BC Gnomonics
Diptych Sundial
Latitude specific or Universal, Hour angle
By including compasses in sundials, hour angle dials are able to be made portable. The diptych sundial is one such example.
​
These dials often include multiple holes through which to thread the cord gnomon, thus preparing it for use at different latitudes. To be accurately used at each latitude, however, the dial face must also be drawn with the corresponding set of hour lines. To accomodate use in many locations, the lower face of many diptych dials includes multiple nested sets of hour lines.
​
Diptych dials, specifically Nuremberg models, feature space ripe for additional dial furniture. These dials may include Babylonian and/or Italian hours, solar calendars, or a plumb bob for leveling the dial.
​
Diptychs may also feature lunar volvelles for telling time with the moon. These devices take into account the age of the moon in order to determine solar time from moonlight.
​
Because these dials use imbedded compasses for alignment, they will face magnetic north rather than true north if the proper allowance for declination has not been made. This will introduce a variable degree of error into the reading.
How to use
1. Open the dial so that the upper leaf is positioned 90º to the lower one, with the gnomon taught between them.
​
2. Level the dial to the local horizon
​
3. If multiple latitude holes are included, thread the gnomon string through the correct one, making sure the leaves retain their 90º angle.
​
4. Use the included compass to orient the dial true north. If magnetic declination is not known, it can be found here (the given value is the difference between magnetic and true north. If magnetic declination is 15ºW, rotate 15ºE of magnetic north to point true north).
​
5. Read the position of the shadow on the hour scale corresponding (or closest) to the latitude of use. This indicated local apparent time.
6. To determine civil time, add the appropriate longitude correction and equation of time correction (more information here). If applicable, add one hour during daylight saving time.