Since the solstice is only a few weeks away, I decided it might be fun to make Ptolemy’s solar angle dial we recently saw.
Initially, I looked into getting a sheet or brass or copper to make this from, but getting one that was reasonably thick (at least $\frac{1}{8}$”) was prohibitively expensive. So my next idea was to shoot for some MDF. However, I then remembered I had some quarter inch polystyrene sitting in my basement. It’s not period, but it’s relatively easy to work with.
I had about a 12″ square, so I popped a screw in the center of it going into a board, tied a string around it, and put a pen at where I wanted it and spun the styrene beneath it (much easier than trying to spin the string). I repeated this with a smaller circle for the inner ring and cut both out carefully with a razor knife.
The cut wasn’t perfect so I sanded the edge of the inner disc and the inside edge of the outer ring until it spun freely.
Next up came time to put the scale on the outer ring. I took a digital T-bevel and reversed the screw in it so that it went into the board1. I taped the central disc back in place and worked around the edge of the ring 180º in 1º intervals (no need to go the full 360º since I don’t plan on measuring anything beneath the horizon).
With that complete, I removed the center wheel and, with a dremmel disc, cut the divisions with each 45º going all the way across. In addition I used a dremmel engraving tip to mark the 45º marks.
Since Ptolemy recommended the piece be made from brass, I gave it a coat of metallic brass spray paint.
At this point, I was out of the styrene and I needed a back board for the central disk to keep it from slipping through. So I picked up a small sheet of lexan and used that.
This was then set in the outer ring and the pointer and shadow marker applied.
This was all painted. To help ensure the dial was aligned vertically, I added a washer on some thread to act as a plumb.
I’d noted in my original post on this that getting it perfectly level wasn’t terribly important, but this was for one that was permanently mounted. Since this one will obviously be set up and taken down, ensuring it’s level each time is important.
At this point, the instrument is essentially finished. I’ll still need to mount it, but for now, a test drive. I took it out and simply holding the instrument, I held it to level and rotated the central disc and sure enough, you can clearly see the shadow of the pointer hand on the other piece. (Sorry it’s out of focus. Getting it aimed, level, and taking a picture at the same time was quite hard.)
You can’t see where it’s aimed from this picture, but this was the result:
It’s between 70º-71º. Pulling up the image data, it was taken at 12:13pm today (June 18, 2018). Checking the altitude of the sun in Stellarium at that time for here in St. Louis, the correct altitude was 71.6º, so I was off by about 1º. This can easily be attributed to not having the piece mounted and level. The plumb was swaying rather considerably from my motion, but overall, I’d call this a pretty good success thus far.
Since the solstice comes up in 3 days, I have till then to get it mounted!