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Author
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Topic: Question on 11.18 problem 5 + aviation sextant
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Rob Pearce
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posted January 27, 2005 07:35 AM
1) I have a question regarding a problem in section 11.18 problem 5. I can't get the answer that is given in the book. The problem is for October 26, but it looks to me like they used the equation of time from the July 26 (5m 42s) table instead of the October 26 table (15m 5s). Or did I simply not do the problem correctly. I got the rest of them correct.
2) I have an old aviators bubble sextant that I had rebuilt and calibrated. How accurate are they? My interest is celestial navigation on land (just for fun)
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David Burch
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posted January 27, 2005 12:43 PM
(1) With StarPilot i get 9:45:01 and it has used an equ of time = 15m 59s...the almanac gives 15m 58s. So yes, this one is wrong.
The right answer is 11 59 60 - 00 15 58 = 11 44 02 and 11 44 02 + 02 01 00 = 13 45 02 and 13 45 02 - 04 00 00 = 09 45 02. thanks for finding this, we will add this to the tech support section, which focuses on actual errors rather than general discussion.
(2) Which model do you have? generally these were made for planes traveling some 100+ miles per hour so not required to have the same precision as a vessel. I have seen some that could do a mile or so, but there are many different designs.
Later in the course you should know precisely, as you can do sights from the backyard with your bubble sextant and compare to known position.
If interest calls for it, we can also devise ways to evaluate accuracy from star measurements... must be in a line however. The method we use elsewhere in the course requires holding the sextant at an angle and you cannot do that with a bubble sextant.
From: Starpath, Seattle, WA
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Rob Pearce
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posted January 28, 2005 09:55 AM
My bubble sextant is an A-10 with an averager.
The dial goes to 10ths of a minute, but I have no idea if it accualy could be that accurate. I would like to play with it some in the back yard though and see what I can do with it. I suspect there must have been tables for bubble sextants, since upper limb and lower limb would not be used. (the star or planet is placed in the center of the bubble, or so I was told). I can't imagine zipping along in a plane trying to use a sextant! One other question, I am at 4,500 feet elevation, does that impact readings?
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David Burch
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posted January 28, 2005 02:56 PM
There is no correction needed for elevation with a bubble sextant.
If you are just putting the body itself in the center of the bubble, then my guess is you would have no dip correction, just take the altutde correction factor from the star column, and do the rest as normal sight reduction... that is, for sun, planet, and stars. I would have to think on this some more for the moon.
I will try to look into this further as you get closer to actually doing the sights or have some in hand.
Might start out with some simple noon sights as they are simplest to interpret. You will get to that section of the course very soon now.
--david
From: Starpath, Seattle, WA
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