# NavList:

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

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Re: Cross-tide strategy (was: DR plotting techniques)
From: Rodney Myrvaagnes
Date: 2003 Oct 25, 18:48 -0500

```On Sat, 25 Oct 2003 21:06:55 +0100, George Huxtable wrote:

>
>>For periodic tides
>>one could calculate the error at the end from missing the cancellation
>>by 90 degrees. It would not be insignificant for the 12-hour trip, but
>>surely would be for anything over a couple of days.
>
>>In my wanderings, the Gulf of Maine crossing is long enough so that
>>tides do cancel out, about 30 to 40 hours.
>
>I don't see the logic in that statement above. In semidiurnal tide areas, a
>"tide", between reversals, takes a bit over 6 hours, so 38 hours would be
>about 6 such tides and should indeed cancel out. But, by that same
>reasoning, a passage time of 32 hours would be 5 whole tides, and
>(depending on the phase at departure) you easily could end up displaced
>sideways from your goal by one whole tide's-worth, to port or to starboard.
>If the passage time is as loosely specified as "30 to 40 hours", then
>cancellation can not be relied on.
>
George is correct here, and I apologize for not correcting myself
sooner. I did realize that the --

integral {sin(x) dx} is cos(x) over any span, and the error at the end
would be just as great or small no matter how many whole cycles had
passed.

--shortly after I sent the last but I was on my way to a play and
forgot about it by the time I got home.

I will try to get back to the remainder ASAP.

Rodney Myrvaagnes                                               J36 Gjo/a

"That idiot Leibniz, who wants to teach me about the infinitesimally small!
Has he therefore forgotten that I am the wife of Frederick I? How can he
imagine that I am unacquainted with my own husband?"

```
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