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    Re: Raw data for bubble
    From: Bill B
    Date: 2007 Mar 19, 17:38 -0400

    >> Bill wrote:
    >> First, to calculate the precise moment a body will be at 90d or 270d do I
    >> not need to know my latitude exactly?
    > Peter responded:
    > Ah; precision precision. You are probably quite correct ... to be
    > precise. Establishing the latitude is, traditionally, relatively easy.
    > You could, for example, have worked it out only a few hours earlier at
    > meridian passage of the sun and are now looking for another position
    > line to get a fix...
    Summer and "few hours" between LAN and a 90/270d azimuth? ;-)
    >> Bill:
    >> If I am that sure of my latitude, why
    >> not use a time site to extract longitude directly?
    > Peter:
    > You would need a number of bodies to get a fix. Here you need only
    > one, usually the sun (although, as with meridian passages, any body
    > can be used, assuming you can see it and the horizon).
    A number of bodies would give you lat AND lon (at least it once did prior to
    the cocked hat discussion ;-) As you pointed out, I would not need a number
    of bodies to get a running fix if I were relatively confident of my
    latitude, just one set of observations at 90/270d.
    After running a number of hypotheticals, it strikes me that your longitude
    might be as good (latitude known) or better (latitude unknown) if you deep
    six the pain of precalculation and use a time sight near Zn 90/270.  It
    gives longitude directly in one set of calculations.
    The beauty is:
    --No pre-computation
    --Only one variable, latitude.  GHA and declination are derived from the
    time of the sight.  Elevation is from observation.
    For example:
    1 May, 2007, Sun
    AP N 40, W 60
    Az 270 (Actual transit)
    1 May, 2007, Sun
    Real position N 41, W 60
    Az 266.73
    1 May, 2007, Sun
    AP N 40, W 60
    Az 269.56
    1 May, 2007, Sun
    Real position N 41, W 60
    Az 266.20
    I correct for variation so I know the compass reading I need to hit 90/270d
    true and make my observations from just before I see the body at 90/270d
    until a bit after and average. Then I do a time sight to extract longitude.
    If I thought I was at N 40 and was at in fact at N 41, and used N 40 in my
    time site at 20:41:45. My longitude would be 59d 59!7, off by on 0!3 or 0.2
    nautical miles.
    Let's say my compass work wasn't spot on, and I missed 90/270 by 3+ degrees.
    Again I thought I was at N 40 and was at in fact at N 41, and used N 40 in
    my time site from 20:21:45. My longitude would be 59d 55!0, off by on 5!0 or
    3.8 nautical miles.
    Not too bad for being a degree off latitude. Real world, say your latitude
    was 6!0 off over your run from LAN and you missed 90/270d by 3+ d.
    Longitude from time sight would be 55d 59!6.  0!4 or 0.3 nautical miles off.
    > Peter:
    > There is one limitation in practice - its a summer-time only trick. In
    > winter months you may not find the sun at 90 or 270d.
    For sure the sun will not hit 90/270 until the declination is approx. 0d. We
    would want any body we use to have a declination that offers an elevation
    where refraction will not become a wild card at 90/270.
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