Since the sun is disk shaped, I think that the brightness of the penumbra (for a straight edge) is an S-shaped cosine function going from 0% to 100% brightness. The 50% brightness point would therefore be in the middle. However, the 50% point may, or may not, be perceived by the eye as the middle.
I have been enjoying the discussion on shadow sharpeners (which I was ignorant of) and want to thank those who have contributed to it. Gordon At 07:25 PM 5/3/99 , [EMAIL PROTECTED] wrote: >I haven't had time to make a shadow sharpener to see if the point is in the >perceived center or not, but in the meantime, I was wondering if anyone >could show, mathematically/graphically, the increasing intensity of light >as one moves from the darker edge of the penumbra shadow to the lighter >edge, and to show at what point on this curve of increasing light the sun >would be bisected by the gnomon? I suspect it is closer to the darker edge >rather than the center. Gordon Uber [EMAIL PROTECTED] Reynen & Uber Web Design http://www.ubr.com/rey&ubr/ Webmaster: Clocks and Time http://www.ubr.com/clocks/
