NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Distorted horizon
From: Jackson McDonald
Date: 2013 Dec 18, 20:07 +0000
From: Jackson McDonald
Date: 2013 Dec 18, 20:07 +0000
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@HistoricalAtlas.com
To: jacksonmcdonald@hotmail.com
Date: Wed, 18 Dec 2013 11:56:03 -0800
Subject: [NavList] Distorted horizon
From: FrankReed@HistoricalAtlas.com
To: jacksonmcdonald@hotmail.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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: http://fer3.com/arc/m2.aspx?i=125822I 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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