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Hello Steven,

I have read this article, but I don't think the author makes his case. Van
der Werf only speculates that the time argument might have been the
problem, because he does not find any other discrepancy in the almanac.
However, he does not show in detail how exactly the wrong longitude could
have come about.

If we accept the theory, we must twist a lot of words in Slocum's text:

1. Slocum does not say that he found an error in the Almanac, he says he
found it in the "tables".
2. Slocum says that a "logarithm" was wrong. RA and Dec are not tabulated
as logarithms.
3. Slocum says a "column" was in error, not the whole almanac.

Furthermore, RA and Dec don't enter the distance computation, unless the
altitudes are computed (and if so, as with some precise methods, only as a
small correction). So the error could only have affected the reduction of
the time sight that was taken in connection with it. May we really assume
that Slocum did not know how to do a time sight? When he re-took the
observation an hour later, he would have taken the distance and altitudes
again. But for both reductions he would have used local apparent time as
obtained from an earlier observation for time, wouldn't he? Of course, he
could have established LAT simultaneously with distance from Moon and Sun.
But this was not easy and certainly not common practice.

And finally, a twelve hour shift of the time argument can affect
declination of the sun by up to 12' around equinox. If we grant Slocum
that he performed at least noon sights regularly, this puts his various
claims about the accuracy of his navigation to within a few miles in an
interesting light.

In short, the problem that I have with van der Werf's explanation is that
we can accept it only if we assume that Slocum did not know what the hell
he was doing.


Steven Wepster wrote:

> Hi all,
>
> An article appeared in the 'Navigation'  journal (Vol. 44 no. 1, 1997)
> in which the author (S.Y. van der Werff) discusses lunar distances and
> particularly the lunar distance observation of Joshua Slocum on june
> 16, 1896, shortly before his landfall on Nukahiva. He comes up with a
> completely different kind of error that Slocum discovered and
> corrected.
>
> Van der Werff's suggestion is that Slocum accidentally took
> the RA/Dec values for a time 12 hours off (12 h too late, actually).
> Slocum's 'discovery', then, was that the tables were 12 hours off.

   

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