Another gnomon teaser?

Following Frank’s teaser about circular-section gnomons there are of course many other gnomons of varying cross sections to be seen. Those of rectangular section on vertical dials have always interested me since it can often be difficult to know where in the shadow the time should be measured

Re: Hail to the Chief

Thanks Frank. Political comment is inappropriate on the Sundial List, but I just couldn't help myself. My apologies to all. Still, at least he passed the cognition test. The other day I did a memory test to see if I still have all my faculties at the age of 73. I missed two questions. One

Re: Hail to the Chief

Dear Geoff and John, > Surely Geoff Thurston is right? Yes. He is! You don't even need to think in three dimensions. The edges of the shadow must run to the base of the gnomon and must touch tangentially. The base is indeed an ellipse. The contact points are always opposite one another (by

Re: Hail to the Chief

Dear Frank, Surely Geoff Thurston is right? The width of the 6am shadow would be one inch. (Assuming 6am and noon are local solar time, and ignoring penumbral effects due to the sun not being a point source.) If the hours are marked out as on Greg McDonagh's Pebble Dial (your article in

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Diese Nachricht wurde eingewickelt um DMARC-kompatibel zu sein. Die eigentliche Nachricht steht dadurch in einem Anhang. This message was wrapped to be DMARC compliant. The actual message text is therefore in an attachment.--- Begin Message --- OOPS Too many 0's The correct answer is 0.506"

[no subject]

Diese Nachricht wurde eingewickelt um DMARC-kompatibel zu sein. Die eigentliche Nachricht steht dadurch in einem Anhang. This message was wrapped to be DMARC compliant. The actual message text is therefore in an attachment.--- Begin Message --- Dear Frank FAKE NEWS If I'm right(and I must be or

Re: Hail to the Chief

Dear John, A moot point... > ... why should any sane normal > person want to help Trump... Ah. I was thinking in terms of "displacement therapy". Thinking about sundials must be better for one's mental health than drinking copious quantities of Diet Coke? Just trying to be helpful :-) Frank

[no subject]

Diese Nachricht wurde eingewickelt um DMARC-kompatibel zu sein. Die eigentliche Nachricht steht dadurch in einem Anhang. This message was wrapped to be DMARC compliant. The actual message text is therefore in an attachment.--- Begin Message --- I partially agree that: "A sundial is an instrument

Re: Hail to the Chief

Very quickly. The reason is the diameter of the Sun and the different distances between a point of the rod and its shadow at noon and at sunset. At summer solstice, for example, if 10 is the distance of a point P from the center of the sundial, the distances between P and its shadows are (if I'm

Re: Hail to the Chief

Dear Frank, Can I help the POTUS? No. Can I answer your question? Maybe but I am not used to working in imperial units. Consider a horizontal cross section of the gnomon at any height above the dial plate. The shape of the section would be an ellipse with minor axis equal to 0.5" aligned