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» Online Classroom   » Emergency Navigation   » Public Discussion of Emergency Navigation   » First attempt at photo moon sight

   
Author Topic: First attempt at photo moon sight
David Burch


 - posted February 11, 2006 09:35 AM      Profile for David Burch           Edit/Delete Post 
This was a first crude attempt at getting a useful "sextant sight" from a photograph, posted here only to show the procedure and to ask for others to take data like this so we can evaluate it. Unfortunately, in this case, I did not have a very precise time and not a very good photograph, but it is a first step. And it is also a Dip short situation to boot (The close shoreline from this elevation added about 0.6' to the dip correction).

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Note first that the shape of the moon, as best i can extract from this poor pic, is completely wrong. it should be either round, or with refraction flattened in the other direction, width wider rather than narrower than its height... but again, this is a very poor photograph. The photo was a bit sharper in the horizontal direction, so i will assume that 2xSD is 76 pixels.

To find Hs, based on the observed SD we would have Hs (LL) = 146p x (2x14.8' /76p) = 56.9' .

If we then use this to do a sight reduction of LL from this known position, i get a = 7.2' A from 303.5. This is not at all so bad, and i am guessing it is mostly due to time uncertainty ( i could have been off as much as a minute), as well as refraction uncertainties... ie this is a 7 mi error, but the moon is only about 1 mile above the horizon, so the poor quality moon image and poor horizon contributions are way smaller than 7 miles. They are just a small fraction of a mile.

so this is encouraging as a possible means of an emergency sight process. Take a pic with your camera (or phone!) and then get an LOP. The few times i tried with the sun and failed was because i did not have adequate shades and filters... so sun sights will take some judicious timing or some experience with filters.

if anyone has the opportunity to take such pictures (sun, or moon or planet with the horizon in view), please do so and send them to us to evaluate. we need accurate time, height of eye, and GPS position.

The real question to be answered is, how high can the sun or moon be and still have this technique give a reasonable answer. We know that this one will work, with the sun or moon so low. It is, for example, a well-known procedure in Emergency Navigation to just time the sunset and call Hs UL = 0°0' and then do a sight reduction. We have done many of these and reported on them elsewhere. There is also a discussion of these sights I believe in the Navigator's Newsletter. The errors are mostly refraction and generally you can get a reproducible result to within 5 miles or so.

The value of this type of sight is you could take it earlier when the body is higher and the refraction is smaller... remember that at the horizon normal refraction is about 34' (diameter of sun or moon), then at H=5° the refraction is about 10' and at H=10° the refraction is about 5', and it goes down rapidly from there. The uncertainty in abnormal refraction corrections from the NA are about ±50%, so if we get up above 10° or so, this uncertainy will not be significant.

From: Starpath, Seattle, WA


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