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    Re: Refraction for beginners
    From: Fred Hebard
    Date: 2004 Jul 9, 17:24 -0400

    On Jul 9, 2004, at 4:06 PM, Bill wrote:
    > Getting back to Great Lakes sailing, it would therefore behoove me to
    > use
    > the ship's barometer or adjust the broadcast pressure (that has been
    > adjusted to sea level) back to local "station pressure."
    Most barometers are now adjusted to sea level even if operated well
    above.  So one bought in Denver would still show pressures of about 30"
    of mercury, even though the actual pressure would be several inches
    below that.
    Regarding the effect of earthly elevation above sea level on parallax,
    it is insignificant.  If you consider a change in earthly elevation of
    1 mile, that is a small part of the earth's radius of perhaps 12,500
    miles.  Parallax might change the moon's astronomical elevation by a
    few minutes of arc.  An alteration in those few minutes of arc of 1
    part in 12,500 is not measurable with a sextant, or perhaps any other
    angle measuring device.  Thus parallax changes are not useful for
    determining earthly elevation, but, on the bright side, also can be
    ignored for navigation purposes anywhere on earth.

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