BC Gnomonics
Regiomontanus Dial / Universal Altitude Dial
Universal, Altitude
The complexity of the universal altitude dial comes from its inscribed hour lines; the body of the dial is relatively simple. Because of this fact, the dial can be made from a wide variety of materials–including ones unfit to support more complex dial structures–such as paper.
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The face of this dial features a gridded triangular scale on its top portion. This is used in conjunction with an adjustable arm called the brachiolum to set both the latitude and date of operation. The lower portion of the dial is adorned with an hour scale made up of particularly spaced parallel lines.
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Some versions of this dial are capable of reading the altitudes of certain stars to tell the time at night. In these cases, the reverse side of the dial features a volvelle.
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Regiomontanus dials also feature a gazetteer of cities with their corresponding latitudes, either on the front or back of the dial.
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These dials are adaptable for nearly every latitude, making them incredibly useful to travelers. The one caveat to this level of universality is that their operation is complex and requires a certain level of understanding that may be beyond someone simply looking for a convenient way to tell the time quickly.
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This dial functions under the same principles as the navicula, though was developed independently.
How to use
1. Position the end of the articulated brachiolum at the intersection of the current date line and the latitude of use.
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The lines of each intersect, creating a grid upon which to set the instrument.
Labels for the latitude scale can be found on the vertical edge of the grid, and date labels can be found at either the top or bottom horizontal edge. The labels of the date scale may be typical calendar months or those of the zodiac.
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2. Hold the dial vertically so that the thread hangs parallel to the hour scale, then tilt it up, causing the thread to fall against the second date scale on the far right edge of the instrument.
Keep tilting the dial up or down until the string indicates the current date on the leftmost side of the date scale. Clamp the thread in this position.
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3. With the thread oriented to the current date, slide the thread's bead onto the 12th hour line.
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4. With the bead set, let the string fall loose. The dial is now ready to be used.
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5. Tilt the dial up so that the shadow of the forwardmost sighting hole falls on the second sight. With this done, the dial is now pointing directly at the sun (the front of the dial is the edge farthest from the date scale used to set the bead).
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6. Take note of where the string is positioned on the hour scale and clamp it down. The position of the bead on the hour scale indicates the current time.
The top labels of the hour scale are used when taking a reading before noon, and the lower labels are used for the afternoon.
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7. 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.
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