Hi Dan-George and all, I enjoyed the video. Diffraction is of interest but rarely relevant to sundialists . We are more concerned with the fact that the sun is not a point source but a bright disc half a degree across. We are concerned about the dark full umbra when the sun is totally blocked and the penumbra when only part of the sun is blocked. This limits the accuracy of sundials, too many shades of grey. Various shadow sharpeners help define the shadow by reducing the solar disc to a point but merging penumbral shadows remain a problem. I described this in a SML email about 17 years ago in a note on penumbral head swelling. See https://www.mail-archive.com/[email protected]/msg03186.html
The phenomenon is easily explores with a finger shadow experiment. Hold your hands so your opposed pointing fingers cast shadows. Observe how the gap between the shadows disappears long before your fingers touch as the shadows swell to fill the gap. The two penumbral shadows merge to create full shadow long before the fingers touch. This is why sundials depending on a gap can give varying results, A gap between two sharply defined point does not give the expected shadow resolution as the penumbral shadows merge. Do the experiment see the penumbral swelling effect. Regards, Roger Bailey From: Dan-George Uza Sent: Friday, September 09, 2016 11:05 AM To: [email protected] Subject: The Brightest Part of a Shadow is in the Middle Hello, what a fascinating video this is! https://www.youtube.com/watch?v=y9c8oZ49pFc Dan Uza
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