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    Re: Distorted horizon
    From: Bruce J. Pennino
    Date: 2013 Dec 18, 20:31 -0500
    
    Brad is exactly correct about using the horizon, as good or bad as it is.
     
    For practical navigation using CN your location error would not exceed a couple of minutes except in truly unusual circumstances. This has been discussed previously, but a scientist by the name of W.J. Peters took 3000 or more sights on cruises over 4 years or more (I can't remember right now) over all of the oceans except where very cold water , or warm water, with very cold or warm air might "merge".  Maximum error due to unusual dip was 2-3 minutes of arc.

    Bruce
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
    ----- Original Message -----
    Sent: Wednesday, December 18, 2013 5:37 PM
    Subject: [NavList] Re: Distorted horizon


    Hi Jackson

    Here are my solutions to your question of what to do in the event of anomalous dip

    1) Frank mentioned that he was standing on Conanicut Island.  He could turn around and trade his barometer for the latitude and longitude.  Sorry, couldn't resist.

    2) If Frank can see Block Island, he can also likely see Orient Point and Montauk Point.  Using his sextant to measure horizontal angle and his three armed protractor will provide a tight fix.

    3) In deep water navigation, with no 'floating islands' visible, the error introduced by the anomalous dip is within the normal margin of error for celestial.  Take the sight and use as is.

    4) Frank could whip out his dip meter or his Circle of Reflection, measure the dip and then use the measured value instead of the tabular dip correction.  This would be the most accurate approach.

    Brad

    On Dec 18, 2013 3:10 PM, "Jackson McDonald" <jacksonmcdonald---.com> wrote:

    Frank,

    Very interesting photos.  Thank you.

    If you were the navigator aboard ship, would you attempt to take a sight, compensating as best as you could for this mirage effect, or would you simply not take a sight?

    JMcD


    From: FrankReed{at}HistoricalAtlas.com
    To: jacksonmcdonald---.com
    Date: Wed, 18 Dec 2013 11:56:03 -0800
    Subject: [NavList] Distorted horizon


    I took this photo on the afternoon of December 13 looking south over the ocean from here on Conanicut Island. The black line represents the expected location of the sea horizon under normal conditions given my height of eye which was about 25 feet. At this scale (8 minutes of arc per pixel), the gap between the black line and the apparent sea horizon varies from about 1 to 1.5 minutes of arc. In other words, it's showing 1 to 1.5' of anomalous dip.
    I stopped to take this photo because I could detect visible undulations in the horizon even with no magnification. You can see some of that in the photo. At 25x magnification, the horizon was mottled and shredded by refraction. The undulations were moving along the horizon at variable speeds that did not correlate with wave motions. There were only very small waves rolling in on the beach there. One of these days, I will try to capture a video of this. I suspect that these "waves of refraction" running along the horizon are sometimes mistaken for ocean waves.
    The second photo taken from a couple of miles east shows Block Island floating above the sea. It's a mirage effect that creates the illusion of sky between the island and the sea horizon and is responsible for obscuring the actual sea horizon. This is almost certainly what's going on in the first photo, too. Refraction is "painting" a narrow band of sky over the actual sea horizon. The distortions within the mirage band create the shredding and undulation along the apparent sea horizon. Of course when nearby islands appear to float like this, most celestial navigators know that the horizon is suspect. With no obvious "floating islands", examining the horizon with binoculars or a small telescope for the "mottling" visible in the first photo should also be a good clue that a mirage is present.
    -FER
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