Hi John,
At last signs of the truth...
Theoretically it's correct that the projection of a circular
disc on to a flat surface parallel to the disc will be a circle.
Unfortunately the sun's apparent size results in the disc becoming
very blurred when you get a couple of hours off of local
The sundial in the court of the Hotel de Ville in La Rochelle (France) has
a node that looks like a circle, actually it is star shaped. But in the
middle there is a hole. That gives a nice and precise sunspot on the dial
face to read the time.
See the resemblance with the 'shadow sharpener'?
But Frank,
This little distortion effect must be quite small. For practical purposes,
you can call the shadow a circle can't you, even though it's a tiny bit
elliptical. Can't you? If the disk is large, this effect becomes almost
insignificant doesn't it?
I'm going to do the simple experiment
I liked your solution to the nodus problem as applied to the Swenson dial.
In reading about the dial on its web-site, I saw that great care was taken in
accurate placement of the dial's lines.
Could you please tell me how the declination of the wall was determined? I
think that if this
Could you also elaborate on why a noncartesian system was used to mark the
placement of the anchor points?
-Bill Gottesman
This radial vs Cartesian discussion has come up before for large analemmatic
dials. Cartesian coordinates are not easy to lay out in the field. The X and
Y distances are
List members might be interested to see the lastest find in the Sundial
cache:
http://www.geocaching.com/seek/log.aspx?LUID=ea705238-eb4c-4987-a068-91de3df
4f8d8
This is the sundial that was made by Roger Bailey and dedicated at last
year's NASS Conference. Unfortunately, none of the pictures
John, and other list readers,-
I first saw discs on almost _all_ the sundials in the sundial garden on
the Deutsches Museum in München (Munich). When I got it, I thought it a
great idea.
That was in 1998, but they are still there.
Here is a picture on the museum's website. Though hard to see,
