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 backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)