I've seen instructions on using a sextant mirror as a signal mirror, but this is the first account I've seen of a signal mirror used as a sextant!

After their ship was torpedoed in 1944, 3rd officer F.L. Newal cobbled up a sextant from a protractor, a biscuit tin, chewing gum, and a metal signal mirror, and headed for Diego Suarez, Madagascar, more than 1200 miles away.

After 42 days, a ship picked them up just 60 miles short of their goal.

Story and side view of homemade sextant

Story and top view of homemade sextant

Wow! Knowledge and skills are what count, more so than gadgets and gear....

Interesting indeed. The two photo's at the links show the protractor in one but not the other. I wonder how the protractor is measuring elevation since it's stuck to the side at the top with gum. Something simple that points to "down" seems to missing in the images. That something would cross the protractor and yield elevation.

How could this be improved with a modern glass/plexiglass signal mirror with a fireball. I may need to breakout my mirror and a protractor, and play around.

You may notice the piece of wire that suspiciously looks like an indicator next to protractor.
The biscuit box has a hinge on the open side and the protractor has a hole at the intersection point of the radials for the angle. The mirror is fixed to some sort of wire so it pivot around the hinge. You aim at the horizon through the hole in the signal mirror and the front sight (stuck to the bottom of the box). Now you pivot the mirror to put the reflection of the sun on the front sight. The indicator shows the angle of the mirror.

Thanks, good explanation, that makes much more sense.

That's pretty epic. Great share.

Impressive!

Chewing gum ... why was chewing gum in the lifeboat?

I expected biscuit tins in a British WW2 lifeboat (food) and a signal mirror (signaling). Less so the protractor, but the there were charts in the lifeboat - maybe the protractor was with the charts for computing navigation compass bearings.

Chewing gum isn't on any modern "ten essentials" lists I remember.

As a check, I electronically searched the ~500 page, 4 volume 1985 USAF AFR 64-4 Survival Manual for "gum", and the only reference to chewing gum was that scented chewing gum could reveal your presence to hostiles.

However, my "go-to" quick reference for military survival kits[1], lists chewing gum in many WW2 survival kits: USAAF E3 kit: 1 stick, USAAF E3A kit: 4 sticks, Navy E3A: 4 sticks, E17: 4 sticks.

The Emergency Parachute Ration tin had gum, and 2 such tins were in the C-1 Emergency Sustenance Vest. Pulling out my manual for that vest[2], it says (p. 10):

"Four Pieces Chewing Gum – Chewing a piece of gum occasionally will help to relieve thirst. The gum also can be used to patch small holes in rubber rafts, in plastic containers, and for other patching purposes."

... and also to make a sextant!!!


[1] Breuninger, Michael S., "United States Combat Aircrew Survival Equipment: World War II to the Present, A Reference Guide for Collectors", Schiffer Military History
[2] Instructions for the use of Emergency Sustenance Vest, Type C-1, Prepared by the Artic-Desert_Tropic Branch HQ, AF Tactical Center.

There's always the chance that someone brought chewing gum aboard...

Could one use duct tape today?

Yes, the best survival gear is the gear between your ears.

However, you can still lose that gear when under stress

That's why the Boy Scouts are taught to STOP[1] when things go wrong, so they don't lose that stowed gear.

• Stop : and calm yourself down, so you don't lose that precious gear
• Think : rummage around in that survival gear to see what may be useful
• Observe
• Plan

While I'm on this metaphor, mnemonics like STOP and catchphrases like "the gear between your ears" are handy containers for the gear between your ears.

I wonder - do we already have a thread on survival mnemonics and catch phrases? Should we start one?

Here are a few: Scout mnemonics that could save a life

On the thread topic of improvised sun navigation devices, there's Finding North with a Shadow Stick and Using an analog watch as a compass.

You are usually far better off with a compass and the knowledge to use it, though.

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.

Duct would have worked too but the gum probably tasted better!

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.

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:

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".

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.

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.