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Trevor,

I wrote:

> But let us consider that lunars were recommended for checking GROSS errors 
of D.R. after many days of sailing - they never could be used for verifying 
the position from one day to another. And facing the real possibility of such 
gross errors, how the navigator was to recognize a lunar observation to be at 
the limit of the 99% error and therefore unusable comparing it with his very 
vaguely known D.R. position?


I accept your answer to this my posting as very reasonable, although when 
reading it in my comfy room and chair, I was recalling the statement that 
many navigation handbooks (published before GPS and Loran-C) repeated again 
and again: "Don't take a small difference between your D.R. position and the 
obtained fix as a proof of the accuracy of your observation! Assess it 
independently!"

But I agree with you that old navigators haven't anything better than lunars 
for many decades and that chronometers onboard were only equivalent to them 
at their best after some two months at deep sea - up to, say, 1870. I agree 
with you that lunars could help the skilled navigator much, even burdened 
with errors that I reported in this thread.

George Huxtable said the same by other words three days ago: "In the days of 
lunars, it's my guess that mariners were not demanding absolute certainty 
about their longitudes. Nor (it's my guess also) were they asking for 99.7% 
probability. If they knew, to within 95% probability, that they were within 
30' of their calculated longitude, one way or the other, that would in 
general be acceptable, in an age and in a trade that wasn't particularly 
safety-conscious."

I find two consolations against Bolte's report of errors of lunars in my 
romantic love for them: Firstly, the average of two simultaneously taken 
lunars to both sides of the Moon were proven by him to be much more reliable 
than a single one; and that you are probably right when you doubt the 
reliability of corner error values statistically deduced from such small sets 
of items.

There were much more numerous sets of lunars collected! I cited the report of 
Parry's polar expedition in 1821-23 about thousands of lunars observed during 
one month. And I have read that Russian admiral Kruzenshtern determined the 
longitude of Nagasaki by the average of 1028 evaluated lunars in 1803 (albeit 
taken on land, which is valid for Parry, too). Where are the workbooks with 
the evaluated observations of lunars?

In merchant fleets they were probably thrown away in harbors. Frank's logbooks 
of whalers prove it. And in navies? Maybe heaps of material lay somewhere in 
British, American and other archives.

But it is not the question to me, sitting in the center of Europe and 
searching for every nautical title for long weeks and months.

I thank to all participants in the thread for the exciting discussion.


Jan Kalivoda

   

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