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    Re: Artificial Horizon Levels
    From: Robert Eno
    Date: 2004 Mar 28, 18:51 -0500

    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.
    ----- Original Message -----
    From: "Fred Hebard" 
    Sent: Sunday, March 28, 2004 1:41 PM
    Subject: Re: Artificial Horizon Levels
    > Robert,
    > It sounds good to me.  You would want a firm, flat surface, clean and
    > without irregularities, such as new sheet of glass.  It also would be
    > important to orient the level between readings on the exact same spot
    > as possible, and you might want to look at the scatter in the readings
    > from having picked the level up and put it back down without rotating
    > it.  Perhaps the physicists will be able to add additional
    > considerations.
    > Even buying an ordinary level for carpentry work, it is surprising how
    > many of them, especially the more inexpensive ones, give a different
    > reading depending upon the orientation of the level.
    > Have you tried to calculate how accurate a sextant reading would arise
    > from having leveled the mirror to 30 seconds of arc?
    > Thanks,
    > Fred

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