Capricorn: Dec.22-Jan.29 The Sundial Primer
created by Carl Sabanski
Capricorn: Dec.22-Jan.29

The Sundial Primer Index

Dialling with QBASIC - Mac Oglesby - Polar Sundial

The QBASIC routines presented here will assist you in designing a double gnomon polar sundial. With the use of two gnomons, one at each end of the dial plate, the sundial can indicate the time from 6 a.m. to 6 p.m.. Figure 1 shows an example of the polar sundial that can be made. Notice the morning and afternoon hour lines are stacked and also carry up the side of each gnomon. As this sundial is designed for the Northern Hemisphere the right gnomon indicate the morning hours that continue up the left gnomon. The left gnomon indicates the afternoon hours that continue up the right gnomon. For the Southern Hemisphere this is reversed and so too is the numbering of the hour lines.

The polar sundial can be used at any latitude by tilting the dial plate so the upper edge of the gnomons is parallel to the Earth's polar axis and therefore point to the celestial pole.

Figure 1: Double Gnomon Polar Sundial

Figure 1: Double Gnomon Polar Sundial

Double Gnomon Polar Sundial - POLSCAL2.bas and POLARSCL.bas

Both these programs provided the same output but "POLARSCL.bas" provides a couple of options and has a lot of remarks to describe its operation. This will be the program used in the following discussion. This program does not produce a graphic output to the screen. It creates a text file of the polar sundial hour lines.

You can get the QBASIC programs as well as 3 DeltaCad files here.

Figure 2 shows the program when it is opened in QBASIC. Take the time to read through the comments. There are a few parameters you can change and these are highlighted by the red rectangles.

The first is located on line 16 and it defines the location where the text file will be written.

filename$ = "c:\filename.txt"

"c:\filename.txt" can be changed to place the file in any directory and give it any name. "c:" defines the disk and is likely not going to change. The path is set to the root but should be changed to a more appropriate directory. Remember to use the backslash "\".

The second is located on line 17 and is used to define how many hours will be placed on the main dial plate. The default value is "4.5". The remaining 1.5 hours will be placed on the side of the gnomons.

Figure 2: Double Gnomon Polar Sundial QBASIC Program

Figure 2: Double Gnomon Polar Sundial QBASIC Program

The final parameter that can be adjusted is in line 25 and is the height of the gnomon "H". The default value is "76.2". As the comment says the drawing can be scaled in the CAD program if it is needed. If the value "76.2" is taken to be millimetres then this is equivalent to 3 inches.

After the text file is created use the program CNVxxxx to convert it to a DXF file. Figure 3 shows the hour lines when the drawing is opened in DeltaCad. As you can see only one set of hour lines for the main dial plate and gnomon are drawn.

Figure 3: Double Gnomon Polar Sundial DXF File Opened in DeltaCad

Figure 3: Double Gnomon Polar Sundial DXF File Opened in DeltaCad

From this point Mac had to develop the drawing to complete the sundial. Figure 4 shows where inverted and rotated the two sets of hour lines to create the second set required for the sundial. Note that the hour lines located on the gnomons are not equally spaced although they may appear to be.

Figure 4: Drawing the Remaining Hour Lines

Figure 4: Drawing the Remaining Hour Lines

Figure 5 shows the completed sundial drawing. The hour numbers and text have been added and the boundaries for the main dial plate and gnomons have been defined. When the drawing is printed the 3 parts of the sundial can be cut out.

Figure 6: Completed Drawing of the Double Gnomon Polar Sundial

Figure 6: Completed Drawing of the Double Gnomon Polar Sundial

Figure 7 shows another view of the same sundial. The gnomon hour lines can be clearly seen. The paper parts have been glued to pieces of wood. The top of the wood gnomons must not extend beyond the lines marked with "6" as these are the shadow casting edges or style. The wood base is constructed to position the sundial at the correct angle.

Figure 7: Double Gnomon Polar Sundial - A Second View

Figure 7: Double Gnomon Polar Sundial - A Second View