BC Gnomonics
The self-orienting armillary dial, most commonly referred to as the (universal) equatorial ring dial, is one of the most precise forms of portable sundial.
This instrument combines elements of both altitude and hour-angle dials. It is through this hybrid construction that it is able to locate true north without a compass. It achieves this by incorporating a latitude scale as well as a date scale; when both are set, the dial is rotated until the nodus' beam locks its rotation at the north-south meridian.
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This dial evolved from the earlier design of the astronomical ring dial, which itself was adapted into the observational armillary sphere. Through these roots, the equatorial ring dial functions as a solar compass, timekeeper, and model of the celestial sphere. When properly set and oriented, the nodus assembly parallels the earth's axis while the hour plate parallels the equator.
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Often, the meridian rings of these dials are inscribed with cities and their accompanying latitudes.
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Sometimes, these dials feature a quadrant inscribed on the meridian ring, allowing the user to determine local latitude.
How to use
1. Set the dial for the latitude of use. This is accomplished by sliding a ring or clamp around the outer edge of the meridian ring, along the marked latitude scales.
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2. Slide the nodus to mark the current month. The nodus is located at the center axis of the dial and is made to slide up and down on a track.
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3. Unfold the equatorial ring, ensuring that its resting position is perpendicular to the meridian ring.
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4. slightly rotate the nodus bridge to an estimation of the current time.
5. Suspend the dial by the crown marking the latitude. allowing it to level itself with the local horizon.
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6. Twist the dial until a beam of light is projected through the nodus onto the inner side of the equatorial (hour) ring. Ensure that the beam falls at the center of this band.
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7. Reference the hour scale on the top of the equatorial ring. Locate the beam's position along it to read current solar time (Note that there are two points where this beam can be seen: one being the correct time and the other being the hour of corresponding solar altitude mirrored across 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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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.
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