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    Re: Some Sites
    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:
    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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