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From: Frank Reed (no email)
Date: Wed Mar 15 2006 - 19:21:55 EST
I wrote earlier:
"Stars, including Polaris, were rarely used in celestial navigation before
the 20th century except occasionally for lunar distance sights."
Greg, you replied:
"I seem to recall reading in a number of places that they were especialy on
land. Hard to say what is right when you hear so many different things!"
Land "navigation" and marine navigation are parallel universes. Everything I
said previously in this thread applied to marine navigation. For exploration
and mapping work on land, there are a number of differences. Stars were more
popular (they're easier to shoot from a stable platform, night is the
prefered time for this sort of work, and more), and lunars lingered longer
(chronometers don't like riding horseback and they don't like being dunked in
streams).
Also, I should add that there is no intrinsic reason why stars were not used
in 19th century marine navigation (the extra work required is minimal). They
just weren't popular.
In another post, you wrote down one equation which can be used to clear a
time sight:
"cos t = (sin h - sin L sin d) / (cos L cos d)"
and asked:
" (1) I thought you had to use co-latitude , co-declination, & co-Ho? (If
this was already done I must have missed it)."
Yes, that step is already done. It just swaps sines for cosines and cosines
for sines. The equation you've quoted above is a direct application of the
fundamental spherical trig cosine law (try deriving it yourself). Note that
there are a number of other equations that will allow you to calculate the
corner angle of a spherical triangle from the three sides. They're mathematically
equivalent so the results are always the same. Using one instead of another
is a question of calculational convenience, and that can be a big deal when
you're working with logarithms.
And you asked:
" (2) This looks suspiciously like every other spherical / navigational
triangle so why do you say stars were not used except for lunars?"
Because that's what's in the logbooks! Celestial navigation (the maritime
flavor) was very much a daytime activity in the late 18th and 19th centuries
--something like 98% of all sights. So why didn't they use stars more often?
Any ideas?
And you asked:
" (3) Are you saying you can't get a good longitude from just the noon
sight?"
I can answer 'yes' to that as you've qualified it: you can't get a "good"
longitude from "just" the noon sight. Noon sights (alone) were not generally
used for longitude. But a series of sights around noon can be used to get a
fairly good estimate of local time. For example you could do an altitude roughly
half an hour before noon and record it and the time on your watch. Let's say
your watch shows 11:35:00. Then shoot the noon sight and record the maximum
altitude. Perhaps you record the time on your watch and find that it's
12:06:10. Finally you wait until the Sun returns to exactly the same altitude after
noon that you observed half an hour before noon (and maybe you're even smart
enough to adjust the altitude for the change in the declination in an hour).
At that moment, you look at your watch and it shows 12:34:30. The average of
the times before and after noon is 12:04:45 so that means that your watch is
4m45s fast on Local Apparent Time. After adjusting for the equation of time,
you can then compare the corrected time with GMT from a chronometer and
you're done. Notice that the time I suggested for the maximum altitude does not
match the time half-way between the equal altitudes. This happens because the
Sun appears to "hang" at its maximum altitude for a rather extended period of
time --the exact time of noon is very hard to estimate from the single
observation used for noon latitude. Also note that the equal altitudes on either
side of noon are extremel sensitive to small changes in altitude so you need a
good observer with a good instrument and a stable platform to make this work
right.
"My reason for asking is that I do a lot of rev-war & pre-1840 re-enacting
and I'm trying to understand exactly what was done
on land & sea."
Aha. Very interesting. I suppose I would just re-iterate that practice on
land could be, and usually was, quite different from maritime practice. Partly,
this is because the problem is very different. At sea, navigation is a
little simpler, from one point of view, because you can sail in a straight line
for an extended period of time. From a different point of view, it's much more
difficult at sea because, of course, there are no landmarks. The reasons for
recording exact latitude and longitude are really very different in the two
environments.
And you wrote:
"I know for example that on land both David Thompson & Lewis & Clark took
Lat via Polaris."
Another difference between land-based celestial navigation and ocean
navigation in the 19th century: the ocean navigators were vastly greater in their
numbers, guiding thousands of vessels across the world's oceans every year, but
can you name any three of them <g>? They're an anonymous horde. By contrast,
the land explorers, though few in number and frequently only marginally
competent in the art (Lewis & Clark), they are often names that we all know
today. The navigation practiced by Lewis & Clark is justly famous because of the
historical significance of the expedition, but their navigational technique is
idiosyncratic, singular, and theirs alone.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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