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Alex wrote in relation to land exploration:

"Why did they use sextants rather than theodolites?
As I understand, theodolite is a much more precise instrument,
because of the stable platform.


Its only disadvantage on land is that it is somewhat larger and
heavier than a sextant.

"Precisely. A theodolite with hardwood tripod legs weighs about 4x times as
much as a sextant. Eg I own a Wild T1a theodolite. The combined weight of
instrument, carrying case and tripod is 12 kgs. The  Zeiss Yachtsman sextant
and case which I use on expeditions weighs 2.8kgs.

Also a theodolite is a much more delicate instrument than a sextant. My
sextants have survived being bucked right off the back of a pack horse and
on a recent expedition one travelled 750km on the back of a camel with only
marginal change in index error over a five week period.

Conversely a theodolite is more fragile with telescope, vertical and
horizontal bubbles etc it is easier to get out of alignment. Nevertheless, a
theodolite is a pretty robust instrument. I have been receiving instruction
in the use of a theodolite over the past six months after years of using a
sextant and I am very impressed with them. (Their accuracy and ease of use,
not their portability. They are incredibly heavy to lug around.)

The explorer Gregory who was also a surveyor never took theodolites on his
exploring journeys. He claimed they were far too heavy and cumbersome. By
this he meant they were too difficult to secure on the back of a packhorse.
The tripod in particular is a difficult item to secure to a pack saddle eg
on modern expeditions I have found the long handled shovel the most
difficult item to secure to a camel saddle.

Also the role of theodolites and sextants should not be confused. A
theodolite is an instrument of surveying, a sextant an instrument of
exploration and navigation. Explorers whether A C Gregory or Lewis and Clark
were not sent out to make exact maps - they were sent out to discover and
record - the theodolite followed the sextant. After Gregory the explorer
came Larry Wells the surveyor, after Lewis and Clark came Mason and Dixon.

And don't underestimate the accuracy of a sextant in the hands of an expert.
I urge you to look at Gregory's results in my paper at
http://users.bigpond.net.au/kjkelly/gregorylunar/A%20Lunar%20Distance%20Calc
ulation%20v1.pdf.

Yes he could get better results with a theodolite but for practical
exploration and navigation purposes what was the point. In all my study of
Gregory and his field lunars I have never found an error of Longitude of
more then 8 nautical miles and that is in a country the size of Australia -
same size as continental USA.

Alex also wrote:
"And you can certainly measure lunar or any other distances with it."

I am interested in how this could be done. I have been using a theodolite
regularly in practice for some time now, measuring horizontal angles between
two land based features, altitudes of celestial bodies and their azimuths
but how do you record the angular distance between two bodies using a
theodolite in that a lunar distance requires simultaneous observations of
two celestial bodies at a fixed point in time. This does not appear possible
with a theodolite (my limited experience notwithstanding.) Happy to be
corrected.

Kieran Kelly
Sydney

   

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