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  • Writer's pictureEvan Boxer-Cook

What Makes an Armillary Dial?

Updated: Oct 5, 2023

To classify sundials is a notoriously difficult task

Pinning down the defining characteristics of anything man-made proves difficult, as just as easily as borders are drawn, exceptions to them can be found and new examples created that highlight the fragility of the taxonomy. In the case of the armillary sundial, two definitions stand to argue. This article aims to explain the underlying ideas, strengths, and flaws of both.

The following definitions agree with the fact that the armillary sundial is a subcategory of the equatorial dial.

Closed Equatorial and Meridian Rings, Polar Gnomon

This stance asserts that to be considered an armillary dial, the sundial in question must feature at least two rings akin to those of an armillary sphere. Specifically, the great circles of the equator and meridian. The dial must also feature a polar gnomon perpendicular to the plane of the equatorial ring.

This definition keeps true to the form of the armillary sphere, grounding the dial form firmly in a skeletal model of the celestial sphere.

This definition essentially argues that to be an armillary dial, the dial in question must resemble an armillary sphere or at least possess its baseline features. This is a sensible enough definition, but begins to show an over-emphasis on closed rings when met with certain dials.

Take for example the equatorial bowstring dial as seen below.

Though it features a polar gnomon, equatorial ring, and meridian ring, it is not considered an armillary dial because the rings are not complete. This could be taken as evidence that the equatorial bowstring simply belongs to a different category, if not for the fact that the definition also excludes dials even more clearly armillary inspired.

This sundial includes the same features as the equatorial bowstring, plus several others that increase its likeness to the armillary sphere. To deny this dial the designation of armillary when it so closely resembles one–on sole account of the fact that its equatorial and meridian rings are not closed–is to contradict the colloquially intuitive in favor of the pedantic.

While the lines between classifications are not necessarily barred from being pedantic, it does the greater system a disservice when this pedantry comes at the price of intuitive understanding. When the designation of armillary dial is tied up in such a specific web of requirements that dials such as the one above cannot rightfully be considered what they are designed to be, an ecosystem is created in which only experts hold enough understanding to be "correct."

With armillary status withheld on a technicality, those familiar enough to recognize dials like this as armillary-esque are made to be incorrect for little reason other than arbitrariness. In doing this, the purpose of language is undermined. If the coloquial is rejected, the accepted term is buried in scholarship and the popular term is deemed wrong despite being colloquially accepted. This constitutes a breakdown of language, and an entirely unnecessary exercise on that part of the learned.

Returning to the definition, the purity of its wording bars many "cut" armillary dials from the classification. Even ones closely resembling of armillary spheres–the single trait this definition values–are excluded. For this reason, this definition is arguably too specific in its requirements.

Polar Hour Plane

Another definition posits that to be considered an armillary sundial, a dial must feature a curved hour plane that is parallel to the celestial axis.

This designation is based more solidly in the latin roots of the word"armillary" than it is on the design influence of the armillary sphere itself. In this definition, dials are considered armillary because of the hour plane's resemblance to an "armilla" or bracelet, with hour lines inscribed along the inner band.

This definition is fairly broad, and encompasses many types of equatorial dials with cut and full great circles alike. In addition, this definition encompases the other equatorial subtype of bowstring dials.

Though, this definition may be considered too broad, as it includes many dials one would not intuitively consider armillary inspired. For example, the first of the following two equatorial dials would fall under the armillary designation.

This level of technical inclusivity includes a great many dials that were intended only as equatorial dials, and even more problematically bars very clearly armillary sphere-inspired dials.

For example, a dial may include a meridian ring, ecliptic band, and even rings for the tropics, but if it has its hour lines inscribed along the top rim of its equatorial disc rather than along the inner face, it cannot be considered an armillary dial.

While this is in keeping with the definition's basis on "armillary" rather than "armillary sphere," the term encompases many dials resembling armillary spheres while excluding others. It also overrides the colloquial connection between armillary sundial and armillary sphere, robbing the lexicon of a term to describe dials resembling armillary spheres. In this way, the use of this method of categorization counterintuitively obscures its own definition.

To emphasise this point, reference the below two dials.

Both of these dials would be considered armillary sundials under this system, though there would also exist no way to differentiate them from each other apart from explicitly stating that the second "looks like an armillary sphere." Without a convenient way to tell these very different dials apart with a term of classification alone, it can be seen that this definition is not only too broad, but disallows the entire category of "armillary sphere-esque" sundials from existing by monopolizing the term most often associated with armillary spheres.

The Bowstring Equatorial Problem

When examining the limits of each definition, a common issue arises. The equatorial bowstring poses a significant issue when attempting to create an all-encompassing definition for armillary dials.

The first definition leaves room for the distinct classification of bowstring dials by specifying that armillary dials must have full rings, but this allows many dials with cut armillary rings to be excluded. The second definition encompasses all banded armillary dials–even ones with cut rings, but also consumes the classification of bowstring dials by nature of its broad approach.

There is no apparent way to classify both armillary and equatorial bowstring dials in wholly inclusive yet mutually exclusive systems. This impasse, however, may be the key to resolving this problem. Perhaps bowstring equatorial dials should not be their own separate category at all, but instead the simplest form of armillary dial.

Proposing a New Definition

Because the previous two definitions of armillary sundial are either too fine or too coarse, I propose a third:

To be considered an armillary dial, a sundial must include at least two rings: those for the equator (upon which hour lines are drawn) and meridian. These rings need not be fully closed. The celestial axis must be defined by either a gnomon or nodus and be anchored at the north and south poles of the dial.

This definition borrows from the first, ensuring that the dial in question at least somewhat resemble an armillary sphere, while preserving coloquial recognition of dials with cut rings. While nearly all dials of this type will feature a curved polar hour plane, the definition is open to those with a standard equatorial disc and hours on the top rim.

It also consumes the category of equatorial bowstring dials. Under this definition, bowstrings are the simplest form of armillary dial, carrying nothing more than the bare minimum of features designating them as such. Despite being armillary dials, bowstrings may still retain their particular subcategory, nested within armillary, nested within equatorial.

The intent of this definition is to create a framework in which a dial recognized as armillary inspired is designated as an armillary dial, while offering a more structured definition than "if it looks like one, it is one." It also relaxes the distinction between equatorial planes and equatorial bands on dials, leaving both as types of equatorial dials and as potential design decisions on armillary dials.

I believe that they key to effectively classifying sundials–as with all things–is to maintain a medium between broad requirements that allow in unrelated dials on technicalities and specifications that are too fine and thus exclude dials that very nearly otherwise meet the standards.

Evan Boxer-Cook


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