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    Re: Celestial navigation: easier basic principle?
    From: Bruce Cutting
    Date: 2020 Jan 26, 12:10 -0700

    Why is it unlikely that any bright star will be within an arc minute
    of zenith?
    Quoting Peter Monta :
    > Hi,
    > ... an easier method to locate longitude and latitude by having our
    >> sightings match this knowledge via time and sighting with any of the
    >> luminaries when at Zenith let's say
    > If you know the time and can see a star at the zenith, then you can indeed
    > determine both your latitude and your longitude from this single event.
    > But there are practical difficulties.  The zenith is not marked on the sky;
    > you really only have the sea horizon as a reference (or spirit leveling if
    > on land).  If the star is the same angular distance to each part of the
    > horizon, then it is indeed at the zenith, but this takes time to verify,
    > and meanwhile the star is moving.  Also, it is unlikely that any bright
    > star will be within an arcminute or so of the zenith, so the available
    > navigational events are severely restricted unless you can deal with very
    > dim stars.
    > So it's better to work with several of the things you can observe easily,
    > namely, angular distances of various bright stars to the horizon, observed
    > separately over the course of a few minutes.  A star not at the zenith
    > gives only a "circle of position", not a position directly (as you don't
    > know the star's azimuth to any accuracy), so you need several of these
    > observations.
    > But, as you say, computation and instrumentation are moving targets, and
    > it's always interesting to see if they can map onto the classical
    > navigation problems in new ways.  Sketch out a complete version of your
    > zenith scheme and try it under the real sky.  (Using a camera, you can see
    > dim stars very near the zenith.)  This kind of playing around can provide,
    > I think, some good intuition about how the classical techniques have been
    > shaped---maybe better than reading a textbook treatment of the techniques
    > already fully formed.
    > Cheers,
    > Peter

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