Well it's 1) How bright the dial face has to be for it to show a shadow when the sun is as low as it can be at the sundial's mounting-location.
2) How un-bright does the dial need to be at noon on the summer solstice, so that it won't be too bright to look at. *As for #1* , you can find that out whenever the sun is as low as the lowest it can get at the sundial's mounting-location. And, if it isn't visible that low at this time of year, due to obstacles like a tree or a house, then of course you can go to a different location without that obstruction. So you can easily find the answer to #1 now. So, at a location where the sun is visible as low in the sky as it can ever get at the mounting-location, try brown, and, if the shadow isn't visible, try light-brown, and then (dark, ordinary, and light) tan, and then yellow. I rather doubt that it will require something as bright as yellow, unless the dial will receive sunlight all the way to sunset. Most likely some brown or tan will be light enough, depending on how low the sun can be and still shine on the dial (which depends on the trees, buildings and other obstructions in your yard. *As for #2: * One thing that you already know is that matte-black shows a shadow when the sun is at its brightest, because it showed a shadow even during your recent winter experiment. So matte-black will show a shadow at the solstice-noon too. Whatever shade of brown/tan/yellow is barely light enough to show a shadow when the sun is at its lowest for your dial-location, that's a first thing to try at summer-solstice noon. But, if that's too bright at summer solstice noon, then you want to find out how bright it's permissible for the dial to be, at summer solstice noon, without being too bright. For that, to be really sure, I agree that you have to wait for the summer solstice. But you can get a good estimate now, if you have a formula for sunlight intensity as a function of solar altitude: Find out, from the formula, and from he noon solar altitudes now and at the solstice, how many time more intense the sunlight is at summer-solstice noon, as compared with noon on some day this week. Find the sine of the sun's altitude at noon on some day this week. That's also the cosine of the sun's distance from the dial-normal (perpendicular line) at noon. Multiply that cosine by the factor by which the sun will be brighter at solstice-noon, as compared to that noon this week. Find the angle whose cosine equals the result of that multiplication. Find the difference between that angle and the sun's angle from the zenith on this week's observing-day. Tip your color-sample southward up from the horizontal, at noon, on this week's observing-day, to get the sunshine-intensity that a horizontal surface would receive on at summer-solstice noon. So, with the color-sample tipped up southward at that angle, find the shade that won't be too bright at solar noon on this week's observing day. Hopefully there's a shade that is bright enough when the sun is at its visible lowest at the dial-location, and still isn't too bright at solstice noon (on a horizontal dial). I'd bet that some brown or tan would be likely to meet both requirements. Michael Ossipoff On Tue, Feb 28, 2017 at 5:09 PM, Steve Lelievre < steve.lelievre.can...@gmail.com> wrote: > John, thanks for the clarification, and your patience with my questions. > > All, I'm off to buy some photographic mattes to do experiements with. This > is all about having a horizontal dial face that is not too bright to view > even in the summer midday sun - so I'll go quiet now and report back after > the summer solstice. > > Steve > > > > On 2017-02-28 1:40 AM, John Lynes wrote: > > Hi Steve, > I'm sorry I've confused you. > > ... > > The take-home conclusion is that there is no single ideal reflectance for > the plate of a sundial. It varies with the sky illuminance. When Weber's > Law prevails, a reflectance of about 60 per cent is likely to be a safe bet. > > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > >
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