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From: Frank Reed (no email)
Date: Tue Jun 14 2005 - 21:16:05 EDT
George H you wrote:
"There's another phenomenon that a navigator might not expect when sailing
in gusty/squally conditions in a North or South direction near noon. If his
vessel is speeding up in the puffs, then slowing in the lulls, then so will
his rate of change of Sun altitude due to those changes in speed. That
changing slope has to be added to the expected parabolic change in Sun
altitude caused by the Sun's transit through the meridian. And the end
result is a wavering of the curve of altitude with time, about its peak. In
an extreme case, the Sun could show more than one maximum altitude.
I don't suggest that that's going to be a cause of serious error. The
effects on the resultant wavering parabola can be averaged out, rather
well, by the proposed folding of the graph, particularly if the series of
measurements extends well away from noon"
As long as you're considering more intricate scenarios like this one, it's a
good chance to point out the advantage of the simple altitude adjustment
technique that I've already outlined. Imagine a scenario where you're sailing
along with a nice breeze making 7 knots from the beginning of your sight series
twenty minutes before noon until right around noon when, suddenly, the wind
dies. Though you might need to think through a few little details, by
adjusting each sight individually, this case can be handled easily.
You asked:
"I haven't seen any response from Frank to this suggestion, and wonder if he
thinks that it is perhaps wrong, or negligible."
At this point, I think it's a minor detail, but I haven't devoted much time
to it (so maybe I'll change my mind later!). What little time I have spent on
this 'lat/lon at noon' business has been focused on refining the small stuff
and trying to reach a happy balance between economy of paperwork and economy
of paper (that is, I don't think there's advantage to this method if it
requires lots of tricky calculations, and at the same time, there's not much
advantage if it requires purchasing and carrying a volume of tables).
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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