BC Gnomonics
Types of Hours
In addition to our system of 24 common hours, there exist several other systems for breaking up the day. Listed below are some significant hour systems, how they work, and how to recognize them when they appear on sundials.
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The following hours are explained as used on sundials. For information on mean and civil time, see L.A.T. to Civil time. Each type can be classified as equal or unequal: equal hours are always 60 minutes long, while unequal hours vary in length over the course of the year.
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Modern Hours
Alternate names: Common hours, Equal hours, French hours, German hours, European hours, vulgar hours
Parent system: Equal
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The modern day begins at midnight with 12am. Solar noon is 12 hours later at 12pm. The full day comprises 24 equal hours, running from midnight to midnight. Midnight and noon anchor the day, and will always occur at 12am and 12pm respectively (12 hours apart). Sunrise and sunset do not occur at set hours, instead varying over the course of the year.
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This hour system is the one most commonly seen on sundials. They are drawn differently depending on the type of dial they appear on, but have certain key traits than can be used to identify them.
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Identifying characteristics:
1. Because 12 (noon) coincides with the sun's culmination, it is indicated when the sun sits true south (assuming the dial hasn't been corrected for longitude). On altitude dials such as quadrants, this means that 12 will be indicated when the dial is at its farthest upward tilt. On portable hour angle dials, 12 is the central hour.
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2. With the sun, functional, time-telling potential is limited to the range of 4am-8pm except at extreme latitudes. This common range of hours is indicative of a modern hour dial. Dials may feature more hours than these, however.
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3. Modern hours may be laid out in two sets of 12, or one continuous span of 24, depending on convention and context, however the former is far more common.
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4. On horizontal and direct north/south vertical dials, these hour lines converse at the base of the gnomon.
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Babylonian Hours
Alternate names: Greek hours,"horae ab ortu solis" (hours from sunrise)
Parent system: Equal
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Babylonian hours are most often drawn on horizontal or vertical sundials, not uncommonly on diptychs, and perhaps most commonly on Nuremberg diptychs. This system reckons time starting from sunrise, counting through 24 hours until the next sunrise. Because of this, the only static hour is sunrise itself, which marks the beginning of the 1st hour.
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Identifying characteristics:
1. Babylonian hours are quite easy to identify, granted you are able to tell them apart from Italian hours. They feature a distinct lean, the lean beginning at the first hour (sunrise), and ending with a perpendicular line at the 12th hour (though the lines may only record up to the 9th or 10th hour).
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2. Babylonian hour lines may include a 24 line before 1, signifying the end of the previous day.
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3. On some sundial quadrants, these hour lines may be parallel.
Italian Hours
Alternate names: Italic hours, Bohemian hours, Welsch hours, "horae ab occasu solis" (hours from sunset)
Parent system: Equal
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Italian hours measure time beginning at sunset, counting through 24 hours until the next sunset. Italian hours can either begin at 0 and count to 24, or begin at 24 and count to 0, the latter indicating hours until sunset. Like Babylonian hours, they are often seen on diptychs with additional dial furniture.
Identifying characteristics:
1. Italian hours can be considered the mirror image of Babylonian hours, featuring an opposite lean. The hour lines end at sunset with 24 or 0 (often a vertical line when on horizontal or vertical dials, though this hour is not always depicted).
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2. Often, Italian hours are displayed alongside or overlapping Babylonian hours.
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3. On some sundial quadrants, these hour lines may be parallel.
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Seasonal Hours
Alternate names: Temporal hours, temporary hours, unequal hours
Parent system: Unequal
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Seasonal hours are perhaps closest in tune with the movements of the sun. Being an unequal hour system, the length of one hour varies over the course of the year. Sunrise marks the beginning of the first hour, the sun culminates at the end of the sixth hour, and sunset occurs at the end of the 12th hour. At the equinox, seasonal hours are 60 minutes long, but can range from 40 to 80 at the winter and summer solstices.
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Identifying characteristics:
1. Seasonal hours on altitude dials are marked by 7 lines, separating the dial plate into six segments with two numerals each; the two hours of the day when the sun is mirrored about the north-south line. These lines are often irregularly spaced relative to each other. On horizontal and direct south vertical dials, these hours appear to have a more uniform spread than modern hours.
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2. Perhaps the most common signifier that unequal hours are present is the fact that the end of the 6th hour marks noon. In the case of altitude dials, this means that the highest sun will cause the dial to mark the 7th hour.
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3. On horizontal and vertical dials, these hour lines appear to converge at either the substyle point or a point beyond the gnomon entirely.
