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A-12 bubble sextant results
From: Paul Hirose
Date: 2002 Dec 20, 20:11 -0800
From: Paul Hirose
Date: 2002 Dec 20, 20:11 -0800
Near dusk today I pulled out my 1942 Link A-12 bubble sextant and took some shots through a window. I made the sights from a standing position and tried to make the coincidences rapidly but not sloppily. However I did use a magnifying glass to read the vernier carefully to the nearest minute. Then I used the USNO's online calculator and a GPS-measured position to obtain Hc. Right away it was evident the instrument had a +10 minute index error. Allowing for that, as well as refraction, I got: UTC Ho Hc diff. 00:12:13 4 31.6 4 30.6 +1.0' 13:42 4 16.1 4 15.3 +0.8' 15:06 4 01.7 4 00.8 +0.9' 16:10 3 50.3 3 49.7 +0.6' The results are amazingly good. In fact, I don't believe they are typical. It doesn't seem likely someone badly out of practice as I am (not that I ever was well practiced) should do so well with a zero magnification sextant. This was probably just a lucky streak. My Ho is still consistently high; a -11 minute index correction would have made the numbers look even better. With the large index error and rapid change of refraction at these low altitudes, I made the index and refraction corrections separately rather than lumping them together in the normal manner. There may be nonstandard refraction at work too; we just had a storm pass through. Then there's the matter of shooting through the window because I was too lazy to go out! I plan to repeat the experiment to get a better idea of my accuracy.