The sundial in the court of the Hotel de Ville in La Rochelle (France) has a node that looks like a circle, actually it is star shaped. But in the middle there is a hole. That gives a nice and precise sunspot on the dial face to read the time.
See the resemblance with the 'shadow sharpener'? Thibaud
At 10:00 13-07-2004, Frank King wrote:
The theory suggesting that a circular disc casts a circular shadow depends on the sun being a point source of light which is isn't! If the model on which the theory is based is refined to take the angular diameter of the sun into account, you find that the shadow is generally degraded into an approximate ellipse whose major axis is less than the diameter of the disc and whose minor axis is smaller still. The actual shape at a given time can be determined by noting the shape that the image of the sun that would form if the sun were projected through a pin-hole at the centre of the nodus (this image really is a true ellipse).
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