BC Gnomonics
Roman Altitude Dial
Universal, Altitude, Seasonal Hours
A portable Roman sundial, this dial reckons the seasonal hours based on principles of approximation. Because of this, its readings are not accurate enough for equal hour use, and this dial was never adapted when seasonal hours fell out of favor. Despite the inherent approximation barring further development, this dial is accurate enough for those using the seasonal hour system.
​
The gnomon assembly of this sundial closely resembles the earliest known portable sundials from Egypt. This Roman (previously Greek) dial may exist as the continuation of the Egyptian dial lineage, adapting the L-shaped dials for universal use.
​
The backs of these dials feature gazetteers with major cities and their latitudes to aid in setting the dial for use.
How to use
1. Rotate the equinox line to the latitude of operation along the rim of the dial. On some dials, a line perpendicular to that of the equinox is used.
​
2. To set the date, superimpose one of the two date cones over the conic base at the bottom of the dial. Pivot the gnomon assembly until its sloping end points to the bottom conic base at the point that corresponds with the current month on the date cone.
​
3. Suspend the dial and rotate it until the gnomon casts its shadow onto the sloped hour scale.
​
4. Read the current unequal hour. For reference, the first hour ends when the gnomon's shadow has passed the first mark, and the seventh hour begins when the shadow begins to retreat from the furthest end of the ramp.
* Note that at the end of the 6th hour, the shadow will not fall on the hour scale cleanly.
​