# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Height of Eye on a ship.
From: Frank Reed
Date: 2022 Dec 2, 08:54 -0800

Jeremy,
you mentioned wave height in your latest post, and I've been meaning to ask you about that for your dip calculations. Your height determinations are basically buoyancy-driven, right? So they give a height above the mean level of the water, which is what you want for many practical calculations (like the air draft you described later!). But for dip we want the height above the visible horizon. The sea horizon is composed of thousands of overlapping wave tops in a zone about a mile deep (visual depth), and so the horizon we see is actually the mean water level plus some form of average wave height (I consider two-thirds of max wave height reasonable but swells complicate matters, of course). Do you subtract off wave height for your dip height of eye? Six feet out of ninety only changes the dip by 0.3' (twice that at 20 feet height), but if you're trying for maximum accuracy it might be worth your time. Here again, measuring/observing dip with sights abeam bypasses the theoretical issue completely.

You wrote:
"My Dip exercise with cadets is a didactic.  I am attempting to teach them how to think critically and pay attention to details.  Dip calculation is an easy and harmless way to convey this lesson."

Yes, that's what I meant: "rite of passage" ≈ "didactic exercise".
And yes, now that you mention it, I do recall you telling this story before and that its real significance was for the air draft. Making sure you don't break the boat is considerably more important than a few tenths of a mile in the celestial fix. And while testing the dip by observation is good science, testing the air draft by experimentation is frowned upon. :)

Frank Reed

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