Project - The diptych dial

DIRECTIONS FOR DIPTYCH DIAL ASSEMBLY

The folding sundial you are about to make is called a diptych dial. Such dials have been made for four or five centuries and were traditionally made of ivory or boxwood. The diptych dial provided here was designed by Dr. Allan Mills, Astronomy Group, Leicester University, UK . The scanned images and text have been prepared by Dr. Randall Brooks, National Museum of Science and Technology, Ottawa, Canada. The template was later updated and redrawn by Anders Bergströmwho also wrote the Capuchin dial project below

Step 1: Print the template dial (provided as JPEG graphics to download) on a piece of A4 or letter sized paper. The downloaded files may be resized if desireable. Paste the paper dial onto a piece of bristol board or heavy card making sure that glue is spread evenly over the entire surface. To download the full size template 

Step 2: Trim along the outside lines on the template diagram.

Step 3: To provide accurate time, the sundial must be orientated properly. The first step is to determine the latitude of the site using an atlas. On the right and left of the lower half of the dial (the base), you will see two scales marked 35°-55°. Once you have determined the required latitude, mark the latitude angle off on both scales drawing lines through each of the X symbols at the top. Cut the flaps off along these lines.

At the top you may want to write the location and latitude and/or perhaps your name, i.e. "Sam Smith, Fecit" (Fecit is Latin for "maker" and often appears on early instruments).

NB: If your site is outside the 35-55° latitude range, you can move the position of the "X" until you get the required tilt to the dial's base.

Step 4: Note the dashed lines; two of these are marked "score on back, fold forward" and two marked "score on front, fold down". Folds must be made along these lines and if you score these lines lightly with a sharp craft knife, the folds will be sharper and the sundial will stand better and look more finished. However, in making these scores, do not cut all the way through the card backing! After scoring, make the necessary folds in the direction indicated.

Step 5: Along the line with "Noon" above and "a.m. p.m." below, make another score line on the back and fold the upper section forward so that the two panels make a right angle.

Step 6: To finish the dial attach a string (preferably elastic string) through holes at the top and bottom at the points where all the hour lines converge. The easiest way to accomplish this is to use a thick needle and thread the string through tying knots on each end so that the string is taught when the dial is folded and ready for use. This string is the gnomon (pronounced no-mon) and casts the shadow to indicate the time.

Step 7: You may wish to embellish the back of your dial with an old-style graphic from a book on sundials, a drawing of your pet or whatever appeals to you. This is where you can individualize your diptych dial!

Positioning your sundial:

The sundial must be orientated with the gnomon pointing north/south and, of course, the dial must be located where a shadow will be cast by the gnomon most of the day (though one can move the dial from window to window as the day progresses if necessary).

Method 1: (The Purist's Method) To determine the orientation without reference to other mechanical devices, North can be found by observing Polaris, the North Star, at night. In orientating the sundial, the gnomon is actually being pointed to the North Celestial Pole which is within 1° of the North Star. Thus, if you can find Polaris at the end of the Little Dipper, line up your dial by pointing the gnomon towards Polaris. You might want to record the orientation for your dial by making light pencil marks on a window sill for future reference. Those in southern latitudes will not be able to use this method as there is no bright star near the South Celestial Pole.

Method 2: (The Practical Method) A magnetic compass may be used to determine the north/south line, but, because of the difference between magnetic north and true north, the dial reading could be out by an hour or more depending on the local difference between magnetic and true north (or south if in southern latitudes).

Method 3: (The Lazy Person's Method) To a first approximation, the orientation can be found by finding the orientation at any time from a clock or watch and orientating the dial so the shadow shows the correct time. However, if left in this position, there could be an error of up to 30 minutes over the year as a result of what is known as the "equation of time". Because of the Earth's orbital motion around the Sun, the solar day (apx. 24 hours) is not exactly the same length from day to day varying by up to ±16 minutes a day. However, if the orientation is carried out on April 15, June 10, Sept. 1 or Dec. 20, this error will be negligible and any orientation made between April 15 and 1 Sept. will be in error by, at most, a few minutes (but don't forget the effect of daylight savings time).

Happy dialling!