Glenn's Pennsic pages
copyright 2001 Fred Blonder
(page maintained by D. Glenn Arthur Jr.)
Permission is granted to copy this design as desired for personal and/or educational use. It's provided "as is" with no guarantee of accuracy and, in fact, (as of 2001-07-28) has not yet been tested in actual use.

[What's new at this site] Last updated 2001-07-28.

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A Perforated Ring Sundial for use at Pennsic

This graphic:

is the pattern for a perforated-ring sundial optimized for use at Pennsic.

This is very similar to the Aquitane dials sold by many merchants, but a lot more accurate because the Aquitane is small, and designed for use in England, the wrong latitude for Pennsic.

To use it, print out the pattern and copy it onto some flexible material that can be bent into a circle. Cardboard will be easiest, brass more impressive; I'll leave the details to you. Size doesn't matter. Well, okay, it does a little. The larger you make the dial the more accurate it will be, all other things being equal. I recommend making it in the range of 3 - 6 inches (8 - 15 cm.) diameter to make it portable, and either wearing it as a bracelet or hanging it from your belt. Anyway, feel free to enlarge or shrink the pattern as you see fit. You can even distort the aspect-ratio, making it wider or narrower, without affecting its accuracy.

The ends of the design mark the top of the dial when it's bent into a closed circle. You may wish to cut your working material a tad longer than the pattern to allow it to overlap for structural reasons, but make certain that the ends of the pattern line up exactly. You'll need to punch two holes, one in the middle of the sunrise line and one in the middle of the sunset line. These holes should be as small as you can make them. After you have the pattern transfered and holes punched, bend the strip into a circle, with the pattern on the inside, (Feel free to decorate the outside in any manner.) taking care that the ends of the pattern are aligned. Attach a cord or chain to the point where the ends join.

That should do it. Now, how to read the time. This sundial measures only the sun's altitude, so you must know whether the sun is on its way up or down. At Pennsic, the sun peaks at 1:23 in the afternoon, and for awhile before and after it's not really changing altitude much, so the timelines are scrunched together (and nearly useless) from about 1:00 to 1:45. You need to also know the date. The lines running along the length of the dial are calendar-lines which run (left-to-right) from August 0 (okay, July 31st) through August 20th. There's a heavy line at the tenth, and lighter lines for the 5th and 15th. Note that the dates don't apply to the entire length of the line. The August 0 line for the morning is the August 20 line for the evening, and vice versa. Likewise the August 5th line for the morning is the August 15th line for the evening.

Now let's look at an example: Say it's August 7th sometime in the morning. Hang the dial by its cord and rotate it until the image of the sun falling through the hole falls on the morning scale, then adjust it so the spot of light is about 2/5 of the way between the August 5th line and the August 10th line. Say the image then falls on the line just above the 10:00 line, that's the 9:45 line (Did I mention there are four lines per hour?) so it ought to be 9:45. If It's not, wait long enough and it will be. ;-)

A few notes on accuracy

Historical authenticity

Yes, I'm invoking the dreaded "A" word. ;-)

Perforated ring sundials certainly are correct for the SCA period. I've heard (unreliable) reports that they were used by the Vikings. To the best of my knowledge, ring-dials with two holes are not period. Neither is adapting a dial to adjust for daylight-savings time.

The whole purpose of this exercise is not to produce an inauthentic sundial, but to produce one that is (almost) as easy to use as a digital watch, so that people might actually use it instead of their digital watches. You won't get split-second accuracy with this dial, but it will keep you on-schedule to within a few minutes, which is all most people bother with anyway.

To summarize a bit of fairly obvious celestial mechanics: The sun rises in the morning, peaks at midday, then sets. When it rises it's moving up fairly fast, and when it sets it's moving down fairly fast, but it slows down in the middle. Between 1:00 and 1:15 it climbs only four-tenths of a degree. Obviously you'll get better time accuracy near dawn and dusk, and probably have no clue about the time between 1:00 and 1:45, other than the fact that it's somewhere between 1:00 and 1:45.

This flaw is shared by all period portable sundials such as the shepherd's dial - a pillar with a small projecting arm - and the nice Pennsic XXV medallion designed by Master John the Artificer. The reason is that these dials measure only the altitude of the sun, not the azimuth. The sun is obviously moving westward at a constant rate all day, and a dial that measures this motion will be able to tell the time with equal accuracy throughout the day. The only way to do this is to know which way is west, and align the dial in the correct direction. The period way to do this is to figure out which way is north, then fix the dial in the correct position; this means that it can't be portable. You may find reproduction dials that include a compass. These work well, but all seem to be based on originals from the 18th century.

Revision History

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