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    Re: Are higher altitude sights inherently less accurate?
    From: Geoffrey Kolbe
    Date: 2012 May 03, 09:18 +0100

    At 03:23 03/05/2012, you wrote:
    >For once I disagree with Geoffrey. He says, "The error how vertical
    >your sextant is becomes rapidly more important [at higher
    >altitudes]. And in consequence, errors in high altitude sights will increase."
    >The opposite is true. The error with sextant "tilt" is max at 45d
    >altitude and decreases to zero at 90d altitude, for any given tilt
    >angle, as the attached table shows.
    >It is true that it's uncomfortable, even weird, swinging around in
    >azimuth as you rock the sextant looking for vertical. Here's were
    >the traditional half-silvered horizon mirror can help. Just keep the
    >vertical division of the mirror perpendicular to the horizon (as you
    >pivot around awkwardly on one foot) and your altitude error should
    >be within reason, just as the attached table shows.
    >Happy highs (altitudes),
    Yes John, you are right.
    I should have thought about the problem in more depth before posting,
    rather than after posting. :-(
    When using an artificial horizon with a marine sextant, the idea (in
    the case of stars) is to bring the two images of the star into
    coincidence. If the mirrors are correctly adjusted, that will bring
    the sextant into verticality  automatically - provided the AH is level.
    Suppose the AH is not level, what happens then?
    For very low altitude sights using an AH, It is the front-to-back
    tilt error of the AH that is important. The side-to-side tilt error
    in the AH has little effect on the resulting altitude. Clearly, for a
    body on the horizon, the side-to-side angle of the AH does not matter at all.
    For very high altitude sightings, the front-to-back tilt error is
    still just as important, but the side-to-side error starts to become
    important as well. For a zenith sighting (if it were possible with a
    sextant) the side-to-side error would (by definition) be just as
    important as the front to back error.
    So for low altitude sightings, only the front-to-back component of
    the AH tilt error is important. For high altitude sightings, the
    total tilt error of the AH becomes more important and for this
    reason, high altitude sightings using an AH are inherently less accurate.
    Geoffrey Kolbe

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