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    Re: Index Error - Real vs. Artificial Horizon
    From: Bill B
    Date: 2013 Feb 28, 22:59 -0500

    On 2/28/2013 7:49 PM, Örjan Sandström wrote:
    >  >In other words, since the index mirror is a few inches above the
    > horizon mirror, >wouldn't there be a parralax error here as well, since
    > we are looking at the >reflected image in the artificial horizon, not
    > miles away at an actual horizon?
    Another way to look at this is follows:
    When you are using an artificial horizon (AH) you are not looking at the 
    mirror, you are look at images of a body a long way away. Try this 
    experiment. Stand fairly close to a mirror and focus on its frame. When 
    the frame is in focus, background objects at a distance will be out of 
    focus and vice versa. Unless you arer using the 0 mag sight tube 
    supplied, I doubt you could focus on the AH frame (with the scope). Even 
    if the apparent distance between the mirrors increases by a few inches, 
    that is just noise when viewing a most celestial objects.
    That is not to say there is no parallax between you and the celestial 
    object depending where you are located on the earths surface, but that 
    has nothing to do with sextant mirrors. (Angles are measured from the 
    center of the earth to the center of the body, not from someplace on the 
    surface of the earth to the center of the body,) There is also a slight 
    bit of parallax between the earth and sun. Even between the stars and 
    earth (google "parsec").
    When you get around to trying a reduction with the moon you will notice 
    an "hp" correction. This is for horizontal parallax. But these are 
    topics for another day.
    Bill B

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