Friday, February 26, 2010

Fun in the sun-dials

I never know where my reading will take me. I am re-reading Lybian Sands, a book by Brigadier R.A. Bagnold, OBE who was also a Fellow of the Royal Society and author of just about the only book on sand dunes that there is: The Physics of Blown Sand and Desert Dunes, reprinted (fortunately) by Dover and easily available. Also be warned that it is very technical. Bagnold was a sapper, as the British say; a military engineer. He was bitten by the bug of desert exploration by automobile in the 1920s-1930s. In the process he invented (or reinvented) the sun compass, a device that substitutes for a magnetic compass, which is of little use in a steel automobile. There are all kinds of branches to this story, but my first thought was "oooh, I gotta build one!" It is, after all, a 360 degree protractor with a big needle in the middle. Yes, but the sun's azimuth changes during the day. so you have to adjust for that. And so I came to sundials (pushed, I may add, by an article in my favorite magazine, Model Engineer, which you may Google).

Sundials are fascinating beasts. For a long time, they were the poor (and middle class) man's only way of telling time. I have found a lot of information on the web. Not all (if any) of it is correct and complete. A lot of it is "buy our sundial" or "download our unchecked software and print out a sundial" type stuff. There are two sites which are commendable: one in the UK, the British Sundial Society,
and the other a NASA link. Now a sundial is just a disk or square with marks on it to tell you what time the sun thinks it is, so where do you put the marks?

Unfortunately, there is complete disagreement by these authorities on how you mark up a simple horizontal sundial. I have now made several sundials; they are not exactly difficult:
You could make them out of plastic, cardboard, or almost anything. Almost needless to say, I made them out of wood. The one on the left is called an equatorial sundial. when properly oriented, its axis should be parallel to the polar axis. This was inspired by the British Sundial (BSS) people. The way their project is stated, it won't work unless the rectangle is transparent. Very hard to find transparent wood, so I had to reverse it a lot. The net of it is that when the pointer is exactly facing true south, it will show solar time. There is no question about the marks: one hour is 15 degrees protractor-wise.

The other dials are all horizontal. They differ in the markings, which correspond to the various authorities. I think the authorities all all wrong. I happen to have a copy of Astronomical Algorithms, by Jan Meeus, unfortunately out of print but available used. Tomorrow's project is to make a sundial per Meeus. By the way, the little triangular piece, called a stylus by Meeus and a style by others, needs to be in proportion to the sundial. The equatorial sundial is about 14 cm across; the others are pieces of a branch and about 6 cm across.

I have found out a great deal about sundials in this week. All this will come in very handy when I build a real sun compass!

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