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Author
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Topic: Problem Using Artificial Horizon
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Bill Riner
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posted June 17, 2005 09:16 AM
I'm trying to use an bowl of water as an artificial horizon, but I'm having trouble. I'm not sure what I should see through the telescope. I see two images of the sun when the sextant is set near 0 degrees. The sextant height of the sun is roughly 35 degrees, so I set the sextant to 70 degrees (double) and viewed the reflected image. I expected to see two images of the sun which I would line up, but I don't even when I move the arm. I think I'm not seeing anything from the right (silvered) side. Any ideas?
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David Burch
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posted June 17, 2005 02:21 PM
First i assume you have read our article on artificial horizons from the back of the course materials.
next, an unprotected bowl of water is a bit tricky unless it is absolutely flat dead calm. any wind at all will ripple the surface and prohibit the sight.
assuming that is not an issue. If you look to the sun in the water through the scope set at 0° 0' you should see only one sun, the superimposed direct and reflected image. If you see two, that are more or less at the same height but offset right and left, then you have a large side error, which should be corrected. this is discussed in the text and in bowditch on sextants that we inlcude in our ecourse or you can download from our resources.
next key issue is the statement "roughly" 35° high, so you went to 70. remember you are only seeing some ± 2 or 3° through most scopes, so "roughly" may not be accurate enough. next time, do a quick sight reduction for time before and after you start, so you will know within a degree what the true height is at any time... although with more practice you will not need that.
so, summary so far: remove side error and compute Hc to remove both of these uncertainties, then we can pursue this more... use your correct lat lon from a hand held GPS, or look up your lat lon from your address, we have links that do that in the resources.
From: Starpath, Seattle, WA
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Bill Riner
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posted June 21, 2005 03:35 PM
I switched to a mirror and it's working fine now. I think I wasn't seeing a second image at twice Hs because I wasn't holding my sextant level.
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David Burch
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posted June 23, 2005 07:29 AM
Yes, that is what we recommend for beginning at this. a good "plate glass" mirror, leveled with business cards or some fine steps using a high quality carpenter's level. Ultimately the plate of water or oil will be more accurate, but to get underway on the sights, the level mirror works best.... also can be used for stars, and water is very hard for that.
From: Starpath, Seattle, WA
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