My Kayaking mate and I have taken our cameras down (see debris above), scanned the images and sent them in. The right hand image is my attempt to process the raw image. The bright white lines are the sun passing through the sky over two months, the gaps are caused by clouds. You can also see the neighbours roofs, trees on the horizon and telegraph poles (plus dots that are probably due to rain getting in).
As I noted, I tried to set the camera to get the best horizon shot, but I ended up aiming the camrea too low, and the bottom half of the image (cropped away) was jts darkness.
Still what I got was pretty good, and you can clearly see e movement of the sun over the weeks.