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    Re: Artificial Horizon Levels
    From: Trevor Kenchington
    Date: 2004 Mar 28, 20:59 +0000

    Robert, Fred:
    It hardly needs tables, nor even much geometry.
    With a truly level artificial horizon, the sextant angle (after
    correction for index error) will of course be twice the observed
    altitude of whatever body you are looking at. With a tilted horizon, it
    will be twice the altitude plus twice the angle of tilt (where tilt
    towards the body observed is treated as positive, tilt in the opposite
    direction as negative).
    Hence, an unrecognized 30-second tilt would introduce a 1-minute error
    into the sextant angle but that would lead to a 30-second error in the
    estimate of altitude. The final calculation of Ho might have a very
    slightly larger or smaller error, depending on misreading of tables of
    refraction and so forth.
    Trevor Kenchington
    Robert Eno wrote:
    > Thanks Fred.
    > With respect to your last question, do you mean to say what error would be
    > imparted by the horizon being off by 30 seconds of arc?
    > If such is the case then the answer -- in a round about way  -- may be found
    > in issue # 41- 42, Fall/Winter 1993-94 of the Navigator's Newsletter. In
    > this issue, Doug Davies, son of the eminent Admiral Davies, affixed an
    > artificial horizon to a surveyor's transit and tilted its southern end about
    > it's east-west axis, to simulate taking observations at higher latitudes. It
    > doesn't exactly answer your question as to what errors might be imparted by
    > improper levelling of the artificial horizon but it does lead one in the
    > right direction.
    > I wonder if there are a set of tables out there that provide this
    > information. I am sure that if one were to sit down and do the math
    > (geometry) one could come up with something.
    > Robert
    > ----- Original Message -----
    >>Have you tried to calculate how accurate a sextant reading would arise
    >>from having leveled the mirror to 30 seconds of arc?
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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