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Re: Waves & Dip
From: Brad Morris
Date: 2017 Apr 24, 07:56 -0400

Hello Mike

You wrote view of the horizon is made up only of crests. Therefore we need to take our sight when our yacht is as close to the crest as possible.

Yes! Think of the view, out along the barrel of the telescope of our sextant. For the point of the example, our HoE is 8 feet. The horizon is 3.3 miles away. When we form a triangle with these parameters, we find that the angle at the horizon is extremely shallow. You cannot see troughs or wave faces at the horizon, you only see crests. If you try to look down the face of the last wave, from the crest, all you see is the wave in front of it.

You also wrote

We are all aware waves and swells are never uniform in height

That is true in an absolute sense, yet not in a statistical sense. Each wave may be different, yet as a whole, waves in a wave field can be categorized by frequency (period) and amplitude. The National Data Buoy Center www.ndbc.noaa.gov maintains an extensive system of buoys. They don't say "waves are snowflakes, good luck", rather they define the three specific wave types. Wind waves, period and amplitude. Swell, period and amplitude. Significant wave, only amplitude. Additionally, they provide the spectral density of the energy, sorting all the energy into frequency bins. So statistically, it is possible to categorize a large group of waves. Those waves being our view down the barrel of our telescope. The assumption is then, that the wave you are upon, at the crest, is statistically similar to the other waves out in the wave field surrounding you.

Brad

On Apr 24, 2017 4:59 AM, "Mike Freeman" <NoReply_Freeman@fer3.com> wrote:

Thanks David,
I am just learning celestial but as soon as I read this it confused me. We are all aware waves and swells are never uniform in height but to attempt to consider this factor while working out a sight must be unrealistic. In my considered opinion we must consider the waves & swells are in fact uniform and the view of the horizon is made up only of crests. Therefore we need to take our sight when our yacht is as close to the crest as possible. I understand the horizon we need to use will be parallel to the celestial horizon and a line drawn from crest to crest achieves this. As does a line from trough to trough but the visible horizon is not made up of troughs and our view in a trough is of the face of a wave. A line drawn from half way up a wave to the crests of the waves on the visible horizon is not parallel to the celestial horizon. It is also clear that in unfavourable conditions as with all position fixing we should not be over confident about the accuracy of any fix obtained.
Very happy to have any of the above corrected.
Mike

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