Specialized sub-types of the OERD
While the equatorial ring dial sits at both the end and peak of portable sundial development, it has two notable drawbacks: functional blind spots surrounding noon and the equinoxes.
While most examples of this dial simply bear these limitations, there are a select few engineered to overcome them, often in unique ways. Before exploring these special cases, let us expand upon the limitations themselves.
Noon shadow
During the minutes leading up to and following 12pm, the dials' nodus beam is hidden by the shadow of the meridian ring. Due to its nature as a microcosm of the celestial sphere, the dial's north/south meridian aligns with the true meridian, and thus the sun's position at solar noon. At noon, the meridian ring's shadow is cast northward, engulfing the nodus.
Equinox shadow
On the days surrounding the equinoxes, the dial's equatorial (equinoctial) ring casts its shadow onto its own inner surface. Because the nodus is positioned on the equator at this time–its line of sight with the sun broken by the equatorial ring–no beam can be observed so long as the dial is properly oriented. This blind spot exists as long as the sun's path coincides with the equator (and thus the equatorial ring). The thicker the dial's equatorial ring, the longer this limitation applies.
Managing the microcosm
Each of these blockages occur because the equatorial ring dial exists as an aligned microcosm of the celestial sphere. The physical construction of this microcosm creates a valuable educational model, but by definition brings limitations at certain instants–when celestial courses align with their physical counterparts within the dial.
Eliminating these blind spots ultimately comes down to stripping away pieces of the microcosm, creating a more skeletal celestial sphere.
Dial 1
This dial lessens the impact of the equinox shadow by cutting away the southern portion of the equatorial ring. However, reducing the ring also reduces the number of successive daylight hours that can be displayed, impacting the dial's universability with respect to extreme northern latitudes.
A related drawback, at least with the dial pictured, is the fact that the equatorial ring is cut at 6am and 6pm, resulting in an inability to display early morning or late afternoon hours. This impacts the dial's function during spring and summer, when there are more than 12 hours of daylight.
Dial 2
While cutting away a portion of the equatorial ring allows the dial to function reliably at the equinoxes, the fact remains that the meridian ring blocks the sun from reaching the nodus at noon. Dial 2 solves this problem by cutting away the southern portion of the meridian. While this dial overcomes both blockages, it is stripped of the portability of the standard equatorial ring dial, relocating its latitude scale to the nadir and eliminating the possibility of suspended use.
Dial 3
Dial 3 takes a different approach, cutting away the western portion of the equatorial ring rather than the southern portion. This entirely frees the dial from the equinox shadow, though fundamentally alters the dial conceptually. In this configuration, it only aligns with the celestial sphere in the afternoon, thereby losing the ability to act as a solar compass for half the day.
Dial 4
Dial 4–a crescent dial as opposed to an equatorial ring dial (using the tip of its claw rather than a light point nodus)–features a partial, swiveling equatorial ring that can model the true equator either before or after noon. Thus, assuming the operator knows the appropriate position for the hour ring, the dial retains its function as a solar compass and microcosm of the celestial sphere.
Note that while the hour plate can pivot to occupy either the eastern or western portion of the equator, this adjustment is not required for the dial to tell time–only for it to maintain its polar orientation.
Because this dial's hour ring is horizontally offset from the suspension ring, it is able to display noon without issue. One drawback of the pictured dial is that unlike dial 3, it is unable to display early morning/late afternoon hours.
Ultimately, the various solutions to the discussed functional gaps reflect differing priorities. In the case of the crescent dial, a much less intuitive celestial sphere is presented, though through this skeletal approach blind spots are eliminated. If a stationary windowsill dial is all that is required, clipping the equatorial and meridian rings solves the same issues while keeping the essence of the sphere intact.
While the standard equatorial ring dial retains certain limitations, doing so allows it to remain truly and completely universal–a trait shared by very few dials. Not only is it a time-finder, but a solar compass and an intuitive model of the heavens. To a lesser extent, there is also the concern of aesthetic appeal and prestige, which a cleanly ringed portable armillary dial undoubtedly carries.
While the models examined are ones made of brass, the equatorial ring dial lends itself to a host of other (less durable) materials as well–materials that benefit from the structural integrity of complete and doubly pivoted rings. Broadly, the universal limitations of the dial are preferable to alterations that would benefit specific use cases but undermine others.
Evan Boxer-Cook
Founder
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