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    Re: Raw data for bubble
    From: Bill B
    Date: 2007 Mar 13, 19:36 -0500

    Peter Wrote
    >>> ...An example of how this can be useful is
    >>> when you are trying to make an observation at prime vertical - unlike
    >>> a meridian passage the body just zooms past this due east or west
    >>> point of azimuth. It is the only way I know of being sure to be able
    >>> to adopt that precise moment.
    > Bill inquired:
    >> Would you please go into more detail on the above paragraph?
    > If you can manage to make an observation with an azimuth of precisely
    > 090d or 270d then you can establish longitude with that one sight,
    > just as latitude can be established with a meridian sight. But how to
    > ensure that the sight is recorded at the right moment?
    > The moment of prime verticality can be calculated for the body
    > (usually the sun) but its a bit tricky to make that the moment when
    > the altitude is recorded. By making a series of sights over a few
    > minutes before and after this moment then plotting the slope the
    > appropriate moment can be adopted for sight reduction. As a bonus all
    > the other advantages of this slope analysis of the individual sights
    > are also made available.
    I am missing something so obvious I am *almost* ashamed to ask.
    First, to calculate the precise moment a body will be at 90d or 270d do I
    not need to know my latitude exactly?  If I am that sure of my latitude, why
    not use a time site to extract longitude directly? Or created an LOP near
    90/270.  Again, if I know my latitude, the LOP will give me longitude.
    Second, if I understood you correctly am am not clear how the slope is used
    to determine the exact moment, or a are you saying with the slope you can
    use a point between observations that  corresponds to the calculated moment?
    May I impose on you to give me a cook-book version of the method?
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