This wall sun clock in my header was taken while I traveled in Germany. I saw a number of these, some simple in design and structure and others much more elaborate with a lovely painting as a background. If the angles are even slightly off you will never get the correct time. The clock has to be aligned parallel to the axis of the earth's rotation. I am not sure how this works with the changing of the seasons and dropping angle of the sun. That is way too much math for me! i.e. "Since the gnomon's style must be parallel to the Earth's axis, it always "points" true North and its angle with the horizontal will equal the sundial's geographical latitude; on a direct south dial, its angle with the vertical face of the dial will equal the co-latitude, or 90° minus the latitude."
Just ONE image in this formula from wikipedia.
I, of course, was more fascinated that it seemed to be an incorrect time when compared with my watch and I was enjoying the beauty of the swatches and swishes.