Ancient Hours Equinoctial and Seasonal
- BC Gnomonics
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The Phalara Dial
As part of a 2010-2011 excavation, a sundial was discovered at the site of ancient Phalara (modern Stylis, Greece). The dial was found inside the remains of an ancient house–a rather large one–also containing pottery fragments, coins, and fishing equipment. For this reason, the owner of the house is believed to have been either a fisherman himself or otherwise involved in the trade of fishing equipment.
The sundial is intensely fascinating for several reasons. It is one of only a few ancient Greek dials (each originating from the 4th-3rd century BCE) confirmed to display equinoctial as opposed to seasonal hours. Several authors believe these early dials (all equatorial in form) represent an early state of Greek temporal culture that predated the popular adoption of the seasonal hour (believed to have been introduced from Egypt). The equinoctial hour was likely introduced to the Greeks from the Babylonians, adapted from their division of the day into 12 beru (1 beru = 2 equinoctial hours). I am of the opinion that when Herodotus notes that the Greeks learned of the "12 divisions of the day" from the Babylonians, he was referring not to seasonal hours or the zodiac, but to their concept of beru.
Particularly strange is this dial's placement within a private home. Equinoctial hours and instruments that measure them are generally thought to have been the exclusive tools of astronomers, used for technical measurements and latitude-independent record keeping. This type of hour would have been of little use in public life, even in the unprecedented chance that its non-astronomer owner conceptually understood the equinoctial hour.
On The Seasonal and Equinoctial Hour Systems
The use-cases of seasonal and equinoctial hours are mutually exclusive. While seasonal hours are used to situate oneself or describe a specified point in time, equinoctial hours are used to describe duration. Because there is no starting point inherent to the equinoctial hour system, this type of hour functions exclusively as a free floating unit of measurement.
The two systems could be likened to a stopwatch and a clock respectively. The equinoctial hour is a stopwatch, able to measure the duration of events, the time lapsed between them, or otherwise count out equal measures once a starting point has been determined. It is a unit of measurement and is deployed when needed–the equinoctial hour is not used to specify the time of day. It cannot do this, for such a feature would require the system to tether itself and proceed with reference to a specific moment in time (for example, how later equal hour systems began at sunrise, sunset, or midnight, as our modern system does. See Types of hours).
The seasonal hour, in opposition, is fundamentally tied to the motion of the sun, with the first hour of the day always beginning at sunrise, and the first hour of the night always beginning at sunset. This system cannot easily be used to specify duration, as the length of an hour is not fixed (one seasonal hour is 1/12 of a given day's period of daylight, which varies throughout the year). Within the seasonal hour system, hours were exclusively conceptualized with respect to the beginning of the day (or night) as a baseline. While one could specify an event as beginning "at the 9th hour," the system could not describe an event as beginning "in 3 hours." Such a function requires counting from an arbitrary point rather than from sunrise, which is a feature not accommodated by the seasonal hour system.
Our modern system has the capacity to fulfill both the functions of stopwatch and clock, due to the fact that hours are both counted from midnight and are always of a set duration. In addition to specifying moments in time (e.g. "I'll meet you at 4 o'clock"), modern hours can be used to measure duration and specify elapsed periods counting from any point within the day (e.g. "I'll meet you in 3 hours" or "the event will last for 3 hours").
Other Ancient Equatorial Sundials
The Phalara dial stands as the best preserved equinoctial-hour-displaying sundial of antiquity. While several mysteries remain as to the status and ubiquity of the equinoctial hour in pre-3rd century Greece, this dial and those like it provide many opportunities for speculation, debate, and further lines of questioning. For the sake of completion and context, I will include brief references to the other known ancient equatorial dials (see Herrmann et. al. for further discussion of these dials).
When the equatorial class of dials is mentioned, equal or modern hours invariably come to mind—an intuitive association, particularly given the overwhelming number of post-15th century equatorial dials of both the portable and stationary varieties. However, in this ancient context the equatorial dial must be conceptually separated from the equinoctial hour*. Equatorial dials feature faces tilted parallel to the celestial equator. It is particularly easy to draw equinoctial hours on such a surface; simply mark out lines radially arranged 15º apart converging at the celestial axis (whether this axis is represented by a polar gnomon or the tip of a horizontally mounted nodus (see figure below).
Although easy to lay out, equinoctial hours to not lay exclusive claim to equatorial dials. Seasonal hours can be drawn on this plane as well, as can be seen in some forthcoming cases.
*Likewise, I consider it similarly important to draw a clear distinction between equal and equinoctial hours. Equinoctial hours are described above: hours of consistent length that are tools and terms of measurement—that is, not "clock" hours. Equinoctial hours do not reference time elapsed since a consistent starting point like sunrise, but rather are mobile and can set referential starting points wherever is most convenient.
Equal hours are also of consistent length, though exist in a continuous, cyclical march from a consistent anchor point. Each type of equal hour system places this anchor point with a different celestial instant: common hours begin counting at midnight, Babylonian hours at sunrise, and Italian hours at sunset. Equal hours can perform the measurements of equinoctial hours with the added benefit of specifying moments in time.
Before moving forward, it is worth learning to recognize equinoctial and seasonal hours when they are drawn on equatorial dials, as the two systems can become difficult to differentiate in this case.
Above is a recreation of the summer side of an ancient equatorial dial, shown as it would appear with both seasonal and equinoctial hours. As a general rule, equinoctial hour lines converge at the celestial pole (If the dial in question has a polar gnomon, this simply means that equinoctial hour lines converge at its base) while seasonal hour lines converge at the horizon*. This horizon line on the dial's face is drawn through the tip of the nodus' shadow when the sun is on the horizon (see figure below). note that this line is not always actually drawn on the dial plate. It may be represented by an incised horizontal line, the top edge of the dial (summer sides only), or lacking entirely.
Another method of differentiating the two hour systems: equinoctial hours are spaced 15º apart while seasonal hours simply divide the daily arc of the sun into 12 segments. If hour lines intersect the horizon line, they must be equinoctial, since the horizon line marks the start of the first and end of the 12th seasonal hour.
*Note that technically speaking, trigonometrically correct seasonal hour lines are not straight lines but rather segments of parabolas.
In most extant cases, one or both of these shorthands will allow you to determine which type of hours the equatorial dial carries, though fragmentary cases can beget confusion.
The Oropos dial presents once such example. The winter site can conclusively be determined to display equinoctial hours, seeing as though the hour lines converge at a point above the nodus' hole. Because the nodus of this dial is horizontally mounted and thus sits on the horizon, seasonal hour lines would converge at the hole itself. That the lines converge above this point signifies that they instead converge at the celestial axis that passes through the nodus' tip (which on the winter side of such a dial, falls above the actual dial face). The summer face, however, is less easily analyzed. Due to the low fracture of the dial, it is difficult to determine whether its hour lines converge at the horizon or at the axis (which, being on the summer side, falls below the horizon/base of the nodus). Although, it is likely that if the winter side reads equinoctial hours, so too does the summer side.
The Olympia dial likewise displays equinoctial hours. The dial features two polar gnomons rather than a single continuous one, resulting in a certain offset. This dial features no marked horizons or date lines.
The British Museum's equatorial dial is in remarkably good condition. It featured a set of polar gnomons and has a clearly marked horizon on its winter side. On the summer side, the horizon is delineated by the upper edge of the dial face. The dial displays seasonal hour lines which cross three date lines on each side. If extended, these lines would converge at the horizon line.
The unfinished dial of Paroikia appears to represent a mistake on the part of its stonemason. While the layout of its hour lines indicate an equatorial dial designed to display seasonal hours, both the summer and winter faces are inscribed on the same side and overlap. Although the dial could technically be used in this state, it would require laborious turnover each equinox. Because no holes exist for its gnomons, however, it is evident that this dial never saw use. It has been speculated that its maker, following a template, misunderstood the direction to "turn" the dial for its second instription, turning its summer face upside down rather than beginning the winter face on its reverse.
While the sundial of Heraclea on Latmos features a conic dial on its south side, its northern face carries an equatorial dial with a horizontally mounted nodus designed to display seasonal hours. The dial is slightly concave, slightly minimizing the period of interoperability the equinoxes typically present for equatorial dials.
Evan Boxer-Cook
Founder
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Remijsen, Sofie. 2021. “Living by the Clock. The Introduction of Clock Time in the Greek World.” Klio 103 (1): 1–29. https://doi.org/10.1515/klio-2020-0311.
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Schaldach, K. “The Arachne of the Amphiareion and the Origin of Gnomonics in Greece.” Journal for the History of Astronomy 35, no. 4 (2004): 435-445. https://doi-org.lprx.bates.edu/10.1177/002182860403500404.
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