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Author
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Topic: Solar Index Correction
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Lanny
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posted February 06, 2006 01:14 PM
Yesterday I checked my sextant's index error using the solar index correction method. In recording the sun's actual semidiameter to test my calculated values, it came to my attention that the actual value can vary depending on which almanac is used. The Nautical Almanac for 2/5/2006 gives a value of 16.3, the StarPilot calculates a value of 16.2, and the almanac from the Lunar Distance site calculates a value of 16.25.
I did 8 checks yesterday, 4 "toward" and 4 "away". For my SD values, I had 5 at 16.25,and the remaining three at 16.2, 16.15. and 16.4. Except for the 16.4 , all were either right on or close to those values calculated by StarPilot and almanac from lunar distance site.
I just thought I would pass this on for any comment. Thanks.
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David Burch
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posted February 06, 2006 04:23 PM
Thanks Lanny for this fine example. I might only add that i am not sure of the precision of the StarPilot SD values, so I cannot comment on relative values of SD data. My guess is the StarPilot SD is only ±0.1' but I would have to check with Luis on that and will report back.
And to my knowledge, the printed almanacs do not quote an accuracy specifically on SD, though they do state that "quantities taken from this almanac are generally correct to the nearest 0.1'," which would imply 0.05' as general accuracy. They go on to say one third of the dec and gha values would have errors larger than 0.05' but only one tenth would have errors larger than 0.1'.
As for the lunar distance site (can you please tell us the link for the one you refer to) they may well be using computations that could in principle be more accurate than those of the printed almanac. As you know, this type of sight requires the most accurate data and actual sights available, and those involved with this in depth tend to use only the highest quality sources.
In any event, your results are at the limit of practical cel nav, and a fine job. thanks again.
From: Starpath, Seattle, WA
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David Burch
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posted February 06, 2006 04:30 PM
I might add for readers not familiar with what we refer to here as "solar IC method," that this is a technique for finding IC by measuring upper limb of the sun relative to its lower limb to obtain an accurace IC, and at the same time provide a check of your work, since it effectively measures the SD of the sun after applying your determination of IC.
And further, that the actual forms we use for this at Starpath were developed by Lanny and are available for download from the Forms section of our online resources.
From: Starpath, Seattle, WA
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Lanny
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posted February 06, 2006 11:33 PM
The link for the almanac from the lunar distance website is in resources under INET Connection/Celestial/online almanac (last item under celestial).
I have another question. My Astra sextant index errors are 2.1 on for Toward and 1.6 on for Away. I was wondering for the sextants that you and Robert have tested if it is common for there to be a difference between the Toward and Away values similar to what my errors are.
Thanks for answering my questions.
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David Burch
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posted February 08, 2006 10:17 PM
I do not have any notes here on that, and i do not recall an issue with that in the past. I will get in touch with Robert, who is at sea these days as much as on land, to see if he has any recollection. Failing that, we could try it here, or if you like you could borrow one of ours for a comparision.
From: Starpath, Seattle, WA
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