BC Gnomonics
Analemmatic Multi-Dial /
Inclining-Analemmatic Dial
Universal, Hour-Angle, Azimuth
This multi-dial features both an analemmatic and horizontal (inclining) dial on a hinged plane; both able to be adjusted to the user's latitude.
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As with all analemmatic dials, a sliding gnomon must be used and adjusted to match the solar declination. The gnomons for the two dials may be distinct, or combined into a single, multi-purpose gnomon.
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Due to the distinct mechanics underlying these two types of dials–azimuth and hour-angle–the dial as a whole is self-orienting. When both included dials display the same time, true north is pointed and the dials accurately oriented.
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In practice, aligning this dial may prove complex. Though an agreement of shadows is all that is needed to achieve this, there are several points around the noon line where this agreement seems to occur. In addition, the correct hour mirrored around noon will also yield agreeing shadows (e.g. 9am and 3pm). Because of this, it is necessary to have either a rough sense of either the time or the direction of true north to effectively align the dial without aid.
The Sundial Primer
Nicolas Bion
C. J. Budd, Christopher Sangwin
How to use
1. Incline the dial to the latitude of use against the included scale.
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2. Locate the current date on the scale of the analemmatic dial and relocate its gnomon to that position (if the analemmatic dial shares its gnomon, the entirety of the horizontal inclining dial slides across the date scale).
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3. Level the dial to the local horizon with a flat surface.
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4. Rotate the dial until the shadows of both gnomons indicate the same time (Note that the shadows will agree at two points mirrored around noon. The user must thus be aware of either the vague direction of true north or whether it is currently before or after noon).
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5. Read the position of the shadow(s) on the hour plate(s). This measures local apparent time. 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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