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#283728 - 02/13/17 12:38 AM Re: Improvisation - sextant from signal mirror [Re: hikermor]
rafowell Offline
Member

Registered: 11/29/09
Posts: 197
Loc: Southern California
Originally Posted By: hikermor
Could one use duct tape today?

I think so - I see four spots that seem to be chewing gum - two to hold the sight wand to the tin, and two to hold the protractor to the tin.

I think duct tape would have done just as well.

For the general function of "attaching things together", duct tape, cord, thread and wire are all popular survival kit items, and of these duct tape seems most apropos here.

Given the choice, I'd use kneadable epoxy putty stick* rather than a wad of chewing gum, but I currently don't carry either. And I wouldn't advise chewing the epoxy ...

*I repaired my broken eyeglass frame with this once.


Edited by rafowell (02/13/17 01:42 AM)
Edit Reason: Asterisk was in a misleading location
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A signal mirror should be backup for a 24 hr, all-weather radio distress signal, such as a 406 MHz PLB (ACR PLB)

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#283747 - 02/13/17 08:30 PM Re: Improvisation - sextant from signal mirror [Re: rafowell]
Phaedrus Offline
Pooh-Bah

Registered: 04/28/10
Posts: 1880
Loc: Great Plains
Duct would have worked too but the gum probably tasted better! grin
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“You can tell whether a man is clever by his answers. You can tell whether a man is wise by his questions.” Naguib Mahfouz

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#283758 - 02/13/17 10:20 PM Re: Improvisation - sextant from signal mirror [Re: rafowell]
Mark_R Offline
Old Hand

Registered: 05/29/10
Posts: 822
Loc: Southern California
I imagine that any clinometer would have worked. There's a long history of attitude measuring equipment that can be improvised. Cross staff, back staff, quadrant (protractor + plumb bob), astrolabe, various clinometers. The catch is having the tables or formulas to being able to translate the data into usable coordinates.
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Hope for the best and prepare for the worst.

The object in life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane

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#283759 - 02/13/17 10:29 PM Re: Improvisation - sextant from signal mirror [Re: Mark_R]
Russ Online   content
Carpal Tunnel

Registered: 06/02/06
Posts: 4438
Loc: SOCAL
Gravity doesn't work well enough in a small boat bouncing around at sea. I was a tad confused by the picture of the improvised sextant.
Measuring the angle between sun/star/planet and the horizon is much more consistent than using gravity. A sextant allows for ship movement while allowing an accurate elevation angle.

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#283763 - 02/14/17 07:11 AM Re: Improvisation - sextant from signal mirror [Re: Russ]
rafowell Offline
Member

Registered: 11/29/09
Posts: 197
Loc: Southern California
Originally Posted By: Russ
Gravity doesn't work well enough in a small boat bouncing around at sea. I was a tad confused by the picture of the improvised sextant.
Measuring the angle between sun/star/planet and the horizon is much more consistent than using gravity. A sextant allows for ship movement while allowing an accurate elevation angle.

I don't think any measure of gravity (local vertical) was required.

The way that type of signal mirror sight works[1] is that you align the reflected light beam from the sun with the line between the hole in the mirror and the hole in the sight.

My guess is that he did that, with the horizon as the target, then recorded the mirror tilt angle, and doubled it to get the sun to horizon angle.

In more detail:
  • (a) Make sure the hole in the mirror is the same height above the biscuit tin as the hole in the sighting wand.
  • (b) Put your eye at the very center of hole in the mirror and align the biscuit tin so that the horizon is at the very center of the hole in the sight. (moving boat, so probably intermittent).
  • (c) Tilt the mirror so that, at the same time that (b) is achieved, the shadow cast in the center of the square mirror light beam by the non-reflective hole is centered on the hole in the sighting wand.
  • (d) Detail someone else to keep an eye on the protractor, and whenever (b) and (c) are simultaneously achieved, cry "mark" (whereupon this other person records the mirror angle as measured by the protractor).
  • (e) Double the protractor angle, and you have the sun-horizon angle at that instant.


[1] Video showing the use of a foresight (heliograph-style) signal mirror aimer:

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A signal mirror should be backup for a 24 hr, all-weather radio distress signal, such as a 406 MHz PLB (ACR PLB)

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#283770 - 02/15/17 12:30 AM Re: Improvisation - sextant from signal mirror [Re: rafowell]
Mark_R Offline
Old Hand

Registered: 05/29/10
Posts: 822
Loc: Southern California
Originally Posted By: rafowell
Originally Posted By: Russ
Gravity doesn't work well enough in a small boat bouncing around at sea. I was a tad confused by the picture of the improvised sextant.
Measuring the angle between sun/star/planet and the horizon is much more consistent than using gravity. A sextant allows for ship movement while allowing an accurate elevation angle.

I don't think any measure of gravity (local vertical) was required.

The way that type of signal mirror sight works[1] is that you align the reflected light beam from the sun with the line between the hole in the mirror and the hole in the sight.

My guess is that he did that, with the horizon as the target, then recorded the mirror tilt angle, and doubled it to get the sun to horizon angle.



I came to the same conclusion. Somebody else would have to read the protractor while he was aiming it. There would have to be some extra mathematics to compensate for the vertical travel of the center of the mirror's sighting hole, but not impossible with a working knowledge of trigonometry (and mathematics tables).

As far as gravity goes, you are right that gravity dependent instruments don't fare too well in rough seas. However, gravity dependent sextants were used in aircraft until the 1960's demonstrating that they could be "good enough".
_________________________
Hope for the best and prepare for the worst.

The object in life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane

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#283772 - 02/15/17 06:26 AM Re: Improvisation - sextant from signal mirror [Re: Mark_R]
rafowell Offline
Member

Registered: 11/29/09
Posts: 197
Loc: Southern California
Originally Posted By: Mark_R
Originally Posted By: rafowell
Originally Posted By: Russ
Gravity doesn't work well enough in a small boat bouncing around at sea. I was a tad confused by the picture of the improvised sextant.
Measuring the angle between sun/star/planet and the horizon is much more consistent than using gravity. A sextant allows for ship movement while allowing an accurate elevation angle.

I don't think any measure of gravity (local vertical) was required.

The way that type of signal mirror sight works[1] is that you align the reflected light beam from the sun with the line between the hole in the mirror and the hole in the sight.

My guess is that he did that, with the horizon as the target, then recorded the mirror tilt angle, and doubled it to get the sun to horizon angle.


... There would have to be some extra mathematics to compensate for the vertical travel of the center of the mirror's sighting hole, but not impossible with a working knowledge of trigonometry (and mathematics tables).

Good point - I had visions of trying to pivot the mirror about the lip of the tin, to keep the sighting hole near the pivot axis, but as you point out, the consequences of pivoting about an axis offset from the center hole are elementary trigonometry - annoying but fully calculable. While trig tables would help, they aren't essential - they can be reconstructed by, for example, Taylor series expansion.
_________________________
A signal mirror should be backup for a 24 hr, all-weather radio distress signal, such as a 406 MHz PLB (ACR PLB)

Top
#283775 - 02/15/17 07:34 PM Re: Improvisation - sextant from signal mirror [Re: rafowell]
Mark_R Offline
Old Hand

Registered: 05/29/10
Posts: 822
Loc: Southern California
Originally Posted By: rafowell
Originally Posted By: Mark_R
Originally Posted By: rafowell
Originally Posted By: Russ
Gravity doesn't work well enough in a small boat bouncing around at sea. I was a tad confused by the picture of the improvised sextant.
Measuring the angle between sun/star/planet and the horizon is much more consistent than using gravity. A sextant allows for ship movement while allowing an accurate elevation angle.

I don't think any measure of gravity (local vertical) was required.

The way that type of signal mirror sight works[1] is that you align the reflected light beam from the sun with the line between the hole in the mirror and the hole in the sight.

My guess is that he did that, with the horizon as the target, then recorded the mirror tilt angle, and doubled it to get the sun to horizon angle.


... There would have to be some extra mathematics to compensate for the vertical travel of the center of the mirror's sighting hole, but not impossible with a working knowledge of trigonometry (and mathematics tables).

Good point - I had visions of trying to pivot the mirror about the lip of the tin, to keep the sighting hole near the pivot axis, but as you point out, the consequences of pivoting about an axis offset from the center hole are elementary trigonometry - annoying but fully calculable. While trig tables would help, they aren't essential - they can be reconstructed by, for example, Taylor series expansion.


I would still prefer a waterproof trig table. Your average joe can do linear interpolation between the table values, without much trouble, if required. But, reduction/regression math is going to be a bit much. I didn't get those till calculus III and computational methods in my second and third years. Unless you had a technical professional (mathematician, physicist, engineer, etc) in the lifeboat, I wouldn't depend on being able to generate a Taylor series out of thin air.
_________________________
Hope for the best and prepare for the worst.

The object in life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane

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#283776 - 02/15/17 07:44 PM Re: Improvisation - sextant from signal mirror [Re: Mark_R]
Russ Online   content
Carpal Tunnel

Registered: 06/02/06
Posts: 4438
Loc: SOCAL
If you're going to that much effort, just get a cheap Davis Mk 3 sextant and get some elevation numbers you can use. Not the most accurate or precise sextant, but better than protractors and gum.

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