The seven lines would represent not only the equinoxes and the solstices but also the changes between the signs of the zodiac, around the twentieth day of the month.

My sundials are designed using my PC and the Postscript computer language, which is ideal for both mathematical and graphical problems. Opposite is a sample of my Postscript program for my ED-16 EarthDial.

Below is the final graphical result, on paper, of the Postscript program.

% EARTHDIAL2004.PS
% Version 1
% HORIZONTAL SUNDIAL WITH CIRCULAR DIAL PLATE
% DISPLAYS LOCAL TIME
% 7 DECLINATION LINES for ZODIAC SIGNS
% NUMERALS ARABIC

% CIRCTEXT
% 22.3.92
% Circular Text from the Blue Book Program 10 pp 167-169

/outsidecircletext
{circtextdict begin
/radius exch def
/centreangle exch def
/ptsize exch def
/str exch def
/xradius radius ptsize 4 div add def

In the course of developing the program, I used the EarthDial Calculator from the EarthDial Project, using the latitude of my town, Newport, in Shropshire, England, for the basic data .The date lines for the summer solstices and the equinoxes, (where the date line runs in a straight line from West to East) are also available from the EarthDial Project. To find the other zodiac date curves, I used a computer program which I recommend, Shadows 2.0, by Francois Blateyron.

(In fact, when I computed all the date curves for Newport's latitude, I found that the winter solstice curve would be beyond the two circles which make the distinctive design of the EarthDial, so for ED-16 there are only six curves ! )

They produced a twice-scale 500 mm.square piezo print for me, then laminated and encapsulated it in matt film, ideal for the variable English weather!

I mounted the dial on a piece of 12 mm. MDF board and then added the ball gnomon, for which I copied the method from the EarthDial Project. Because ED-16 was made at the smaller scale, instead of a ping-pong ball, I chose a 18 mm. wooden bead for the gnomon.

I also added two small circular mirrors which reflect the sky and show that the more the sky is blue and clear, the darker is the gnomon's shadow.

Setting up ED-16

The first steps was to choose a site where I could get a clear view as possible of the sun from dawn to dusk. I chose a flat roof on the South side of our house. Then I levelled the EarthDial and aligned the dial, North to South, using one of the three methods explained in the Planetary Project. I preferred the "Use the Sun" method, and went to the http://www.spot-on-sundials.co.uk/calculator.html website to give the correct solar time for the particular day. I checked at 9 a.m.. 12 noon and 6 p.m. ( remembering to allow for British Summer Time (B.S.T.)). Then I photographed the final dial.

My EarthDial

The last stage was the most difficult. The best position for ED-16 was quite far from my computer and, in any event, getting signal cables to a webcam was difficult. I decided that the answer was a wireless webcam, with a range of at least 20 metres. I used a Logitech Quickcam Cordless and enclosed it in a ABS plastic box with a small aperture for the lens. Suspended under a wooden gantry, it was about 70 cm. above the dial. Perversely, in order to provide sufficient power to the webcam all day, I needed A/C power but fortunately it was available from the outside wall for garden appliance!

As far as the computer software was concerned, I wanted to be able to send EarthDial images every 15 minutes to my website and to EarthDial Central and also to produce, if possible, animations of ED-16, through a full day and eventually, through the differing seasons from the summer solstices via the equinox to the winter solstices.

I found the I-Spy software from Surveyor Corporation, which I now use for both my (almost) realtime EarthDial images and also provides a method of making log files, in order to create present and future animation.