From: David Pike
Date: 2018 May 10, 04:02 -0700
Your slides 2 and 3 had a central ray originating at the location of the observer, defining the height of a right triangle with two dashed-line sides; that is not there. Assume the observer is in the center of the ship. His position defines the angle between the ends of the shadow. The right triangles are defined by his two observation lines of sight.
I made the slides about ten years ago for more effect than accuracy. Looking at slide 2, I realise I drew it for an into-Sun course so that I could show a side view of the R34 rather than a cross section. In fact, downloading EM Maitland’s ‘The log of the R34’ from Kindle, which is more a diary than a navigational log, he says the shadow was on the starboard side almost immediately underneath. The actual position of the shadow can be worked out from the course, time, date, and approximate position. First thoughts are that it couldn’t have been more than just over 60 degrees down, but I’ll work this out when I next get chance. Then, assuming the shadow was almost beam-on (they were heading west and it was around noon), it shouldn’t be too hard to calculate airship height using geometrical drawing from schooldays many decades ago. The fact that the observation cabin was nearer the front than the rear of the airship will complicate the drawing slightly. DaveP