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: The taxonomy of refraction
    From: John Huth
    Date: 2011 Jan 8, 18:09 -0500
    Marcel - 

    I thought I'd share back what Andy said about those photos on the board in case anyone else is curious.   

    The first photo indeed displays an inferior image.   He asked about the wind conditions, which I didn't think about, but there was little wind in the first photo.   He (and now I) believe that there was local heating right at the surface of the water, I'm assuming because there was little mixing due to wind and wave conditions.    

    The inverted image is compressed, which he says is due to the curvature of the Earth (another al-Biruni measurement?? - did he have a digital camera in the 11th century?).   He said that one can think of it as a reflection off of a slightly convex surface - this blew me away, but I guess it makes sense. 

    The appearance of the low lying parts of the island as the day wears on is because the island is higher than the sea horizon in the last photo - an example of what I think Frank meant when he said that you can see the horizon "breathe".   

    Best, John H. 

    On Fri, Jan 7, 2011 at 6:01 PM, Apache Runner <apacherunner@gmail.com> wrote:
    Marcel - 

    In your case, yes.   In this case, I'm not so sure.  I think that that island is just on the edge of the geographic range, so high points poke above the horizon and low points disappear.    I'm not entirely sure, but I've seen this island break up into small pieces and also appear as a continuous strip.   Also, it slants at an angle from my house, so the closest point is about 5 miles away and then slopes off to about 8+ miles.   From the height of my house - 25 ft, I can sometimes see the close part as a continuous strip and then it breaks up into islets and then disappears.   Under other conditions, it appears to be continuous all the way to the end.   I'm assuming that a lot of this is due to the effective horizon shifting under differing atmospheric conditions.   

    I sent those photos to Andy, as you suggested and gave him the conditions.   I'll report back what he has to say.


    John H. 

    On Fri, Jan 7, 2011 at 10:08 AM, Marcel Tschudin <marcel.e.tschudin@gmail.com> wrote:
    > I guess my first-order thinking is that the apparent horizon is going
    > further out as the day progresses...
    I presume you mention this in the context of the second photo having
    gaps next to the lighthouse and the adjacent building. I could imagine
    that this is only a "colour feature" from how the "missing" parts are
    illuminated by the sun. The way how they are iluminated may have
    turned them to the same blue colour as the sky. I observed this some
    time ago in the context of "ships in the sky" where part of the sea
    received the same colour as the sky. You find these photos here

    Keeping up with the grind

    Keeping up with the grind
    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