Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Re: Dip uncertainty
    From: Trevor Kenchington
    Date: 2004 Dec 6, 15:25 -0400

    Alex wrote:
    >>further anomalous curvature between George's head and the man behind
    >>were minimal, the angular anomaly corresponding to that vertical
    >>displacement would be reduced by the greater distance.
    > I think this argument is wrong.
    > The dip is the angle between the ray from the horizon that enters
    > your eye and the horizontal direction.
    > Refraction distorts the angle, not the "height of the ray".
    > The error due to refraction of the taller observer in our example
    > is the SUM of the error due to refraction of the shorter observer
    > and the additional error caused by refrraction on the interval
    > between the shorter and taller observer.
    Try making a drawing of it, Alex. I think you will find that that
    confirms my version.
    Bruce has made the point in a less equivocal way: Imagine a prism placed
    in the light ray to cause the extra, anomalous refraction. That prism
    will displace the light through an angle which depends of the geometry
    of the system (and the optical density of the glass) but an angle which
    is constant. Now view the light emerging from the prism from a point
    close to the prism: The angle between the light reaching your eye and a
    straight line to the source of that light will be large. But if you back
    off from the prism, that angle will be far smaller -- not because the
    prism is bending the light any the less but because the distance from
    the prism to the light source subtends a smaller angle at your eye as
    you back away from both of them.
    > Let me give another, somewhat indirect argument supporting Georges and my
    > point of view. We know that the errors in altitudes due to refraction are
    > very large on low altitudes. So large that low altitude observations are
    > not recommended. (This was recently discussed on the list).
    > The errors in the dip due to refraction are much smaller (they affext
    > all altitudes, big and small).
    > Now suppose you do your observations from VERY high place, say from
    > a space ship orbiting the Earth.
    > Then the error in the dip due to refraction is one half of the
    > total error due to refraction (just by symmetry: the ray travels
    > the same distance in the atmosphere before and after it touches the
    > horizon). So the error in the dip in this experiment is clearly greater
    > than the error of the dip when the observation is made from a ship.
    > Which proves that this dip error cannot decrease with height:-)
    It proves nothing of the kind.
    For anomalous dip to decrease with height of eye, we have to assume that
    the amount of anomalous refraction decreases with height above the sea
    surface. The observer on the bridge of a large ship thus sees a ray of
    light which bent unexpectedly sharply (or the reverse) when rising from
    the horizon to a height of about 6 feet (i.e. to beside George's head)
    but which then conformed more closely to the expectations of the dip
    tables as it rose to 24 or 60 feet up. That reduction in non-standard
    refraction, within what is otherwise a single near-surface air mass, is
    clearly normal (though not universal) in the real world.
    A hypothetical observer aboard the International Space Station, however,
    would observe the Earth's horizon through a deep ocean of air, with many
    layers of different refractive index, the surfaces of which would not be
    normal to his view. That many-layered system involves a potential for
    anomalous refraction after the light ray passes George's head and before
    it reaches the Space Station which greatly exceeds the potential for
    anomalies over the tiny distance between the horizon and George's eye,
    let alone that between the back of George's head and the sextant of the
    (now very annoyed) ship's captain who is trying to get a Sun sight while
    that ****ing little sailing boat keeps blocking his view of the horizon!
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site