The Sextant: Is it a poor design?
Scott McDiarmid
I'm new to the study of celestial navigation. I have progressed to the
point of understanding the primary design and intent of the sextant; to
measure the elevation angle of an object by using reflected images of
the object and the horizon.
My question is this: Wouldn't it be easier and less error-prone to
measure elevation angle using plumb, rather than generating a reflection
of the horizon and correcting for dip?
When identifying stars, I routinely measure the star's elevation angle
with a protractor-like device with a weighted bar that hangs straight
downward due to gravity regardless of the sighting angle of the
device. The bar is set against a curved scale and thus indicates the
exact sighting angle, and therefore, elevation angle of the object.
Since the angle observed is from a truly horizontal plane (exactly
perpendicular to the bar), no eye-height correction is necessary. Also,
since it is not necessary to be able to see the horizon, angles can be
measured at any time, day or night, and from any location on land or
sea.
All other corrections would apply to both types of measurement and thus
cancel out. The same would be true for difficulty of observing due to
motion such as waves.
As a rough test, I measured the elevation angle of the evening sun,
first with a sextant, and then with the plumb-based device described
above, and got similar results, which I validated using the software
package SkyGlobe.
As I have said, I'm new to this. Am I missing something, or making some
incorrect assumptions?
Thanks -
- Scott
Paul Kamen
scot...@erols.com writes:
>...Am I missing something...
Yes, I believe you're missing the fact that "down" as measured by a plumb
bob (or similar) will point at right angles to the local wave or swell
surface, and only point in the "real" down direction if averaged over a
long period of time.
It's interesting to look at how this problem is addressed in aviation,
where a good horizon is rare. the navigator turns a dial on the "bubble
sextant" to keep the celestial body superimposed over a bubble, and hits a
button to make a tick on a strip recorder every time the alignment looks
right. Many sights over several minutes average out the error in local
gravity caused by aircraft motions. But it's still not nearly as good as
marine practice, using a real horizon.
--
fish...@netcom.com
http://www.well.com/~pk/fishmeal.html
-"Call me Fishmeal"-
Mark Hawkins
In article <33BD96...@erols.com>, Scott McDiarmid
<scot...@erols.com> writes
Scott,
You are 100% correct in what you say, and indeed there is a device which
employs the method you describe. It is known as the Astrolabe and was
used by Columbus.
More recently, Robin Knox-Johnson made a passage accross the Atlantic
using an Astrolabe which he had made (he had an origanal copied from the
maritime museum in Greenwich".) Robin sailed a route very simular to
Columbus, and used only this replica. He also had a transponder on board
which reported his position back to shore. Robin was not privy to this
posion data until he reached land and the it was compared his plot.
Without looking up the details, I understand that the worst case of
astrolabe vs technology was abou 21 miles!
There are disadvantages. The Astrolabe is very difficult to use for the
stars. and as the crew of the "Mathew" recently found, it is a very
difficult instrument to use anyway!
You can of course have a go at the astrolabe yourself. The National
Maritime Museum at Greenwich, UK has made a "Do it yourself" astrolabe
kit. It does not cost a fortune, and I am sure if you drop the museum an
email (you wil need t go to a search engine for this) I am sure they
will post you one in return for a card number!
The sextant is not a poor design, it has evolved over many years. You
can visit the national maritime museum and see the story of the sextant
and all the previous instruments.
On a completely different subject, in your studies, you will realise
that time is very important to finding Longitude. I can recomend
"Longitude" by a New York Journalist "Dava Sobel" it gives a very god
account of the works of John Harisson and is marine chronometers. I have
read it and it lives up to the very good reviews given by the navigation
institutes.
--
Mark Hawkins MRIN *******************************************
PRINCIPAL * http://www.atlantis-sailing.demon.co.uk *
ATLANTIS SAILING LTD *******************************************
Tel +44 181 398 8684
Sailor
How would you utilise a plumb bob once you are on a vessel that is pitching
and rolling? The answer of course is that you need a steady horizon.
Al Saunders
Consider the platform on which you are mounted when in a vessel at sea.
The ship is pitching and/or rolling (heeling) and/or yawing and/or any
of or all of them simultaneously. In addition the ship may be
stationary or in constant motion or accelerating or decelerating in any
or all of the directions at once. In addition the ship may be moving
vertically and/or horizontally at a constant speed or accelerating or
decelerating or be in a constant position. All of this time the plumb
bob is endeavouring to locate itself relative to its suspension point
(which is moving as described above) and the center of the earths
gravity (which may or not be truly at right angles to the sensible
horizon). It is only pointing at right angles to the wave or swell by
random accident and only momentarily. Add to this the time lag of
movement of the plumb due to inertia and the plumb will almost never be
pointed toward the true vertical.
Imagine yourself on this vessel in a reasonable seaway with waves coming
from a different direction than the swell and you would be hard-pressed
for your eyes to even follow the random oscillations of the plumb (even
in six inch swells and six inch waves). In addition you pointed out the
use of a protractor. Can you read the angle between the plumb and the
body with an accuracy of 1 minute of arc?
I hope that I have dispelled any thought that your suggestion is
practical. The thought of using a bubble sextant or any kind of
artificial horizon is absolutely inane. I happen to own a Fairchild
bubble sextant and have taken it aboard many friends boats. When they
actually try to keep the bubble centered and the body even in visibility
never mind the center of the bubble when you are only in 1 foot waves,
they laugh at themselves for thinking that it could be practically used.
I admire your approach to trying to come up with a better ĺ§ousetrap';
this one just isn't it. Good luck in your pursuit of the practice of
celestial navigation.
--
Regards, Al Saunders, RIN
http://www.globalserve.com/cbeeson/~alzarc.htm
"One accurate measurement is worth one thousand expert opinions"
Scott McDiarmid
Thanks for the responses! They were quite informative.
- Scott
James A. Chamblee
scot...@erols.com asked:
> As I have said, I'm new to this. Am I missing something, or making some
> incorrect assumptions?
>
So how would your plumb bob idea work on a pitching and rolling deck? Also,
how could you be sure that the instrument was truly vertical? With a
sextant, we swing the instrument to check this.
Not being critical, just asking.
Jim
Md.
Mark Edward Balcom
Scott McDiarmid wrote:
>
> I'm new to the study of celestial navigation. I have progressed to the
> point of understanding the primary design and intent of the sextant; to
> measure the elevation angle of an object by using reflected images of
> the object and the horizon.
>
> My question is this: Wouldn't it be easier and less error-prone to
> measure elevation angle using plumb, rather than generating a reflection
> of the horizon and correcting for dip?
>
> When identifying stars, I routinely measure the star's elevation angle
> with a protractor-like device with a weighted bar that hangs straight
> downward due to gravity regardless of the sighting angle of the
> device. The bar is set against a curved scale and thus indicates the
> exact sighting angle,
>
> - Scott
what you are describing is an old instrument known as an astrolabe. It is
not very accurate on a moving vessell and reguires two people to get a
reading. That devise was replaced many years ago. Good thinking though. I
like the way you reasoned that out. Bubble sextants don't work well from
the deck of a pitching, rolling vessel, either.
Mark
S/V Kaiulani
Scott McDiarmid
Sailor -
Not a plumb bob, but a plumb-based device, something similar using the
same principle as a gimbaled, oil-filled compass or inclinometer - well
damped.
- Scott
Scott McDiarmid
Jim -
This was the same reason Sailor and others have responded with, and
seems to be the general consensus; that the motion of the sea is the
problem. But, rather than a true plumb bob, I meant a plumb-based
device (such as an oil-filled, gimbaled compass or inclinometer which
are (most of the time) fairly effective at correcting for the motion of
the sea).
Roger M. Derby
Paul Kamen wrote:
> scot...@erols.com writes:
>
> >...Am I missing something...
>
> Yes, I believe you're missing the fact that "down" as measured by a
> plumb
> bob (or similar) will point at right angles to the local wave or swell
>
> surface, and only point in the "real" down direction if averaged over
> a
> long period of time.
Oh, Paul! What you said. While the plumb bob may respond slightly to
local masscons, it generally points toward the center of the earth. A
"vertical" line painted on the bulkhead will be perpendicular to the
wave, but the plumb bob WILL point down. (This assumes you and it
aren't swinging about in an undamped fashion.)
Roger
==============================================================
mailto: der...@flash.net web: http://www.flash.net/~derbyrm
Michael T Kennedy
for real, the stars are often partly obscured by overcast and the motion
of the (small) boat makes it a real challenge to get any accurate
sights. What you do often is to shoot as many stars as you can see,
taking the bearing as well as the elevation, and then when the light
becomes too bright (you usually shoot stars at dawn so that you can
identify them first), you go below to reduce your sights. You discard
any that are way out of the triangle and hope that you have a good fix.
In Transpac 81 the only star I could find was Polaris on one night.
Every other night was overcast. The Polaris sight, plus my morning and
afternoon sunlines got me to Hawaii second overall. The Polaris sight
was my only latitude and came through a hole in the clouds about two
nights before landfall. That was pre-GPS and celestial navigation is a
dying art I suspect.
Mike Kennedy
yacht Audacious
Michael T Kennedy
Scott McDiarmid wrote:
>
> Jim -
>
> This was the same reason Sailor and others have responded with, and
> seems to be the general consensus; that the motion of the sea is the
> problem. But, rather than a true plumb bob, I meant a plumb-based
> device (such as an oil-filled, gimbaled compass or inclinometer which
> are (most of the time) fairly effective at correcting for the motion of
> the sea).
The motion of the sea is not the only problem. When you are doing this
for real, the stars are often partly obscured by overcast and the motion
of the (small) boat makes it a real challenge to get any accurate
sights. What you do often is to shoot as many stars as you can see,
taking the bearing as well as the elevation, and then when the light
becomes too bright (you usually shoot stars at dawn so that you can
which stars they were. You discard any that are way out of the triangle
and hope that you have a good fix. In Transpac 81 the only star I could
find was Polaris on one night. Every other night was overcast. The
Polaris sight, plus my morning and afternoon sunlines got me to Hawaii
second overall. The Polaris sight was my only latitude and came through
latitude from noon sights but the track to Hawaii is right under the
sun's azmuth in July. Hard to get a noon sight altitude when it is 90.
Paul Kamen
scot...@erols.com writes:
Same problem. Ocean swell periods can be as long as 20 seconds. Even if
the amplitude is very low, "down" as measured by any device using weight
and buoyancy will give you a direction normal to the local swell surface.
(Assuming boat speed is slow compared to swell velocity - which it is.)
Paul Kamen
Roger M. Derby <der...@flash.net> writes:
>...but the plumb bob WILL point down. (This assumes you and it
>aren't swinging about in an undamped fashion.)
Nope.
Consider the boundary conditions on the free surface of the gravity wave.
All body force accelerations are normal to the surface!
So a plumb bob in a small boat on a very big non-breaking wave will point
at right angles to the local wave surface - and not towards the center of
the earth (with same assumption about no undamped swinging around).
(There's a device called a "Froude gauge" that's used to hammer this
concept into the thick skulls of undergrad naval architecture students....)
Scott McDiarmid
Thanks, Mike...interesting (and somewhat unnerving!) anecdote about your
Transpac experience!
- Scott
Scott McDiarmid
Paul -
Yes, I think I understand. I have a mental picture of the boat, bow
down, yet still moving forward horizontally, while riding the down side
of a swell; sort of like the way a helicopter often takes off - pitched
forward as it also moves forward. It makes sense that the plumb would
sort-of trail behind (for a while, anyway), and thus point perpendicular
to the swell.
...One last question (actually, two), and then I'll stop beating this to
death:
If the swell is sufficient to cause error with the plumb-based device,
wouldn't the vertical component of the swell cause error in the dip
correction due to the inability to accurately determine eye-height - and
is this insignificant?
- Scott
Al Saunders
artificial horizon. the dip correction is zero. When measuring to a
true horizontal (at 90 degrees to a vertical) there is no depression of
the visible horizon as you are not using a visible horizon.
Regards, Al Saunders, RIN
Steven Goldfarb
In <33BFA3...@erols.com> Scott McDiarmid <scot...@erols.com> writes:
>Sailor -
>Not a plumb bob, but a plumb-based device, something similar using the
>same principle as a gimbaled, oil-filled compass or inclinometer - well
>damped.
>- Scott
OK -- if we want to get totally nuts... you can buy a small gyroscopic
device designed to stablize a camera -- it might help average out the
readings and provide a steady enough platform. Of course, it's one more
piece of equipment to fail.
--sg
--
---------------------------------------
Steve Goldfarb, s...@stevegoldfarb.com
Digital Productions: http://stevegoldfarb.com/
For fun: http://stevegoldfarb.com/sandbox/
Terry Schell
s...@panix.com (Steven Goldfarb) writes:
>In <33BFA3...@erols.com> Scott McDiarmid <scot...@erols.com> writes:
>>Sailor -
>>Not a plumb bob, but a plumb-based device, something similar using the
>>same principle as a gimbaled, oil-filled compass or inclinometer - well
>>damped.
>>- Scott
>OK -- if we want to get totally nuts... you can buy a small gyroscopic
>device designed to stablize a camera -- it might help average out the
>readings and provide a steady enough platform. Of course, it's one more
>piece of equipment to fail.
>--sg
Perhaps, but don't all of these averaging devices assume that you
will spend an equal amount of time on each side of a wave face? The
gyro would probably average over such a large time interval (could
even be days) that this might be manageable, but just some damping
device will have the problem that you spend much more time on the "front"
of a wave than on the back (particularly when the wave train is moving
in the same direction as the boat).
Terry
Doug Hawkins
In article <5pu3u2$n...@panix2.panix.com>, s...@panix.com says...
>
> In <33BFA3...@erols.com> Scott McDiarmid <scot...@erols.com> writes:
>
> >Not a plumb bob, but a plumb-based device, something similar using the
> >same principle as a gimbaled, oil-filled compass or inclinometer - well
> >damped.
>
> device designed to stablize a camera -- it might help average out the
> readings and provide a steady enough platform. Of course, it's one more
> piece of equipment to fail.
Actually, the "plumb-based" device was a precursor to the modern sextant.
Someone else probably knows the name, if it's important. Also, the
Germans used a compressed-gas powered, gyroscopically-stabilized sextant
during WWII. There's a cool sextant book at West Marine talking about
these devices. I'm not sure why they aren't used anymore, but my guesses
would be based on reliability, uncertainty and expense. Militaries can
often afford far more esoteric equipment than sailors.
Doug
Scott McDiarmid
> Scott McDiarmid wrote:
> >
> > If the swell is sufficient to cause error with the plumb-based device,
> > wouldn't the vertical component of the swell cause error in the dip
> > correction due to the inability to accurately determine eye-height - and
> > is this insignificant?
> >
> Al wrote:
> >
> When using a plumb (if possible) or abubble or any other form of
> artificial horizon. the dip correction is zero. When measuring to a
> true horizontal (at 90 degrees to a vertical) there is no depression of
> the visible horizon as you are not using a visible horizon.
Sorry, Al -
As usual didn't express myself correctly. I meant, if the swell causes
error with the plumb-based device, couldn't the vertical component of
the swell cause error in the dip correction when measuring with the
sextant - Although I learned today that this error is effectively
cancelled out because the increase in height going to the top of the
swell is exactly offset by the decrease when going down into the trough,
so you can just use the eye-height measurement obtained at the dock.
Paul Kamen
tsc...@s.psych.uiuc.edu (Terry Schell) writes:
>Perhaps, but don't all of these averaging devices assume that you
>will spend an equal amount of time on each side of a wave
>face? ...
That's one problem, but an even worse problem is that the inertial
reference frame is rotating at 15 degrees/hour, so the gyro will never
stabilize on "down." Unless we want to re-invent the "north seeking"
gyrocompass.
Paul Kamen
scot...@erols.com writes:
>...I learned today that this [chenge in dip] error is effectively
>cancelled out because the increase in height going to the top
>of the well is exactly offset by the decrease when going down
>into the trough, so you can just use the eye-height measurement
>obtained at the dock.
That's true, but for a different reason. As a practical matter you usually
won't be able to see the horizon at all from the bottom of the swells, at
least not from a small boat. So the reading is taken from the top of the
swell. But the horizon is also made of swell tops, not bottoms - and hence
the nominal dip corrections work fine.
Terry Schell
do...@well.com (Doug Hawkins) writes:
<snip>
>Someone else probably knows the name, if it's important. Also, the
>Germans used a compressed-gas powered, gyroscopically-stabilized sextant
>during WWII. There's a cool sextant book at West Marine talking about
Was this a gyro-stabilized artificial horizon or just a stabilized
version of a normal sextant? Was it for aircraft or marine use (very
different needs)?
A small, wind-up gyro stabilized sextant might be cool. :-) It could
really help with those rough seas shots (for those of us who aren't
too good at them).
Terry
Mark Hawkins
In article <5pv2pj$b5h$1...@vixen.cso.uiuc.edu>, Terry Schell
<tsc...@s.psych.uiuc.edu> writes
Going slightly of the subject, British submarines used to use gyro's in
a sextant which was built into a periscope. As you can imagine, the
periscope was only just above the surface of the water thus to low to
get a usable horizon even in the slightest of sea states. A mirror
placed on a gyro stabalised platform produced an artificial horizon and
this would be used for observing heavenly bodies.
Marc Auslander
Have you tried to use your plumb bob at sea? In waves? In big waves?
When you need to brace yourself so you can free your hands to take the
sight?
--
Marc Auslander <ma...@watson.ibm.com> 914 945-4346 (Tieline 862 Fax x4426)