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

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Re: Predicting transit using Bowditch section 2010
From: Jim Thompson
Date: 2003 Dec 23, 08:34 -0400

```Bowditch section 1801 completes the picture.  Section 2010 should have
referenced section 1801, and as Trevor points out, the authors of section
2010 should not have used the number of seconds in their example.

Section 1801 explains the difference in precision, and explains how to add
or subtract EqT to or from LAN 12h 00m 00s to get the precise time of
Meridian Passage, if the navigator needs to know it to the nearest second.
The trick of course is to interpolate for EqT depending on how many time
zones away the observer is from Greenwich when LAN occurs.  That's the part
I did not have clear.  So according to Bowditch 1801:

1. Meridian Passage occurs before noon, so LAN will occur after the mean
time of meridian passage.
2. So [LAN (always 12h 00m 00s) - EqT] = [12h 00m 00s - 4m 23s (interpolated
to 1600)] = 11h 55m 37s ZT.
3. And meridian transit at our Lo = [11h 55m 37s + 00h 12m 32s (DLo)] = 12h
08m 09s ZT, which is correct.

(Thanks Pierre -- your private email got me on that track.)

Jim Thompson
jim2{at}jimthompson.net
www.jimthompson.net
Outgoing mail scanned by Norton Antivirus
-----------------------------------------

> -----Original Message-----
> [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM]On Behalf Of Trevor J.
> Kenchington
> >
> > Jim Thompson wrote:
> > Here is the relevant section from the 2002 version of Bowditch: "2010.
(snipped)
>
> These are instructions for finding the _latitude_ of a moving object
> whose longitude cannot be known precisely. My guess is that they are
> intended to tell the navigator how to estimate when to step out to the
> bridge wing with his sextant and begin observing, _not_ as a way of
> determining just when the Sun crosses the meridian. That is, the method
> described is only intended to give the clock time of LAN to the nearest
> minute. That would explain why the instructions suggest that the time of
> the Sun's crossing the Greenwich meridian in GMT can be used as the time
>   (in LMT) of her crossing any other meridian -- an adequate
> approximation to the nearest minute but clearly not precise to the
> nearest second.
>
> So why, having set out to calculate to the nearest minute, does Bowditch
> quote an answer to the nearest second? I'd suggest that the authors of
> this section have fallen into a common trap that catches many writers of
> textbooks by seeking to present a degree of theoretical perfection that
> is irrelevant to the real world. In this case, their attempt at
> additional precision actually introduced an error.

```
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