X photos 2003-07-04

arcticpenguin

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The X-shaped reflections in the photos were created by reflection of sunlight off slightly non-planar surfaces. There is a whole grid of them, so I assume it's off a man-made structure.

I haven't come up with a specific solution yet that would take into account the low angle of the sun and the high latitude. I'm not even sure if the reflecting surface is vertical, like windows on a building, or horizontal like skylight domes. Actually domes wouldn't be a good shape for that reflection profile.


http://www.randi.org/jr/070403.html
 
Quote from an architect friend:
My guess would be reflections off insulating glass windows... the glass layers can get sucked in a bit due to pressure changes, and if they are fully sealed they never would equalize to the outside pressure... so the outer glass is not flat.
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Now, lessee...do I edit arcticpenguin's past post to remove the link to the future commentary, which is now the present commentary, and let him post a present post with a link to the present commentary, which in the future will be a past post referring to a past commentary?

:wink8:
 
Pyrrho said:
Now, lessee...do I edit arcticpenguin's past post to remove the link to the future commentary, which is now the present commentary, and let him post a present post with a link to the present commentary, which in the future will be a past post referring to a past commentary?

:wink8:
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Since the URL includes the date instead of a 'past' or 'future' status, no action is necessary.

Reflections from skyscraper windows are usually not noticed when the sun angle is high because of the severe foreshortening.
 
So how do you access next weeks commentary?
(I promise not to transgress)
 
I'm feeling optimistic so I'll go ahead and say God put this mysterious grid of "x" shaped lights on that building to help all the heathens rethink their secular and heathenistic faith. [/sarcasm]
 
I have a friend and colleague who has many of the same beliefs and opinions as me. I brought Randi's Commentary to his attention awhile back, and we occasionally discuss things mentioned there.

On the topic at hand, this is what he sent me yesterday:

I was walking to my bus yesterday afternoon and went South on Lyon from Sparks to Queen Street. The sun was pretty much dead on from the west, so it was shining on the windows of the tall building on the east side and reflecting them onto the shady side of the building on the west side. Most of the reflections were slightly distorted but recognizable as windows. But one of the reflections looked just like the X's on the picture.

So I think it's pretty obvious that we're dealing with reflections from windows in a building opposite the "screen" in question (the buildings in the picture, or in my case the building on the west side of Lyon). The only thing I haven't worked out in detail in my mind was the "X" shape: I think it's because the window in question was slightly concave but rectangular, and the "screen" was close to its focus point but because of the rectangular shape different parts were focusing differently. But I can't think 3-dimensionally well enough to imagine where all those beams of light are intersecting.
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To which, I replied:

I agree with you as far as the basic mechanism is concerned. In the Forum, the poster suggested that the windows were slightly concave as well. He suggested it was due to negative internal air pressure (relative to the outside pressure). Sounds possible, especially with modern, fully-sealed buildings.

As far as why the shape 'X', how about this: You mentioned that the 'screen' surface is at the focal point of the concave 'mirror' windows. If the windows formed a perfect paraboloid, the image would be a small dot (the Sun) on the 'screen', but the 'mirror' windows are rectangular, and the window frames would affect the shape of the mirror (as you noted). The corners, being further from the centre, would affect the curvature less (be 'looser', if I have things right) than the centres of the edges (being closer, and thus more 'taut'). So along the diagonals, the light would be better-focussed on the 'screen' than between the diagonals. If I have this right, in actual fact, the edge areas come to focus further out, so if the buildings were further apart, the pattern would look like a Maltese cross whose petals don't actually join in the centre, instead of an 'X'. Sound plausible?

Upon reflection last evening, I have a couple of notes to make on what I said:
  1. I think I was mistaken about the shape of the 'mirror' window - it wouldn't be a paraboloid, even if the window was circular. Wouldn't it be closer to a catenary in cross-section?
  2. Regarding 'loose' and 'taut': I was thinking of the flexibility of the glass almost like a guitar string, whose degree of flex is altered by pressing it against a fret. The further the span (e.g. corner-to-corner), the more the glass can flex, and thus the shorter the focal length will be. I referred to this as being 'looser'.
    [/list=1]

    Any comments?

    Anyway, it'll be interesting to see what Randi comes up with tomorrow.

    bPer
 
I agree with the sucked-in-glass explanation. Double-pane windows are sealed - wouldn't they pull a partial vacuum between the panes, to lessen thermal conductance?

The seals have to be very good (and thus never equalize to the outside pressure), because otherwise moisture would get into the window and condense. Windows in houses do this when the seal is compromised.
 
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