BC Gnomonics
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Universal Altitude Dial /
Organum Ptolemei /
Trigon Altitude Dial
Universal, Altitude
The orthographic projection of this dial rotates above a latitude scale, allowing for truly universal operation anywhere in either hemisphere.
Though brass examples have been made, this dial is most closely associated with the work of Peter Apian.
The instrument is not only a sundial, but also a referential model that can be used to solve several problems exclusive of observation.
This projection can be found on some universal astrolabes (such as the universal astrolabe De Rojas). Rather than tilting the whole instrument to the altitude of the sun, only the sighting assembly is turned over the stationary dial body. In this way, the function of the dial is much like the alidade of an astrolabe.
The sighting assembly takes the shape of a pivoting trigon, with either a plumb bob or rule indicating the hour.
This instrument is effectively a side view of an armillary sphere, similar to the Analemma of Vitruvius.
How to use
1. Rotate the projection disc so that its index points to the latitude of use along the outer rim.
2. Suspend the dial, leveling it to the local horizon.
4. Point the gnomon to the current azimuth of the sun and adjust the trigon up or down so that the shadow of the forward sight falls directly on top of the back sight.
5. Note the plumb bob at the back of the trigon. Keeping the dial steady, secure the string in place where it hangs against the hour plate.
6. Locate the current date along the edge of the dial face. Trace out a line perpendicular to the date of operation until it intersects with the plumb bob string.
7. Once the point of intersection is found, reference either the top or bottom of the hour scale to determine the solar hour. Depending on the position in the day before or after noon, a different hour scale is used.
8. 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.