NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Dip uncertainty
From: Bruce Stark
Date: 2004 Dec 5, 21:36 EST
From: Bruce Stark
Date: 2004 Dec 5, 21:36 EST
Alex,
This IS interesting. I've had the notion you only want a lower height of eye when there's fog, or for some other reason you have to get the horizon close to see it clearly.
My thinking goes this way: Ocean temperature would change the density of air near the surface. Updrafts and downdrafts from clouds would tend to keep that air in flux, so it would vary in density from place to place. Coming through that air, the light from the horizon is bent more than it normally would be. But, as seen from the bridge of a ship, the light has been coming through "normal" air for miles. The "abnormal" bending is a long way off. So there's only a slight difference between where the navigator sees the horizon and where he would see it with normal refraction.
But the small boat navigator is right there in the midst of the abnormal refraction. He sees the horizon in line with the tangent of the bend the light makes just before entering his eye.
Set a prism between you and a line on the wall. Put the prism near the wall and look at the line from across the room. You'll see the line not far from its true position. Bring the prism close to your eye and you'll see the line a considerable distance out of place.
Bruce
This IS interesting. I've had the notion you only want a lower height of eye when there's fog, or for some other reason you have to get the horizon close to see it clearly.
My thinking goes this way: Ocean temperature would change the density of air near the surface. Updrafts and downdrafts from clouds would tend to keep that air in flux, so it would vary in density from place to place. Coming through that air, the light from the horizon is bent more than it normally would be. But, as seen from the bridge of a ship, the light has been coming through "normal" air for miles. The "abnormal" bending is a long way off. So there's only a slight difference between where the navigator sees the horizon and where he would see it with normal refraction.
But the small boat navigator is right there in the midst of the abnormal refraction. He sees the horizon in line with the tangent of the bend the light makes just before entering his eye.
Set a prism between you and a line on the wall. Put the prism near the wall and look at the line from across the room. You'll see the line not far from its true position. Bring the prism close to your eye and you'll see the line a considerable distance out of place.
Bruce
