fix and thus qualify for 
the small advantage given to vessels using celestial. But considering it 
requires quite a bit of digital technology, it might be a hard sell. 

 -FER 



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