Re: Photograph of the Princess of Wales (Frank King) Frank is being his usual pedantic self, which is always welcome, but the police statement can more charitably be taken to say that shadows fall in a continuation of the straight line from the light source to the illuminated object. The edge of a shadow is always in line with an edge of an object and the light source. If a featureless item such as the edge of a gnomon casts a shadow you can't tell which point along the edge cast each point of shadow edge. Distortion created by camera lenses may render some straight lines as curves, but it seems to me fair to use ray tracing to spot photographs that have been tampered with.
Since the sun is, more or less, a point source, I would certainly expect all rays traced back from shadows, past the object, to converge on the light source. As for the double horizontal, yes the edges of the two shadows point in different directions but so do the shadows on the dial of the two hands of a clock. It doesn't seem to me a meaningful point. By the way, why was the replacement gnomon on the double horizontal in the current BSS Bulletin made so long? Ray tracing indicates its top section could never cast a shadow on the dial. This reader has so far failed to see how a shadow can be in the same direction as the light source, if by that Frank means that it is between the object and the light. Perhaps Frank will enlighten us at next month's annual British Sundial Society Conference. Chris Lusby Taylor On Mon, Mar 11, 2024 at 11:00 AM <sundial-requ...@uni-koeln.de> wrote: > Send sundial mailing list submissions to > sundial@uni-koeln.de > > To subscribe or unsubscribe via the World Wide Web, visit > https://lists.uni-koeln.de/mailman/listinfo/sundial > or, via email, send a message with subject or body 'help' to > sundial-requ...@uni-koeln.de > > You can reach the person managing the list at > sundial-ow...@uni-koeln.de > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of sundial digest..." > > > Today's Topics: > > 1. Photograph of the Princess of Wales (Frank King) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Mon, 11 Mar 2024 09:01:46 +0000 > From: Frank King <f...@cl.cam.ac.uk> > To: Sundial List <sundial@uni-koeln.de> > Subject: Photograph of the Princess of Wales > Message-ID: <e1rjbxx-009ehy...@mta2.cl.cam.ac.uk> > Content-Type: text/plain; charset=us-ascii > > Dear All, > > There is much talk about the recent photograph > of the Princess of Wales and her children. > > In one newspaper I read that "a former digital > forensics officer for Dorset Police said... > > 'In a true image, the shadows will all be > in the same direction as the light'" > > Huh! > > He has clearly never looked at a sundial! > > He may mean 'in the opposite direction' but > that isn't true either. Moreover, shadows > do not necessarily align. > > A Double Horizontal sundial works precisely > because its two shaodws are (generally)in > different directions. > > Of course, a shadow CAN be in the same direcion > as the light. I'll leave that as an exercise > for the reader :-) > > Frank King > Cambridge, UK > > > > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > sundial mailing list > sundial@uni-koeln.de > https://lists.uni-koeln.de/mailman/listinfo/sundial > > > ------------------------------ > > End of sundial Digest, Vol 215, Issue 2 > *************************************** > -- Chris Lusby Taylor
--------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
