NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: instances of the use of "horizon grazing" technique
From: John Huth
Date: 2009 Oct 23, 13:37 -0400
George -
I think the principle of a "horizon grazing" star is the following:
If you're in an "emergency" situation, and you know the declination of a star that's circumpolar, you can measure the altitude of the star at its lowest point in the sky. It relies on the small angle approximation, so that a person could use a ruler at the end of a string, like a kamal, to get an altitude and hence figure latitude. E.g. Merak (Decl approx. = 56 degrees) would be a "horizon" grazing star for me in Boston (42 N), where one could, in principle, use the small angle approximation for an altitude of 14 degrees, although that's pushing it, and isn't terribly accurate. Maybe this is a poor example, but that's roughly the idea.
This technique is mentioned in a number of books, and I've never heard of anyone using it in practice, which is why I was curious. I think that perhaps the number of opportunities of a bright star that is perhaps 6 to 10 degrees above the horizon is very limited. As you point out, below about 6 degrees, sighting a star is problematic. I think the lowest I've seen Venus is around 6 degrees.
John H.
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From: John Huth
Date: 2009 Oct 23, 13:37 -0400
George -
I think the principle of a "horizon grazing" star is the following:
If you're in an "emergency" situation, and you know the declination of a star that's circumpolar, you can measure the altitude of the star at its lowest point in the sky. It relies on the small angle approximation, so that a person could use a ruler at the end of a string, like a kamal, to get an altitude and hence figure latitude. E.g. Merak (Decl approx. = 56 degrees) would be a "horizon" grazing star for me in Boston (42 N), where one could, in principle, use the small angle approximation for an altitude of 14 degrees, although that's pushing it, and isn't terribly accurate. Maybe this is a poor example, but that's roughly the idea.
This technique is mentioned in a number of books, and I've never heard of anyone using it in practice, which is why I was curious. I think that perhaps the number of opportunities of a bright star that is perhaps 6 to 10 degrees above the horizon is very limited. As you point out, below about 6 degrees, sighting a star is problematic. I think the lowest I've seen Venus is around 6 degrees.
John H.
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NavList message boards: www.fer3.com/arc
Or post by email to: NavList@fer3.com
To , email NavList+@fer3.com
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