Thanks, everyone. I see now how an analemmic gnomon can be used to read the EOT, if the equatorial band shows both mean and apparent time and you do the subtraction in your head. I'm most interested, though, in something which allows a direct reading of EOT...
Mike, your solution sounds very interesting, but I confess I don't really understand it :-) I take it the motor is to keep the hole pointed toward the sun. Presumably this could be done manually, as on a shepard's dial? I'm unclear though on what's drawn inside. Is it a single curve of EOT as a function of declination, or a family of curves? Ken On Sun, Feb 3, 2013 at 12:02 PM, Ken Baldwin <kenneth.bald...@gmail.com>wrote: > Hello, > > I'm a new list member, and have a beginner question: > > Are there examples of sundials whose sole (or primary) purpose is to > compute the Equation of Time for the current date? > > - I know that this information is often provided as a graph in the > furniture, but why should I have to know the date and perform the look-up > manually? Can't I use the position of the sun to do the computation for me? > > - I know that the EOT correction can be incorporated into the layout of > (some) hour lines, but I'm more interested in having dials which show true > solar time. I'd like a separate device dedicated to computing the EOT. > > - I know that I can construct an analemmic noon mark to show the EOT for > that day, since it's simply the east-west component of the analemma, but > I'd like a design that can be read at any daylight hour. > > It seems to me that it should be possible to build such a dial, since the > EOT is a function of date, and date lines can be read from many sundials. > In principle, I can just re-label the date lines with corresponding EOT > values and interpolate. > > I hope that makes sense. But since I haven't seen anything like that in > introductory sundial books, I must be missing something... Is it that the > shadow length can't be read accurately enough to get a reasonably precise > EOT estimate? Or is it just too hard to make a readable layout, given that > solar altitude is ambiguous between two dates, and that the component of > the EOT due to the eccentricity of the earth's orbit is out of phase with > the equinoxes and solstices? > > Thanks in advance, > Ken Baldwin > Corvallis, OR USA > >
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