NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Some Sites
From: Gary LaPook
Date: 2009 Feb 20, 00:20 -0800
From: Gary LaPook
Date: 2009 Feb 20, 00:20 -0800
Let me know how your Bygrave works out. I have had a notification on
ebay for bygrave but I never got a notice of the one you bid on. Do you
have the listing number, I would like to take a look at it if it is
still on ebay.
There is no height of eye where you have to worry about dip with a bubble octant since you are not using the natural horizon. However, the refraction correction get smaller at higher altitudes due to less air between you and the star and you should look at the refraction table in the Air Almanac or in H.O. 249 which I have attached.
You don't need a theodolite to determine Index Error, just keep taking sights and work them from your actual position and the intercept is the IE. "Toward" means that the sextant is measuring too high an altitude so the IE is plus, etc. Keep track of them and after you have many such intercepts find the average and standard deviation and use the average as the IE. I put a piece of tape on my octants and write the index error, not the index correction. This is because sometimes you might like to use marine practice and apply the IC to the observed altitude and sometimes, using aeronautical practice, you might like to apply it to Hc. Since the signs of the IC are opposite in these two cases and it would be possible to forget what case you were working with when you determined the IC this avoids any possible confusion if you always list IE and then you can figure out what sign to use when you are working your sight. (I hate to be picky but it is "sight" not "site.")
gl
bruce hamilton wrote:
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Navigation List archive: www.fer3.com/arc
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There is no height of eye where you have to worry about dip with a bubble octant since you are not using the natural horizon. However, the refraction correction get smaller at higher altitudes due to less air between you and the star and you should look at the refraction table in the Air Almanac or in H.O. 249 which I have attached.
You don't need a theodolite to determine Index Error, just keep taking sights and work them from your actual position and the intercept is the IE. "Toward" means that the sextant is measuring too high an altitude so the IE is plus, etc. Keep track of them and after you have many such intercepts find the average and standard deviation and use the average as the IE. I put a piece of tape on my octants and write the index error, not the index correction. This is because sometimes you might like to use marine practice and apply the IC to the observed altitude and sometimes, using aeronautical practice, you might like to apply it to Hc. Since the signs of the IC are opposite in these two cases and it would be possible to forget what case you were working with when you determined the IC this avoids any possible confusion if you always list IE and then you can figure out what sign to use when you are working your sight. (I hate to be picky but it is "sight" not "site.")
gl
bruce hamilton wrote:
Inspired by an industrious Chief Mate stuck at anchor several thousand miles away (I'll bet the ship is spotless), and having a clear beautiful night here, I took the following shots with my A-12. The observant might note the lack of odd numbers at the end of the minutes, but it seems to work out that way. Index error 0 Assumed to be zero until I rent a theodolite. Height of eye n/a At what height does one have to worry about height of eye with a bubble sextant? I shall consult the air tables before I take this thing up in a plane, Temperature 2 deg C Pressure 1024 mb Time Z-8 AP 49d 16.0m N 123 07.0m W Sirius 20:29:42 23d 42.0m Betelgeuse 20:35 :22 47d 42.0m Saturn 21:27 :40 22d 34.0m Alkaid 21:42 :51 32d 52.0m Mirfak 21: 52 :11 49d 52.0m Arcturus 21 :56: 50 09d 42.0 m Still no sign of comet Lulin, but it is still only at an altitude of 12 degrees and not very bight. I can see Arcturus which is at the same Alt, but no comet. I might have to get out of the city to get a look. I lost the bid on an actual Bygrave on E-bay, but 400 pounds UK was a bit crazy. I'm happily back to pasting my model one and just need some laser printer transparency material to finish the scale. Bruce Vancouver
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Navigation List archive: www.fer3.com/arc
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