BC Gnomonics
The astronomical ring dial is the direct precursor to the equatorial ring dial, only differing in its method of adjusting for solar declination. To set the date, a central alidade (or pair of sighting vanes) is pivoted around the celestial axis and fixed at an angle to the equatorial ring equal to the sun's declination for the date of use.
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When compared to the equatorial ring dial, the astronomical ring dial is seen to require an additional pivoting ring on which to mount its alidade. If the dial uses an alidade, this ring may be spoked, or filled in with an additional instrument such as an astrolabe.
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Often, the meridian rings of these dials are inscribed with cities and their accompanying latitudes.
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When the dial is used, its innermost (alidade) ring pivots on the celestial axis. Because the declination of the sun is physically represented by the tip of the alidade, this dial may also function reasonably well as a demonstrational armillary sphere.
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 scale.
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2. To set the date, rotate the alidade or sights along the inscribed date scale
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3. Unfold the equatorial ring. Suspending the dial by the ring topping the latitude selector, pivot the alidade's polar-mounted ring and rotate the dial so that a beam of light shines directly through both sights.
A proper alignment will project the shadow of the uppermost sight onto the lower one (the sight closest to the sun should be the one indicating the date on its scale). (Note that there are two points where this alignment 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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4. When the sights are aligned with the sun, read the current hour as indicated by the edge of the polar ring.
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5. 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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