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    Re: Altitude with artificial horizon
    From: Greg Herdt
    Date: 2021 May 21, 13:40 -0600
    Most embarrassing if I missed the original intention of the question.  A couple of comments on this new and interesting thread, though: I suspect that John is correct and first order there is no impact to the artificial horizon itself with altitude.  There will be some vapor pressure impact on the curvature of the fluid ("wetting") near the walls of the A.H. (similar to some of the oil vs water observations reported on Nav List), but I imagine that is minor unless you have a smaller A.H.  I suspect that the oblateness of the Earth first order would be a latitude effect rather than an altitude effect.  Interesting points.  

    On Fri, May 21, 2021, 12:31 PM John Clements <NoReply_Clements@fer3.com> wrote:

    Ooh! This question is a GREAT OPPORTUNITY for me to put my foot in my mouth in front of the best in the world. Let's see if I can get my whole leg down my throat!

    I believe you're asking about the effect that your altitude above the surface of the earth has when using an artificial horizon. The first-order approximation is this: it doesn't matter at all.

    More generally, the basic idea is that you're treating the earth as a sphere, and trying to determine the angle between "straight up" at your point on the earth and the celestial body that you're sighting. The artificial horizon gives you a direct measure of the direction of the center of the earth, and shouldn't be affected by your altitude.

    When you're sighting with an actual horizon, and you're above that horizon, even by a few feet, the angle between the horizon and "straight up" is significant; that's why you need to account for dip.

    Naturally, there are lots of problems with this, but very few of them should rise to the level of concern. That is, I think that most of these errors are swamped by the inaccuracy of your measurements.

    1) The earth is not spherical.

    2) The direction of gravity is *not* the direction of the center of the earth.

    3) The plane of the horizon is also not perpendicular to the direction of the center of the earth, and it's not perpendicular to the direction of gravity either.

    Apologies as always if I'm telling you things that are either (a) false, or (b) obvious!

    John Clements

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