Recently the Tesseract Catalogue 109 was announced on this list. Item nr. 13 is a polarizing sundial by Charles Wheatstone. A virtually identical dial is in the collection of the Greenwich museums, see: https://collections.rmg.co.uk/collections/objects/265579.html. I have never understood how this sundial works. The hour scale shows 2 x 12 hour numbers in a semicircle. So whatever pattern is observed in the black glass reflector, it is obviously supposed to rotate over 180° in 24 hours, which is half the angular velocity of the sun itself. How does this frequency division-by-two come out? Can anybody explain?
Thanks! Frans Maes
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