Tuesday, September 23, 2014

Solar calendar

Yesterday was the equinox, equal day and night. It's not really equal day and night for a couple of reasons. One, daylight is generally considered to begin when the first part of the disk of the sun is visible above the horizon, and to end when the last of the disk is no longer visible. Whereas, the equinox refers to the center of the disk of the sun. Two, the curvature of our atmosphere causes refraction that allows us to see the sun before it is actually above the horizon, but I digress.

For some time I have been thinking about building a device that is kind of the equivalent of a sun dial, but instead of telling the time it would tell the date. We all realize that the fact that the earth is tilted relative to the plane of the ecliptic gives us our seasons. As the earth moves around the sun that tilt causes the sun to appear higher in the sky at some times of the year than at others. I wanted to use the changing height of the sun, and thus the change in the location of the shadows that it casts, to mark the passage through the year. It would work kind of like this.
I also wanted it big enough so that the day-to-day changes would be observable as a way of connecting with the movement of the earth through space.

As it turns out I have a wall in the courtyard of my home in Florida that was crying out for some decoration, So here it is.

My plan was to cut pieces of wood in short arcs and assemble them end-to-end. I was discussing the project with my friend Jim Nemec, who is, among other things, a boat builder. He allowed as how that was the incorrect way to do it, and that I should cut thin strips and laminate them into a curve. As it turns out, he was correct.
I bought a piece of mahogany and ripped it into strips a little over five feet long.

Then I cut some particle board to form an arc of a circle with a ten foot diameter, the arc being a little longer than my mahogany strips, and screwed the particle board to my workbench.

Jim and I then slathered a bunch of epoxy onto the strips and clamped them to the form.

The next day I popped it off, sanded it and trimmed it.

Then I finished it with boat varnish.

As luck would have it, I finished it just in time because, as I said earlier, yesterday was the autumnal equinox (in the northern hemisphere or the vernal equinox in the southern hemisphere). That was important because I want the equinoxes to be exactly in the middle of my calender. Thus, I had to note the position of the shadow yesterday, and mount it such that it fell right in the middle, and such that the brass rod that I attached to the wall was at the center of the circle determined by the arc of the calendar.

Getting it in the exact position, both technically and aesthetically was no mean feat and required the assistance of my brother John and his wife Doreen, holding...looking...holding...marking.... But it's up, and here it is.

I have a measuring tape on the calendar so I can record the position of the shadow each day. I'm not sure how I'm going to mark the days on the calendar, But I have a year of recording before I need to figure that out. I'll report back next September.


  1. It's an intriguing idea. I'm wondering if the pointy thing sticking up on the sun dial, the gnomon, is so long to compensate for the tilting of the earth, which produces the seasons. I probably should have looked it up before I wrote. One would think that if if were possible to make a sun calendar, there would be a lot of them, because it seems like a useful thing. Maybe it takes a bunch of big rocks, like Stonehenge. Again, I should probably have looked it up before I wrote.

    1. I would say that the length of the gnomon more determines the size of the sundial because it determines the length of the shadow cast. The gnomon is meant to be parallel to the spin axis of the earth or, equivalently, equal to the latitude of the sundial. This is, of course, just an approximation because it is only perpendicular to the sun's rays when the sun is over the equator, i.e. at the equinoxes.

      For my solar calendar, the length of the brass rod, simply determines the period of time each day during which the calendar may be read. For mine that is about twenty minutes either side of local solar noon.

      I claim no rights of invention since I am quite confident that any agricultural society living outside of the tropics had an analogous device that they used to determine when to sow crops.

      While it lacks to portability of a smartphone app there is no long term contract.