Thanks for the update! Mystery (probably) solved.
Wow, I’m surprised that hasn’t caused a window-trail-shaped lightening of the paint following the sun’s path. You’d expect there to be more UV damage along the trail.
Thanks for that enlightening post. Really interesting picture of the effect on that wall.
I don’t think that would happen because a/ the Sun’s elevation changes slightly every day and b/ this is Britain. Sun is a relatively rare phenomenon.
There’s an art installation consisting of a highly reflecting curved wall in which the sun’s concentrated light burns a path through the grass underneath. It wasn’t intentional, but it worked out that way.
(I wrote a column on such light-concentrating art installations around the world. None of the designers appear to have taken into account that effects of light concentration on their surroundings. Nor architects. We can only be glad that the Chicago sculpture “Cloud Gate” – AKA “The Bean” – is all convex curves rather than concave ones.)
There is a device called a Campbell-Stokes recorder that consist of a perfectly spherical solid glass ball, which focuses the Sun’s light onto a card at the back, showing how much sunlight there is each day. These recording cards have to be changed regularly, because the elevation of the Sun changes constantly over the year.