For simplicity, this book has only horizontal dials and ones in the northern hemisphere. The southern hemisphere, and
dials other than vertical, use similar principles. See also: The Sundial Primer page here by Carl Sabanski.
A wide gnomon has two styles, look at the picture below, one on the left, one on the right.
In its simplest form, the hours
from 0600 to 1200 Local Apparent Time use the style on the left in the below picture.
from 1200 to 1800 Local Apparent Time use the style on the right in the below picture.
|What to do when the gnomon has significant width?
So there are four quadrants on the dial plate, and there are four quarters in the day. If you write a program for wide
gnomons, for example in DeltaCAD's script language, you use one dial center for early morning and afternoon Local
Apparent Time, and another dial center for morning and evening. So, when drawing an hour line, you need to check if
that hour line angle places it in "early morning", "morning", "afternoon", or "evening" quadrants. That is simple
because the hour line angle will be from 0 to 90, >90 to 180, >180 to 270, and >270 to 360.
A program to generate a dial can be designed to consider style width, or, the result can be printed out, and the four
quadrants placed where they need to be. That is the approach I have used, as very few of my dials use a gnomon of
What if there are analemmas (figure of 8 or double S) instead of hour lines. Around 6am and 6pm LAT, parts of the
analemma will use one dial center, and the rest will use the other dial center. That makes it fun if you are writing a
graphical program, such as the BASIC scripting of DeltaCAD.
if a segment of the analemma falls after 0600 LAT then use the relevant SouthWest dial center and West style
if a segment of the analemma falls before 0600 LAT then use the SouthEast dial center and East style
if a segment of the analemma falls before 1800 LAT then use the relevant SouthEast dial center and East style
if a segment of the analemma falls after 1800 LAT then use the SouthWest dial center and West style
The above spider dial, or double-S-analemma dial, does not show the analemma curves around 0600 and 1800 LAT
(local apparent time). Some other diallists on the sun dial listerv have already done so, with examples written in SciLAB
and in PERL. The problems with an analemma and a wide gnomon together exist around 0600 and 1800 LAT, and are
discussed in Supplemental Shadows, available for free.
The following does show double S analemmas using a wide gnomon. I am still working on the double S analemma
when the analemma crosses the true east west line, for now, the macro issues a warning if an analemma spans the
east west line, otherwise all is good.
|What about analemmas as opposed to straight hour lines?
choice 9 of the
DeltaCAD main H dial
choice 10 of the
DeltaCAD main H dial