The lines on the walls are extensions of the hour lines on the ground. When the shadow of the style falls along, say, IX on the ground, it will also fall along IX on the wall, if the sun is low enough in the sky.
This Google Maps link gives an idea of what a low sun would 'see', and therefore how the shadow would fall. https://www.google.nl/maps/@49.2509693,4.0351274,3a,75y,356.8h,86.51t/data=! 3m6!1e1!3m4!1st2YJ7kq0VC3Ddwrnf01oCQ!2e0!7i13312!8i6656 Here is a thumbnail of what the links should show: Rudolf, Heemskerk 52-30 N 4-40 E Namens Willy Leenders Onderwerp: a strange sundial 2. The hour lines on the vertical surfaces are divergent and do not come together at the point where the style intersects the plane.
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