NavList:

Frank Reed you wrote:
 “It's apparently the shadow of a Henkel He 111 bomber. What do you think: is the identification correct? Is that the correct airplane?”

Possibly.  The He111 had a swept back leading edge, which doesn’t appear in the photograph, but it did have little curvy bites taken out of the trailing edge (TE) close to the fuselage, which might be imagined to appear in the photograph.   It also had an elliptical tailplane.  Once again, if you look at it long enough the parallel tailplane starts to become an ellipse.  It certainly doesn’t look a Dornier17, and probably not a Ju88.  If anything, it looks like something produced by Bristol.  If 1940 is correct, probably a Blenheim.  The only thing that worries me about something by Bristol is the curvy TE.

You also wrote: “Does that "look-down angle" impact the altitude estimate?”

In the simplest analysis, and certainly if you were using a sextant angle to calculate the height, no.  It took me a while to figure this out while studying Maitland’s Log of the R34 east to west crossing of the Atlantic in 1919.  On Earth, the Sun’s rays can be considered parallel.  Therefore, If the aircraft is flying level with the ground, we are looking at a parallelogram.  One set of parallel lines is the Sun’s rays crossing nose and tail (or port and starboard wing tip) and the other set is the aircraft length (or wingspan) and the shadow on the ground.  As opposite sides of a parallelogram are equal, the shape of the aircraft’s shadow on the ground is always the same size of the aircraft (neglecting the smudgy bit round the edge, which we’ll come to next)

You also wrote “how high was the aircraft when the photo was taken?”

Unfortunately, there’s no sextant angle of the length of the shadow given, so we must look at the smudgy area, the penumbra.  I’ll leave it to the pixel counters to come up with a more accurate value, but if we use a penumbra angle of 32’ and estimate the smudged area as about 2ft.  Aircraft height = 2/tan 32’ ft = 215ft.  Does the look down angle affect the width of the smudge?  Possibly, because we’re not looking at a parallelogram anymore.  Oh dear! It’s bedtime, and I’ve run out of old envelopes to draw on the back of in any case.  DaveP