NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Latitude by Lunar Distance
From: Frank Reed CT
Date: 2006 Nov 12, 00:21 EST
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From: Frank Reed CT
Date: 2006 Nov 12, 00:21 EST
Wolfgang, of Jaeger's article, you wrote:
"In our latitudes the error may amount to as much as 40 nm - due to the
fact that the moon is not as high in the sky"
Only for certain observations. This can all be understood most clearly in
terms of the relevant "cones of position". If I measure a lunar distance with a
star roughly at the same altitude as the Moon, then the resulting cone of
position intersects the Earth's surface more or less vertically, and the result
is just as accurate as if the Moon were high overhead.
And you wrote:
"on the condition that sextant reading is accurate to 10 arc seconds (which
is the smallest division of the scale of the Plath Navistar
Professional for instance - and Plath only guarantees an accuracy of +/- 20 arc
seconds)."
Is that a formal, legal guarantee from Plath? In other words, does it
say on paper somewhere "guaranteed to +/- 20 arc seconds"? If so, then should we
not presume considerably better accuracy in practice? Otherwise, they would be
dealing with a high number of returned sextants. Guarantees are usually given by
companies confident that the majority of their product falls within the limits
of that guarantee, wouldn't you say? I suppose in practice, many of
the instruments might be worse than that and no one ever bothers to check! As
for your comment that the smallest division of the scale is 10 arc seconds,
that's not relevant. It tells us nothing about accuracy of
observations. Many sextant micrometers have no vernier at all --the
"smallest division of the scale" in such cases is 1 minute of arc. But you
can still read them to tenths of a minute of arc. The Russian SNO-T is a good
example of this.
And you wrote (referring to Jaeger's article):
"Best results can be obtained when the moon is near culmination and when
the star observed is above or almost above it."
Does he really say that? If so, then it's clear that he only made it
half-way to understanding why this method of navigation works (and this would
make sense, given the era). Each lunar distance observation yields a line of
position where the full "cone of position" intersects the Earth's surface.
Finding the star directly above the Moon is not critical. I should add that I
considered this significant, and would have agreed with Jaeger, when I first
started pondering this topic back in June. He's coming from the perspective of
traditional lunar distance calculations (again, very understandable, given the
era).
And you wrote:
"When taking the distance to a second star the second star distance
should form a right angle with the first (which is self explanatory) and this
may be a rare condition. "
Rare? No. It is not critical at all that this be a right angle. Anywhere
from 60 to 120 degrees will not introduce much error and it can be on
either side of the Moon. At night, this condition should be easily satisfied at
almost any time by at least pair among the navigational stars. If you
want to apply this method in daylight, you would have to wait for the Moon and
Sun to move some distance across the sky or use another independent line of
position to cross with the available lunar distance LOP.
And you wrote:
"There was no reaction or discussion
of the method in the "Annalen" afterwards; it obviously sank like a "lead duck"
as we say in this country - only to be discovered by Frank almost a century
later." "
I'm not surprised. Any discussion of lunar distances in any way was
considered a sort of 19th century hang-over in 1912. It's a bit like discussing
celestial navigation, of any flavor, in 2006. Of course, today we have the
luxury of cheap calculation, and celestial is no longer a primary means of
navigation. If it floats your boat, you can try all sorts of things that were
difficult a hundred years ago.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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To post to this group, send email to NavList@fer3.com
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