Just wondering...on the oceans, navigators use a sextant for out of sight of
land navigation. How is it done on the Great Lakes where they're not
operating at sea level? Of course, I'm talking about the absenceof
electronics. Anyone know of any literature on this?

Thanks,
Matt A.

Matt Ashbrook wrote:

Just wondering...on the oceans, navigators use a sextant for out of
sight of land navigation. How is it done on the Great Lakes where
they're not operating at sea level? Of course, I'm talking about the
absenceof electronics. Anyone know of any literature on this?

Thanks,
Matt A.

The answer is simple: They don't need to be at sea level
The actual difference in angle to any stellar object would be measurable,
but not with a sextant. Not with these small differences in height.

Peter
--
"Outside of a dog, a book is a man's best friend: and inside a dog,
it's too dark to read." -- Groucho Marx

I believe there is no correction for elevation. The angle that is being measured is the
height above the horizon, which is presumed to be a plane tangent to the surface of the
Earth. There is a correction for the elevation of the observer above water level. As
long as there is a clear horizon, the altitude is not relevant.

Actually, the angle of interest is the difference from directly overhead. Since we know
the spot on Earth directly below the star at a particular point in time, the difference
from being directly overhead implies a "circle of position" of the appropriate radius on
Earth. For example, if we assume the North Star is directly over the North Pole, then if
it is observed 20 degrees away from directly overhead, the observer must be on a circle 20
degrees away from the Pole, or 70 degrees North latitude.

Check out any basic celestial navigation book - this will be explained much better than I
can.

Off course, I don't know how much celestial was ever done on the Great Lakes - you're not
very often out of sight of land.

--
-jeff
"Constant Vigilance!" - Frances W. Wright



Matt Ashbrook wrote:
Just wondering...on the oceans, navigators use a sextant for out of
sight of land navigation. How is it done on the Great Lakes where
they're not operating at sea level? Of course, I'm talking about the
absenceof electronics. Anyone know of any literature on this?

Thanks,
Matt A.

Bruce Woodburn a écrit :

Elevation is not relevant. Marine navigators correct their observations for
their own height above the water (a height difference of 25ft is
significant) but not the height of the tide (which can be 25ft around here).
Think of the Great Lakes as a REALLY high tide. Aviators did not need to
correct for their elevation when using their (bubble horizon) sextants.

Bruce

Hello from Pierre ex captain with french degrees !

Faulse about the corrections : we have to correct for altitude and SPEED too as the
corriolis actions are may be evil in computation depending where and how fast you are :
even the bubble may drift ... in a jet (jet plane such as a Concord !) with a jet (jet
stream at hi alt ..!).

At sea it was also avail some devices as artificial horizons and gyro stabilized ... to be
used in or with sextants (or octants)!
But may be all this is off subject as you are using small units ...

Have fun, I was just passing ... reading ... !

*****************
Pierre BONNARD
http://pierrebonnard.free.fr/index.htm

Matt,

That is an interesting question. I know, I asked it 40 years ago. At
that time I was told that lake ships used celestial out on the big
lakes. Years later - due to a lack of work on the coast - I came out
here to run ore boats.

When I asked about when they do sights, the entire deck crew looked at
me very strangely, and one finally laughed. I turned out that one was
the only one that had any idea what I was talking about.

I have pulled down stars since I have been here, and attitude correction
is not required to get reasonable accuracy. The references that I have
about overland celestial (prior to loran and GPS) do not make any point
of altitude. Lake Superior being just over 600 feet really would not be
an issue compared to Lewis and Clark locating rivers nad mountains in
the wilderness.

I do hope that this is actually an answer to what you are asking.

By-the-by, do not put a real @ddress on the usenet. The spambots will
plague you forever.

Matt Colie A.Sloop "Bonne Ide'e" S2-7.9 #1
Lifelong Waterman, Licensed Mariner and Pathological Sailor


Matt Ashbrook wrote:
Just wondering...on the oceans, navigators use a sextant for out of sight of
land navigation. How is it done on the Great Lakes where they're not
operating at sea level? Of course, I'm talking about the absenceof
electronics. Anyone know of any literature on this?

Thanks,
Matt A.

Surfman,
The same can be said for navigation in the Med or Baltic Sea, etc. Dead
reckoning works ok until out of site of land, then currents, weather, etc.
screws it up. These Great Lakes are really inland seas and can be horrible
to navigate on due to the extreme weather and sea state changes.

Incidently, may I ask where you hale from?
Greg Luckett

"Surfman" wrote in message
...
I think you might be able to make a correction for your elevation. I
also am assuming and having never been much of a laker, that dead
reckoning would work. Can't be too long before you hit shore there no
matter where you are hey?

On Sat, 30 Aug 2003 06:31:32 GMT, "Matt Ashbrook"
wrote:

Just wondering...on the oceans, navigators use a sextant for out of sight
of
land navigation. How is it done on the Great Lakes where they're not
operating at sea level? Of course, I'm talking about the absenceof
electronics. Anyone know of any literature on this?

Thanks,
Matt A.






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If you know the lake height above (or below) sea level, celestial navigation
works just fine.

Some lakes are seriously large enough that potato navigation really won't
work. Check out Lakes Huron, Michigan, and the grand daddy of them all,
Superior!


--
Carl Brookins

www.carlbrookins.com
INNER PASSAGES
A SUPERIOR MYSTERY
http://www.Minnesotacrimewave.org/

"Surfman" wrote in message
...
I think you might be able to make a correction for your elevation. I
also am assuming and having never been much of a laker, that dead
reckoning would work. Can't be too long before you hit shore there no
matter where you are hey?

On Sat, 30 Aug 2003 06:31:32 GMT, "Matt Ashbrook"
wrote:

Just wondering...on the oceans, navigators use a sextant for out of sight
of
land navigation. How is it done on the Great Lakes where they're not
operating at sea level? Of course, I'm talking about the absenceof
electronics. Anyone know of any literature on this?

Thanks,
Matt A.

Why is any correction for the lake height needed? The altitude of an object above the
horizon (corrected for the observers height above water level) should be the same.

-jeff

"carlbrookins" wrote in message
...
If you know the lake height above (or below) sea level, celestial navigation
works just fine.

Some lakes are seriously large enough that potato navigation really won't
work. Check out Lakes Huron, Michigan, and the grand daddy of them all,
Superior!


--
Carl Brookins

www.carlbrookins.com
INNER PASSAGES
A SUPERIOR MYSTERY
http://www.Minnesotacrimewave.org/

"Surfman" wrote in message
...
I think you might be able to make a correction for your elevation. I
also am assuming and having never been much of a laker, that dead
reckoning would work. Can't be too long before you hit shore there no
matter where you are hey?

On Sat, 30 Aug 2003 06:31:32 GMT, "Matt Ashbrook"
wrote:

Just wondering...on the oceans, navigators use a sextant for out of sight
of
land navigation. How is it done on the Great Lakes where they're not
operating at sea level? Of course, I'm talking about the absenceof
electronics. Anyone know of any literature on this?

Thanks,
Matt A.

Jeff,
You have to correct to sea level. Not the level of the lake. Lake
Michigan is (or was when I was doing my navigation class, I believe it is a
couple of feet lower now) 579 feet above sea level. You need to add that to
your height off the water to due your calcs.
Hope this helps.

Jim D.

Jeff,
You have to correct to sea level. Not the level of the lake. Lake
Michigan is (or was when I was doing my navigation class, I believe it is a
couple of feet lower now) 579 feet above sea level. You need to add that to
your height off the water to do your calcs.
Hope this helps.

Jim D.