"Eventually" is definable. The umbra cone is related to the cotangent of the diameter of the sun, typically 32'. This number is 107. At twice the distance of 107 times the diameter of the round gnomon or 214 times, the dark umbra shadow is reduced to zero. Then the shadow is penumbral and indistinct. One meter from a 1/2 cm gnomon still gives you a dark umbra shadow.
Roger Bailey Walking Shadow Designs ----- Original Message ----- From: "Mac Oglesby" <[EMAIL PROTECTED]> To: "Sundial Mailing List" <[email protected]> Sent: Sunday, February 10, 2008 6:36 PM Subject: Re: Monumental Sundial > > Carl Sabanski wrote: > > (snip) >> The >>longer the shadows become relative to the length of the stick the less >>distinct the shadow is. I viewed the shadow from a thin wire rod on the >>snow and very quickly there is no distinct shadow at all that could be >>used. > (snip) > > Thanks for your comments and photos, Carl. > > For what it's worth, my experiments with shadows indicate that the > smoothness of the surface receiving the shadow has a huge impact on > the quality of the shadow. With a smooth white matte surface, the > shadow of a fairly distant taut wire may be faint and indistinct, but > often it's still possible to make a good guess as to where the center > of the shadow lies. Of course, there's a limit to how far one can > read a shadow -- eventually it does disappear altogether -- but it's > surprising how useful the shadow of a rod or taut wire can be, given > a proper receiving surface. > > Best wishes, > > Mac Oglesby > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
