Making Time

As one of my summer projects for the Edgerton Center, I’ve been working on the designs for a stained-glass sundial. My initial inspiration was John Carmichael’s Gecko Dial, an incredibly beautiful sundial window.

Stained-glass window sundial made by John Carmichael.

Originally I hoped to make something very similar; however, my advisor pointed out that a sundial in a window is useful for only some portion of the daylight hours. Even on the sunny south side of a building, the building itself necessarily blocks the sun at some points in the day. So I looked around at other stained-glass sundials, trying to find something that could be mounted outdoors to get maximum sunlight. Géza Marton’s Ólomüveg Dial seemed like a perfect choice — it’s a traditional disk sundial like you’d see in a garden, but it’s made of translucent glass and mounted up high so that it can be read by a viewer from below.

Raised stained-glass sundial in Budapest.

Though the Ólomüveg Dial can be read during all sunny daylight hours, it does have one downside: it only marks solar time, not clock time. Because Earth has an elliptical orbit, the planet moves faster when it’s closer to the Sun, and more slowly when it’s farther away. The change in the speed at which Earth moves means that the length of a day marked by the sun can be anywhere from 18 seconds slower to 30 seconds longer than the 24-hour day marked by the clock. This may not seem like much, but the seconds gained or lost each day add up, so that clock time can be different from solar time by as much as 15 minutes at certain points in the year.

It’s not strictly necessary to make a sundial read clock time instead of solar time, but I felt that a sundial that shows clock time would be more interesting and technically impressive than one that shows solar time. If you read a sundial and it’s off by fifteen minutes, that’s not nearly as cool as one that matches the clock.

Most types of sundials don’t compensate to match clock time, but there are some kinds of sundials that can. Analemmatic sundials, shepherd’s dials, ring dials, and Capuchin dials all have parts that can be moved to adjust for the changing height of the sun in the sky during the year — but to make reading the sundial as simple and user-friendly as possible, I wanted to make a sundial that didn’t require any physical adjustments.

While looking through more galleries of dials, trying to find something that was the best of both worlds — simple and accurate — I stumbled onto the spider sundial. Spider sundials are a variant of horizontal sundials that have a fixed gnomon, twelve concentric circles (one for each month) and wavy radiating lines for the hours. You read it by looking at the circle corresponding to the appropriate month; where the shadow intersects that one circle will show the current time.

Spider sundial made by Jürgen Giesen.

The right-hand sundial is a spider sundial; this is Jürgen Giesen’s North Finder. In April, the indicated time is 10:30, but in October, it’s 10:00.

Reading a spider sundial is a little more involved than reading a standard radial sundial because of the addition of the concentric circles, but adding one extra step in lieu of moving parts seemed like a reasonable trade-off. I used Jürgen Giesen’s spider sundial applet to create the template for a spider sundial with a vertical gnomon, using Boston’s latitude and the longitude for Eastern Standard Time. (The green line marks sunrise and sunset; spider sundials can have lines for every hour of the day, but I figured having a rise/set line would be less confusing than having half of the dial be full of unusable markings.)

Spider sundial template, made using an applet from

After taking a very high-tech screenshot of the applet, I tested out color combinations, trying to find something that would look good when cut out of stained glass. A friend suggested that the part of the sundial on the far side of the night line should look like a starry sky, in order to make the day/night distinction more obvious. I especially liked that idea because it felt like a nod back to the Gecko Dial.

Three color options for the sundial.

I settled on the third option (the gradients are pretty, but they’d need a lot of different shades of glass to make) and cleaned up the design in Illustrator to make a full mock-up. Next, I’ll be making a laser-cut version of the mock-up so I can get feedback on whether the markings are easy to understand, what size the sundial should be, and what adjustments will make the assembly easier to construct. Ideally the final version of the sundial will be made of waterjet-cut glass fit into in an aluminum frame (like this dragonfly), but I’ll need to do a whole slew of feasibility tests first.

Colored mock-up of the full sundial.


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