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After I finished another little project today at  the Blunt-White Library at
Mystic Seaport, I went hunting for uses of Sumner's  method in Norie's
Navigator. Norie, for ye who do not know, was a standard  navigation textbook in the
UK, every bit as popular as Bowditch was in the US,  and for what it's worth,
the iconic circumnavigator Joshua Slocum learned  navigation from a copy of
Norie. There is no mention of Sumner's method at all  in the 1860 edition of
Norie. In the 1872 edition there is a short problem  called "Verification of the
Latitude by Double Altitudes Using Sumner's Method"  (which leads me to
believe that the textbook used with that navigation notebook  we were discussing was
none other than Norie). This method does use two  different assumed latitudes
separated by 10 or 20 minutes, like the Sumner  method we know, but that's
all. It is entirely a calculational method. There is  no plotting of lines of
position nor even a mention of their theoretical  existence or significance.
Needless to say, this is information about a  navigational textbook method only
and may not reflect current usage. Norie, at  least, was updated more often and
more extensively than was Bowditch's  Navigator, which was largely unchanged
from 1837 to 1880.

Coincidentally,  in the Seaport's copy of the 1860 Norie, there is a plate on
the inside front  cover which reads "Gift of Captain Victor Slocum", but no,
it was not his  father's copy; just one from the same era.

Here's a puzzle (not from  either Norie above). Quoting an article:
"The principal error to which these  results are liable in all these problems
arises from errors in the observed  altitudes, and this error in some
situations of the objects may become very  large, and ought therefore to be guarded
against. It depends in a great measure  upon the angle contained between the
verticals passing through the objects [in  other words, the difference in
azimuth between the two bodies], being usually  the least when this angle is a right
angle. [...] Whenever indeed this angle  appears to be less than 30 degrees,
an error of some consequence may  occur..."

Anyone have a guess what this particular author is talking  about?

-FER
42.0N 87.7W, or 41.4N  72.1W.
www.HistoricalAtlas.com/lunars

   

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