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Mr Frank Reed wrote recently:

"Land-based surveying had its own needs and preferences. I am under the
impression that "Navigation-L" is mostly about navigation at sea. Not that
land surveying is irrelevant (it isn't) but it's a different animal."

This is an interesting point and raises questions about what types of issues
are worthy of discussion on this list. When I was encouraged to join this
list several years ago I was told that it was for people interested in
celestial navigation  - an analogy is the American Foundation for the
Promotion of Navigation of which I am a member.

Celestial navigation techniques cover four areas
*Maritime navigation
*Terrestrial navigation
*Marine hydrography (which is a combination  of the above) and
*Surveying

All of these topics have been discussed at some point in this forum, thus
the contention that the Nav L list is mostly about navigation at sea is a
bit adrift, witness the extensive and on-going interest among list members
about the work of Lewis and Clark. Unless I am very much mistaken neither
spent much productive time at sea.

Mr Reed's assertion that land surveying is a different animal confuses
exploration  and surveying. They are different. A captain on the quarterdeck
with a sextant in his hand confronts the same problem as an explorer with a
sextant in his hand surrounded by 600 miles of uncharted desert. I have used
a sextant extensively on exploring trips in inland Australia and also on H M
Barque Endeavour and the techniques and principles, as far as I can see, are
identical i.e. you use a sextant to bring a celestial body down to a
horizon. The terrestrial sight uses a double elevation shot but after you
take off IE and divide by 2 the rest is identical - same reduction
techniques, same clock, same tables. It is all based on the solution of the
celestial triangle all the rest is detail.

The small practical  differences between the two areas of navigation were
the subject of an article I wrote for the Navigation Foundation newsletter -
A "Lubber on the quarterdeck"  Issue 78 Winter 2002-2003.

Mr Reed also remarks: "... there is no reason why star sights for latitude
should be more accurate than sun sights (except that you have more star
transits available in principle and you could average them, but that doesn't
work at sea)."

Quiet true. However the ability to average the sights is critical and
probably what Gregory was driving at. Also I have done numerous noon sights
in the field as well as star sights and in my case I have always got far
better results from the stars. The reason why, I know not but I could guess:
1) Bringing a star down onto its reflection in the mercury pan gives the
observer a tiny target to aim at. I think superimposing the small point of
light substantially reduces the margin of error

2) When bringing limbs of the sun into coincidence in the pan it is hard to
get an exact "kiss" unless the sextant has absolutely zero side error. If
there is any side error the point of contact or coincidence can be become
confused by guesswork.

3) Looking into the noon-day sun in places  like Northern Australia for
example is not for the faint hearted. The face quickly begins to fry and I
always end up squinting particularly through my left eye and the right eye
is often left with mesmerising haloes from accidental sun flashes. Sweat
running down your forehead into your eyes is also a problem. I would back a
star sight for
accuracy anytime.

Mr Reed wrote;
"MANY readers in the year 2004 make the huge mistake of believing that
Norie, Bowditch, etc. describe the *practice* of navigation. They do not.
Although Bowditch and Norie and others recommended shooting stars,
navigators at sea rarely did so. Equal time in a navigation manual does NOT
imply equal time at sea!

Now of course, I am describing the evidence from the logbook collections
that I personally have seen. These are logbooks from American commercial
vessels. Do you have evidence from other vessels? Not just suppositions
based on recommendations in Norie... but real evidence. That would be
fascinating.

But 19th century land-based astronomy is a very different subject,"

Deriving a view of 19th Century navigation based solely on the evidence of
American commercial vessels ignores that great navigating and technically
innovative body the Royal Navy. If we pick two of its navigators Lt Matthew
Flinders and Captain Philip Parker King we see two men who tried to shoot
lunars from shore where-ever possible using artificial horizons. Both were
superb maritime celestial navigators, accomplished hydrographers and
experienced lunarians. They combined all four of the points I raised above
as encompassing celestial navigation.

The art of navigation like this list is possibly more multi faceted than Mr
Reed believes.

Kieran Kelly
Sydney
Australia

   

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