Hello John,
This thread just gets more and more intriguing! It'll be fascinating
to see the end result. IMHO, you'd be well advised to make scale
models to 'proof out' your designs. For example, fuzziness of the
nodus shadow is likely to be a relatively minor factor affecting
accuracy--much more important is likely to be the inability of the
nodus to be at the correct location, since children (of all ages)
don't stand straight, or still, or even at the proper footprint.
One reason why most analemmatic dials have hour points only is due to
the unpredictable behavior of the human gnomon. If you like the
appearance, by all means place quarter-hour lines on your 'human
horizontal dial' for aesthetic reasons, but don't expect a sundial
with a human gnomon or nodus to give very accurate time.
Good luck,
Mac Oglesby
P.S. And think about exactly what do you consider to be the nodus.
The center of the human's head? Or what?
Hi Patrick:
Your idea of a wall to receive the morning and afternoon shadows is a great
solution to the problem. But I will have to give it careful consideration
if I use it or not because of the additional expense and effort. (Also,
vandals love to paint walls).
This dial will have the typical "bow tie" shape of a vertical nodus
horizontal dial. The decision must be made on where to terminate the drawing
of the dial face on its eastern and western edges.
One critical factor in determining the size of the face is the "fuzzy
factor" of shadows. In previous threads we have discussed that no point on
the dial face should be much farther than 100 times the width of a spherical
nodus; otherwise the shadow becomes too indistinct and fuzzy.
If a person's head nodus measures about 8 inches in diameter, then the
maximum distance from head to dial face cannot exceed( 8 x 100)/12=67 feet.
Does this look correct to you?
John
>Message text written by John Carmichael
>
>>This means that the shadow of the head nodus will reach infinity at
>sunrise and
>sunset. These dials can not tell time in the early morning and late
>afternoon because the shadow is too long. I'll have place a size limit on
>the dial face.
>
>Patrick, have I interpreted this correctly?<
>
>Yes, exactly so.
>
>Now I'm not sure of this next bit without doing the maths (sorry you call
>it 'math' - singular - in the US, don't you? Another of the tiny
>differences in our common language!!).
>
>If you were to build a circular wall around the dialplate then the shadow
>of the observer's head as it came off the dial plate would go up the wall.
>This was a common practice with meridian lines in European churches in the
>17th & 18th Centuries. In the case here the shadow would never rise up
>above the observer's height and you would thus be able to accommodate hours
>that otherwise would cause the shadow to stretch out to infinity.
>
>I believe I am right to say that in such a situation the shadow of any
>point on the gnomon of a horizontal dial would fall along the hour line or
>along such a line on a vertical wall as described. Each of these
>'extended' hour lines on the vertical wall would actually be at an angle to
>the horizontal - something close to the angle of latitude but varying for
>each hour.
>
>This needs some further thought if you were to be interested, but if you
>have a circular - or any other shape for that matter - horizontal dial to
>hand you might like to tape a strip of paper around its circumference to
>make a sort of vertical wall and sight along the gnomon to mark on the
>strip the 'extensions' of the hour lines. For any particular hour these
>will be the same for any 'angle of sunlight'. Give it a go and see what
>you think.
>
>I suspect that this would also work if the wall wasn't circular but I'd
>have to think about that.
>
>It's possible that such a wall round the dial would give visual interest
>too - though it might be too expensive to construct one to a height to
>accommodate all adults but you might be able to do it for children...
>Though to be fair I suppose you wouldn't need this 'wall' for hours other
>than those which bracket sunrise and sunset throughout the year...
>
>If one constructed such a dial it might be easier to mark out these line
>extensions by trial rather than by calculation - rather as one often does
>for a ceiling dial!!
>
>It was just a thought anyway. [You have to know when to stop thinking in
>dialling!! Just say if you don't want to be confused by these added
>thoughts by a Limey who won't even have to build this dial! :-)]
>
>Patrick
>
>
