Dear Thomas,
I like your design. You ask for ideas
and comments. Here, in order, are the
steps that I use myself for a big wall
sundial:
1. Make a careful survey of the wall.
The wall will NOT be flat. No wall
is ever flat!!! It won't be vertical
either. No wall is ever vertical!!!
[Warning: if the wall leans forward
slightly and if it is slightly convex
you will discover that the January
point on the 1 o'clock analemma is
off the left-hand edge of the wall,
well to the left of its theoretical
position if you assumed a flat
vertical wall. Real life is tough!]
2. Decide on the nodus height. If the
wall is convex you may choose to reduce
the height a little to ensure that your
big analemma is entirely on the wall.
3. Construct the nodus support and the
nodus and attach them to the wall.
You will need to worry about stiffness
and resistance to wind loading. You may
need to check with building inspectors
that it is safe and won't fall down.
4. AFTER the nodus is in position you
should mark out a 1m grid. Remember,
the wall is NOT flat (even if you think
it is) so what IS this grid? I use a
"best-fit vertical plane" and the origin
(0,0) of my grid is the sub-nodus point
marked on this plane. This may be a
little inside the wall or a little
outside it. That doesn't matter.
5. Take a look at:
http://www.cl.cam.ac.uk/users/fhk1/PSQ.jpg
to see an early stage of marking out the
1m grid. The white spots are spaced at
1m intervals and the top row of spots
form the horizon line.
This line follows the curve of the wall.
One of the spots is the closest I could
get to the sub-nodus point. The sub-nodus
point was, in fact, about 3mm inside the
wall so this white spot was 3mm closer to
the nodus than the actual sub-nodus point.
Using an up-market surveying instrument,
little crosses were marked on the white
spots. I use an (X,Y,Z) co-ordinate
system where (0,0,0) is the centre of
the nodus. The Z-axis is horizontal
and normal to the best-fit vertical
plane. Positive-Z is outwards from
the wall so X is to the right and Y
is upwards as expected.
Each white cross had (X,Y) co-coordinates
which were (m,n) where m and n are integer
numbers of metres.
Note that the Z co-ordinate varies a
little from point to point. I repeat:
NO WALL IS FLAT!!!
6. You now have a big grid. You then have
to mark some trial points using this
grid. I would start with that January
point on the 1 o'clock analemma. I am
VERY nervous about that point!!
The Z-value at that point is the local
nodus height. This will probably not
be the same as what you assumed when you
used your computer to draw the plot!
If the wall is convex the Z-value will
be larger than you expected and you will
have to shift the point.
7. A very useful check is to plot the
actual position of the shadow of the
nodus during the course of the day.
In your case for about three hours
at this time of year.
I make a mark about every 10 minutes
and survey the marks. I then compare
the survey positions with the positions
I had computed.
It is best to do this in summer (I
appreciate this is not possible yet!)
because this is when the shadow gets
furthest from the sub-nodus point and
the variations in Z-values have most
effect.
7. Once you have gained confidence in the
ins and outs of your wall you can start
marking out your dial furniture.
[Another warning: all this can get very
frustrating because you keep discovering
new dips and bumps in the wall. That
said, it is very rewarding if you get
it right!!!]
Have fun with the sun. I wouldn't like to set
out a big wall dial at this time of year myself.
I prefer warmer weather!
Best wishes
Frank H. King
Cambridge, U.K.
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