NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Night moon sights
From: Frank Reed CT
Date: 2004 Jan 30, 17:26 EST
From: Frank Reed CT
Date: 2004 Jan 30, 17:26 EST
Peter F wrote:
"But don't the waves right up to the horizon also reflect light? Remember that this shimmer of reflected light is most noticeable when the sea is particularly calm, so at about 3nm away the wave height should be negligible?"
Let me try to describe it differently. Imagine you're standing on the deck of a vessel out at sea. The Moon is, let's say, directly to port and 45 degrees high. First suppose that the sea is smooth as glass. In that case, instead of a shimmering "road" of light leading out to the horizon, you would see a simple clean image of the Moon directly below it 45 degrees below the horizon. The angle of incidence of the incoming light rays from the Moon equals the angle of reflection of the ray that you see coming from below the horizon. If you could really find such a perfect sea, you could treat the ocean as an artificial horizon and measure the angle between the Moon and its reflection (dividing by two gives its altitude).
Now blow...
Wind makes waves. The sea is no longer a flat reflecting mirror. Instead, the waves break the sea's surface into a multitude of facets, each like a small tilted mirror. A wave under the Moon towards the horizon has facets in the trough that are facing towards you and facets that are facing away. Part way down into the trough, there is a facet (like a small mirror) that sits at just the right angle so that a ray from the Moon is reflected towards the observer. The next wave after that also has such a facet, but it's a little further back in the trough of the wave. Wave upon wave out towards the horizon has a spot in its trough that is tilted just right so that the reflected ray is aimed at the observer's eye. Instead of just one image of the Moon (as in the case of the calm sea above), we see lots of little images (actually pieces of images) from all those little facets. And in every case, the geometry is just the same as in the case of a flat sea: angle of incidence = angle of reflection.
Eventually, as you get towards the horizon, the reflected ray from the Moon that would head towards us by the surface in the wave trough out there would be blocked by the crest in front of it. There would be no way to make the angle of incidence equal the angle of reflection in a way that would beam the reflected ray towards the observer on the vessel's deck. At that point, you would no longer see a clear reflection of light from the Moon. And none of the waves after that would contribute to the light you see either. The "road" of shimmering light reaching out towards the horizon would stop short... as if the pavers have run out of moonbricks.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
"But don't the waves right up to the horizon also reflect light? Remember that this shimmer of reflected light is most noticeable when the sea is particularly calm, so at about 3nm away the wave height should be negligible?"
Let me try to describe it differently. Imagine you're standing on the deck of a vessel out at sea. The Moon is, let's say, directly to port and 45 degrees high. First suppose that the sea is smooth as glass. In that case, instead of a shimmering "road" of light leading out to the horizon, you would see a simple clean image of the Moon directly below it 45 degrees below the horizon. The angle of incidence of the incoming light rays from the Moon equals the angle of reflection of the ray that you see coming from below the horizon. If you could really find such a perfect sea, you could treat the ocean as an artificial horizon and measure the angle between the Moon and its reflection (dividing by two gives its altitude).
Now blow...
Wind makes waves. The sea is no longer a flat reflecting mirror. Instead, the waves break the sea's surface into a multitude of facets, each like a small tilted mirror. A wave under the Moon towards the horizon has facets in the trough that are facing towards you and facets that are facing away. Part way down into the trough, there is a facet (like a small mirror) that sits at just the right angle so that a ray from the Moon is reflected towards the observer. The next wave after that also has such a facet, but it's a little further back in the trough of the wave. Wave upon wave out towards the horizon has a spot in its trough that is tilted just right so that the reflected ray is aimed at the observer's eye. Instead of just one image of the Moon (as in the case of the calm sea above), we see lots of little images (actually pieces of images) from all those little facets. And in every case, the geometry is just the same as in the case of a flat sea: angle of incidence = angle of reflection.
Eventually, as you get towards the horizon, the reflected ray from the Moon that would head towards us by the surface in the wave trough out there would be blocked by the crest in front of it. There would be no way to make the angle of incidence equal the angle of reflection in a way that would beam the reflected ray towards the observer on the vessel's deck. At that point, you would no longer see a clear reflection of light from the Moon. And none of the waves after that would contribute to the light you see either. The "road" of shimmering light reaching out towards the horizon would stop short... as if the pavers have run out of moonbricks.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
