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    Re: Plumb-line horizon vs. geocentric horizon
    From: Pierre Brial
    Date: 2005 Feb 10, 07:42 +0400

    Good morning,
    Some words to introduce myself:
    I'm new to this forum, and this is my first post.
    I live in Reunion Island, in the Indian Ocean (E55?30' S21?00), where I
    work as engineer in surveying.
    Being at first interested by land navigation, I have been a GPS addict
    since may be 1995. Nevertheless, this let me frustrated to not to
    understand the traditional way of positionning, so I learn Celestial one
    year ago. I practice it at first with a theodolite, and then bought
    recently a Cassens & Plath CPSailing sextant (it is in fact a disguised
    Astra IIIB).
    Then come back to the topic:
    Fried Squash a ?crit:
    > Due to the oblateness of Earth, the plumb line differs
    > from the actual center point of Earth by a
    > navigationally significant amount, up to 12 minutes of
    > arc.
    > But this fact does not effect the practice of
    > celestial navigation because the lat-lon grid itself
    > and the Almanac's GP data are based on the plumb line.
    It takes me a while to understand this, but I agree.
    > The local plumb line may differ by additional amounts
    > as well, called deflections, due to surface anomalies,
    > but not by a navigationally significant amount.
    Near mountains or steep islands, the deflection can be high enough to
    affect geodesical measurements. In the place where I live the deflection
    is 0.7', which is considered one of the highest in the world. As I
    understand that a bubble sextant is not that accurate, this should not
    affect navigation. Nevertheless, If you need accuracy, you have to take
    it into account when you use a theodolite.
    Best regards
    Pierre Brial

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