Hanging Sundial

Sundials are the ultimate in analogue timekeeping - there's ironic poetry in the way something as insubstantial as a shadow can direct the lives of men.

Today, in this digital age, the low precision of a sundial might seem to make them redundant, but, still, many of us make them in schools, or for fun with a stick on the beach. They surround us, as decoration, monument, and art.

The one true disadvantage of a sundial is its lack of portability - not only do they tend to be heavy, they need to be aligned with the North before the work, and they are restricted geographically; each sundial will only work at a specific latitude.

Imagine, though; you need to know the time of day, but your job moves you around an area. Then you need a hanging sundial!

Every sundial works the same way - a fixed point (the gnomon) casts a shadow onto a marked dial.

In most sundials, the gnomon is a stick of some sort, casting a shadow across a dial. The dial traces the apparent movement of the Sun in two dimensions; as it crosses the sky East-to-West, and as it rises and sets in the sky. That traces a curve on the dial.

The hanging sundial only traces the movement in one dimension - the rise and set. The user of the dial traces the movement East-to-West by turning the sundial as it hangs. The gnomon is a pin across a gap in the disc of the sundial, which casts a shadow on the inside of the disc, which is marked with the hours of the day.

Unfortunately, the Sun's apparent path across the sky is inconstant - higher in the summer, lower in the winter. The hanging sundial deals with this by having three scales traced around its edges - one for midsummer, one for midwinter, and, on the other side, one for the equinoxes. You simply choose the dial closest to the date you are reading the time for.

Plotting the path of the Sun takes a lot of sums. Angles, sines, all sorts of buttons on the calculator.

Fortunately, there are any number of sundial calculators on the interwebs.

Unfortunately, most of them very helpfully churn out templates for the gnomon and dial of a traditional sundial.

I did eventually find one website, though, that provided the data I needed (the elevation of the Sun) as a separate number;

https://www.esrl.noaa.gov/gmd/grad/solcalc/azel.ht...

I had to churn through the dates and times individually to get the required angles, but I got it.

Having drawn the basic design of the sundial in Inkscape, I used the NOAA data to mark the dial by drawing angles from the point where the gnomon would fit (see the second image).

I did this three times - midsummer and midwinter went on the ring that makes one side of the sundial, the equinoxes went on the other face, in a mirror image to the first, since the dial was to be flipped in construction.

I have included the SVG I created with Inkscape, but you will need to re-draw the markings (unless you happen to live about 52.5 degrees North of the equator...).

To finish making the sundial, you'll need...

• plywood
• something to cut it
• wood-glue
• clamps
• a scrap of paper
• a paper-clip
• superglue
• something to cut the paper-clip

When you have cut out the pieces of your sundial, they need to be aligned as you glue them.

Glue your layers, then roll up the scrap of paper and thread it through the larger holes in the pieces. Open up the paperclip and thread it through the smaller holes in the outer layers of the sundial.

Clamp it and let the glue dry.

Cut the paperclip to that it fits the sundial flush, and fix it in place with a drop of superglue.

Add a length of thread or string to hang it, and you're done.

Using the sundial is easy - you hang it up, edge-on to the sun, and check where the gnomon's shadow falls on the dial.

In my laser-cut version, the inside of the dial is very dark, so the shadow doesn't show up very well. It will show up better if you lighten the inside of the dial. You could paint over or sand away the burned surface, but if you plan to sand it you will need to make sure you put your hour marks away from the inner edge, or you will sand off the numbers!

My sundial didn't work.

Well, it worked, but it was wrong, out by a couple of hours.

Somewhere down the line, I got my numbers wrong. I haven't worked out where, yet, but it was probably a combination of rounding off my town's latitude and forgetting to allow for British Summertime ("Daylight Saving").

So, if you make one of these, check your maths!