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Frank Reed and Fred Hebard have recommended reading the following-

>In the Spring 1997, in "Navigation: Journal of the Institute of Navigation"
>there is an article by Siebren Y. van der Werf  of Groningen, Netherlands
>entitled "The Lunar Distance Method in the Nineteenth Century: A Simulation of
>Joshua Slocum's Observations on June 16, 1896".

I have now had a chance to read it too, and agree that it's a really useful
account of lunars and how they work. Would another listmember like a copy?

The following will only be useful to those who have a chance to read van
der Werf's article.

=============================

Fred cast doubt on van der Werf's claim to have discovered the error that
Slocum described in his tables when he took a lunar nearing Nukahiva in
1896, and I agree with Fred about that. I think that claim is quite
unfounded.

Van der Werf quotes Slocum's claim that "The first set of sights ... put
her many hundred miles west of my reckoning by account... Then I went in
search of a discrepancy in the tables, and I found it."

But then, he fails to quote Slocum's crucial next sentence- "In the tables
I found that the column of figures from which I had got an important
logarithm was in error." Why did he leave that out? Well one possibility is
that the sentence contradicts the claim in the article to have discovered
the error, in a table which is unrelated to any logarithm.

Frank Reed writes- "He also agrees that Slocum was probably just confused
when he talked about discovering an "error in his logarithms"". Where does
van der Werf say that? He says no such thing, that I can find, and avoids
all mention of logarithms.

He blames the error on a misreading by Slocum of the time-argument of the
almanac table for the Moon's right-ascension and declination. This is
because Van der Werf checked that table against a computer prediction, and
thought that he had found a 12-hour error in the times. Which indeed he
had, because he (not Slocum) had got the time-argument wrong, and at first
failed to appreciate that the table in the almanac (just like all such
tables in the almanac for the previous hundred years) was based on
astronomical time and date, in which the new day started at Greenwich noon:
not civil time, which starts at midnight.

Van der Werf acknowledges that other relevant tables were clearly based on
astronomical time, and states "But for the tables of the moon's right
ascension and declination, the place where the change of the date is
indicated suggests the use of civil time." As relevant extracts from that
table are included in the paper, it's easy to check that claim. And it has
no basis whatsoever, that I can see. Change of date, and change of time
from 24 hrs to 0 hrs, happen at the same moment, just as one would expect
of astronomical time. There's no reason to be misled into thinking anything
else.

Slocum would have been astounded, and confused, if he had found that one of
the tables in the almanac had been switched to civil time. Astronomical
time is what he was expecting, and that is what he got. The only confusion
about the matter was van der Werf's. It is, frankly, ludicrous to suggest
that Slocum may have wrongly though that this one table was for some reason
expressed in civil time.

And yet, van der Werf, in putting forward that explanation for Slocum's
error, feels sufficiently convinced to say- "Only this can be the "error"
that Slocum mentions."  A sweeping statement indeed, and to me, it's
nonsense.

======================

Without any decent clock to determine an accurate time interval, Slocum
would have needed to find the apparent local time at the same moment as his
lunar was taken, to obtain longitude using his lunar determination of
Greenwich time. An altitude of either the Sun or the Moon would have served
that purpose, and altitudes of both had to be measured anyway for use in
clearing the lunar. Normally, a mariner would choose to work with a Sun
altitude for this purpose, because the arithmetic is much more
straightforward. Only if he, perversely, chose to work with the Moon
altitude instead would he have needed those Moon R.A / dec tables that van
der Werf makes such a big thing of. The only reason I can think of for NOT
choosing the Sun would be if the Sun was so near the meridian as to make it
useless for determining apparent time. But van der Werf states that "the
observation must have been made in the (local) afternoon." In which case,
why on earth would Slocum have chosen the Moon over the Sun, on which to
base his calculations for apparent time? Unless he did, those Moon R.A.
/dec tables would have served no purpose at all.

However, in spite of the shortcomings listed above, the article is well
worth reading.

George.

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contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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