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Hi Tracy,
I've attached a photo of the date scale I did for an
analemmatic sundial at a swimming pool. The footprints are intended to hint that
you should stand on the centreline. The date scale is a circle. Find the date on
the circle, mentally follow the east-west line to the inner arc, then follow a
radial line to the centreline. Stand there.
The entire sundial, including this diagram, was drawn
without a ruler, and with no calculations, except for one tiny detail. You need
only an east-west line, a protractor and a length of string!
The month names were painted by children, hence the
delightful hand-made feel.
This doesn't give you the analemma, but it does
graphically show how the place you stand relates to the date. The date scale
divides the circle in 365.25 daily arcs (28 Feb has 1.25 of them) with the
solstices on the N-S line. The one tiny detail for which you need a calculation
is the very small offset from the centre of the date circle to the point from
which you measure those daily arcs. It needs to be offset to compensate for the
eccentricity of the earth's orbit. If done correctly, this places the autumnal
equinox on September 23rd, not 21st as it would be if the orbit were circular.
But this correction is so small you may ignore it in practice if you
wish.
I thought I'd invented this construction (it isn't in
Cousins or any of the other common sundial books) but have found that it has
been known for centuries!
Chris Lusby Taylor
51.4N 1.3W
----- Original Message -----
Sent: Sunday, October 29, 2006 9:52
PM
Subject: date scale
Hello Everyone
I really like the drawing you made John for your
customers - it helps a lot. Thank you.
Thanks Tony for the picts and Powerpoint presentation -
it is wonderful.
Thanks Chi-Lian and Linda for your
comments.
After reading the advice I have received, I think that I
will make my date scale the conventional way - although I think the analemma
looks way cooler! It looks as if the first day of summer and the first day of
winter are at each end of the scale. Is this true always? Also, does one stand
directly on the centerline, or on the month "block" which falls to the left
and right of the center line?
More on my previous obsession of using the figure 8 in
the date scale:
If I am not mistaken, an analemma, on the ground for
instance, can be made by marking the position of the sun (cast by some point)
at the same time for a year every few days or so. I am referring to the
website: http://www.cerrilloshills.org/analemma/path3.htm So,
if I were to mark the path at noon, local time, for a year, the figure 8
should fall on the N-S line? It appears as though the months outlined on the
figure 8 match the date scale in placement except at the top portion of the
figure 8 where the months seem to flip from the left side to the right side. I
can't help but think that the straight line date scale mimics the analemma in
month placement. That's why I figured that I would make the figure 8 instead
of the plane straight line. Also, I thought about positioning some type of
gnomon that would cast a bright spot (or shadow) on the date scale which could
follow the figure 8 path exactly (at noon) just like the noon analemma sundial
featured in the website above.
I am fascinated by the analemma and would love to
somehow use it in my sundial, however, I definitely want my sundial to be
correct and accurate - no false sundials here! But I keep seeing the analemma
used as a date scale in pictures on the internet. One I found looks like it is
bronze.
If I shouldn't use the analemma in the date scale, where
could I put it in the sundial properly? Or, should I just make a separate noon
analemma sundial like that in the website referred above?
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