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Aquitaine Sundial Ring

by J.Edwards

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My birthday is coming up this weekend and because I won’t be able to make it home to visit my family then, they decided to throw an early birthday dinner for me while I was home for a short visit. Among the much appreciated gifts, including some new hiking gear and a few surfing documentaries, was a portable sundial from my father. It is a form of ring dial, named after Eleanor of Aquitaine, who is said to have given one to Henry II in 1152, so that he would be equipped to know when to leave the hunt to rendezvous for their secret love trysts (or so they say). I haven’t been able to dig up any historical information on the ring itself, or how true the lore behind it holds, but I do find the concept behind the device quite interesting and certainly appreciate it from a horological perspective. 

The ring is actually composed of two separate rings, one inlaid into the other, with a hole drilled into the outermost ring to serve as a reverse gnomon. Most impressive for me, is how easily the ring can be calibrated to read the time correctly in each month of the year, simply by rotating the outer ring to correspond with the appropriate time of year stamped around the circumference of the reference ring.  

Like most mass-produced sundials, my Aquitaine Sundial Ring is calibrated to read true solar time time along the 40th parallel. I live on the 42nd parallel, but I doubt I would be able to register much of a difference between this ring and one calibrated for my latitude, even if I were to hold them side by side, due to the small size of the scale that is used to indicate the hours on the inner circumference of the ring. It is able to approximate the time near enough for me to at least predict the number of daylight hours I have left in the day before it gets dark though. In testing the ring outside at this time of year, I found that reading the time proved to be a bit troublesome because of the glare coming off of the snow underfoot. I’ve since tied the ring to my rucksack and am looking forward to field-testing it further when I hit the hiking trails again in the spring.

Now if only I could find a way to shrink my Sunset Wheel.

The Aquitaine Sundial Ring is available to order from:

5 Comments

  1. J.Peter
    Posted December 8, 2008 at 10:30 pm | Permalink

    Happy Birthday

  2. Posted December 9, 2008 at 8:26 pm | Permalink

    Thanks J.Peter!

  3. Dan
    Posted November 5, 2009 at 12:44 pm | Permalink

    The first record of a sundial comes from the chaldean astronomer Berossus in 300 B.C. His sundial was a hollow half-sphere that had a bead-shaped pointer. During the day the shadow of the bead moved in a circular arc across the inner surface of the sphere. This arc was divided into twelve equal parts, and were called temporary hours because they changed with the seasons. Equal hours were only decided upon when clocks came into common use in the 13th century.

    The accuracy of a sundial (when compared to a watch or clock) will depend on several things. First, the sundial always reads standard time. When daylight time is in effect, you must add one hour.

    Second is your longitudinal position in relation to your time zone meridian. For instance, if you live in the Central Time Zone the meridian is 90° (Atlantic is 60°, Eastern is 75°, Mountain is 105°, Pacific is 120°, etc.). Every 15° of longitude equals 1 hour of time, so for every degree you move west of your time zone’s meridian you need to add 4 minutes to the sundial’s apparent time (when you move east of the meridian you will need to subtract 4 minutes for each degree).

    Third is the angle of the pointer (or gnomon). For accurate time, the angle must be the same as the latitudinal position. A sundial located in eastern Ontario, for instance, needs to have the gnomon set to an angle of 45°. In southern Ontario the angle might be 42° and in the prairies 50°. Most manufactured sundials (including the Aquitaine ring) are set to an average angle of 40°.

    Last is the correction for the equation of time. Because the earth’s orbit around the sun is not perfectly circular, there will be deviations in the speed of the sun’s motion across the sky from season to season. The two most extreme points are in February when you must add 15 minutes to the apparent sundial time, and early November when you must subtract 15 minutes. Most of the spring and summer months are accurate to within 5 minutes.

    It is clear that even a clock will not give you the true reading of time for your location, because the same time is used within a 15° longitudinal area. The sundial is precise without being accurate, while a clock is accurate without being precise. Regardless of all the variables, if you have two sundials that are made the same and reading time in the same location, then they will display the same time. That, if nothing else, is reassuring.

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  4. hatcher
    Posted December 26, 2010 at 8:36 pm | Permalink

    i have one but i dont know how to read it.

  5. Posted December 27, 2010 at 11:42 am | Permalink

    @Hatcher Align the hole in the center ring with the according letter of the current month on the outside of the ring. Position the ring so that the sun shines through the hole onto the inside of the ring. The position of the pinpoint of light on the inside of the ring indicates the time.

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