The
Sundial Primercreated by Carl Sabanski |

The vertical sundial must be designed for the particular latitude (ø) where it is to be used. The style height (SH) of gnomon is equal to the co-latitude or 90° minus the latitude. The hour line angles (X, HLA) can be calculated as follows: X = arctan {cos ø * tan (h)} where h is the hour angle, in degrees, given by: h = (T and T The sun can only shine on a vertical direct south sundial in the Northern Hemisphere and a vertical direct north sundial in the Southern Hemisphere between 6 A.M. and 6 P.M. Visit the "Sun Charts" page for some interesting information. Table 1 shows the calculation performed for a sundial located at latitude 50°N. Notice that the hour line angles for the am and pm hours are symmetrical about the noon hour line. Click here to download a spreadsheet that will perform these calculations for you.
Figure 1 is a plot of the hour lines for a vertical direct south sundial in 15-minute intervals. A drawing like this can be used as a template to lay out a dial plate. It also shows the dial mounting details.
When you have determined how large a dial plate you want then you must give some consideration to how large the gnomon should be. The height of the gnomon will determine the path the shadow will take over the dial plate throughout the year. The following three figures illustrate this for three gnomon heights using a dial plate that is fixed in size.
As the gnomon height increase the shadow covers more of the plate yet remains on the dial plate. For a very large gnomon the shadow will extend beyond the end of the dial plate for portion of the year. The sundial software package "SHADOWS" generates sundial layouts very quickly and is very good for doing this type of design comparison.
For an image complete with shadow click here. |