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!

Step 1: The Concept

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.

Step 2: Uh-oh... Maths!

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.

Step 3: My Sundial

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

Step 4: Construction

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.

Step 5: Using the Dial

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!

Step 6: ...and Now the Bad News!

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!

Comments

[KarenSDR.]

I can think of a few reasons why it might be off. The main one is what timearl said: the angles change a lot throughout the seasons. Dials of the type you made generally have a fairly wide interior to display the different angles, or a way of rotating the hole to change the angles, which should be combined with the changing angle in the interior. I took a bracelet years ago and drilled a hole and engraved lines on the inside. Both the suspension point and the hour lines vary with the seasons. I tried it just now and got about 9:30. I added an hour for daylight savings and got 10:30. I checked the clock: 10:25 But I had to look at the side of the interior that reflects the season. If I had looked on the other side it would have been an hour off.

This picture may give you a better idea than my photos. Those diagonal lines are actually steeper than what it shows in the drawing. https://en.wikipedia.org/wiki/Astronomical_rings#/media/File:BauernringDrecker.jpg

Other things that can throw it off are the equation of time, and timezones. I happen to live pretty close to the solar center of my timezone. But if I travel a hundred miles east or west, I have to adjust for that. Solar time is specific for each longitude, but clock time is averaged together in a time zone. So depending on where you live east or west, you could be off by half an hour or more. In some countries it could be several hours.

The equation of time is the result of the earth's orbit being an ellipse rather than a circle. At some times of year the sun is "fast' or "slow" by fifteen minutes.

[ikerninya]

Urgente!!!
Alguien me puede explicar como se ve la hora, es decir, si por ejemplo me marca un 10 que hora sería y en cada cara que se mira.

[timearl]

I found this really interesting and was sorry it didn’t work. I thought of a few possibilities: Is the hanging hole in the right place? Is it out by the same amount all year round? (it’ll take a year to answer that one). Could it be the way you hung it? It’s not clear what you mean by “edge-on to the sun”, but a flat vertical sundial (in the northern hemisphere) faces south.

Then I had a couple of correction/upgrade suggestions (I thought it might need to be a sphere rather than a ring, and also there’s the longitude and height of the sun to think about.

That got me more interested,so I went to Wikipedia and looked at sundials (there are at least 6 different types). From there I found astronomical rings, one use of which is as a sun dial. Yet again, there are several types of astronomical rings, but fewer than there are of sundials. Anyway, it looks like an astronomical ring is what you’re actually (on the way to) making, so don’t
give up, but it'll take a bit more work to get it right.

[Kiteman]

By "edge on", I meant that the edge with the hole in would cast its shadow onto the inside of the opposite side of the ring.

I haven't been back to this project (the maths hurt my brain), but your point about hanging is interesting - thinking back, I don't think I made allowances for the missing mass of the hole the light shines through. Maybe that side was "light", and tipped upwards?

[timearl]

That's possible, could you just add on a similar amount of wood and see if it corrects it?
Anyway, the way you used it sounds more like a sun ring than a sundial. I'm no mathematician (or astronomer), but it's possible that they use different calculations or figures. Have a look at the wikipedia entries for "sundial" and "astronomical rings", they're both really interesting, and you might get some more clues on how to get yours working.

[XTL]

That calculator uses +ve West of GMT meridian (for satelites) but has been replaced by another for +ve East(on th eground). Is your calculation for the wrong side of GMT - might account for the error. Also don't forget 'the equation of time' - in my part of the wolrd (174Longitude) this can be up to 30 minutes correction but closer to zero if in UK.

[Kiteman]

I'll try the new version at some point...

[nancyamy]

Since I don't have Inkscape, I can't see the SVG. Can you give me the dimensions of the plywood circle please? Presumably you could calibrate your clock by marking it after you check where the sun shines, yes? Thanks for this--I love clocks and can't wait to make this.

[XTL]

svg files can be viewed in modern browsers directly.

[Kiteman]

Would a PDF help?

The dial is 100mm across, but the size isn't that important, especially if you're going to add your own marks (which, yes, would work).

[mrsmerwin]

This might make a nice science project. The kids could mark the time once an hour during the day time. It would take a while to get the markings for other seasons of the year but I think it would help them understand the relative positions of the earth and the sun. I think I will try to find materials to make one.

[Kiteman]

Cool idea!

You could make it from any think material - maybe corrugated card, with a strip of white paper glued around the inside?

[mrsmerwin]

Thanks. That may work. We are starting a S.T.E.A.M. program at one of the local elementary schools. This might be a good long term project.