Sundials

A year or so ago I bought a book called Ornamental Casting by Robert Whitmoyer. One of the projects in the book is a sundial. He shows how to cast them, but he doesn't delve into how they are layed out or used. I thought it would be neat to make a sundial for myself, so I added this to my huge list of projects.

Then I was asked to do a presentation on casting metals in a home shop. (The presentation will take place in early April 2003.) I wanted to make something that people would find interesting, so I decided to cast a sundial. The first thing I had to do was to find out how to lay one out. The numbers I mean. I already knew how to lay out common geometric shapes.

The graphic above shows the layout for one of the sundials that I made. (This is not the first one that I made, but I had designed it in a CAD program, so I thought I'd use it here.) To lay out the positions for the numbers I had to determine the angle between the 12 o'clock mark and each hour. In the graphic the angle θ is the angle between the 12 o'clock and 11 o'clock marks. This angle is found using this formula:

θ = tan-1(tan t * sin Φ)

In the equation t is the time in degrees and Φ is the latitude in degrees. (The equation can also be used with radian measurements for the angles.) The latitude is the easy one. I live at roughly 41° North latitude, so Φ is 41°. The time is found by multiplying the number of hours between 12 o'clock and the time being calculated times 15°. So for 11 o'clock t is 15°, 10 o'clock is 30°, etc. Half-hours are found by using 7½° instead of 15°. For 11 o'clock the equation would look like this:

θ = tan-1(tan 15° * sin 41°) ≈ 9.97° ≈ 9° 58"

This is also the angle between 12 o'clock and 1 o'clock, so only one side of the sundial must be calculated. But there is one caveat. This will result in a sundial that is corrected for latitude only. For a sundial to be accurate it must be corrected for the longitude also. This is beyond the scope of this page though. The sundials will be off by 18 minutes (they will read 11:42 am at 12 noon) as I've made them.

This is the first sundial that I made. Well, actually it's the fourth try with this design. It is a little over 11" in diameter, the outer edge is ½" thick, and the recessed area is 3/8" thick. The thingie that sticks up is called a style, and its angle is equal to the latitude. The front surface of the style intersects the center of all the layout lines. I sanded the tops of the letters and the outer edge, and my girlfriend stained the recessed area and the sides and back of the style.

When one of my friends saw the sundial, she said she was looking for something like that for another couple. So I decided to tinker with some different shapes. Here are some pictures of the pattern and the finished sundial:

The pattern is an equilateral triangle with sides 12" long. The back is 1/4" thick hardboard and the raised center portion is 1/16" thick balsa wood. I cut the letters out of the balsa wood, and they are 1/2" tall. I tried several different things to make hour marks, and I found an interesting solution. They are eyes that are glued onto small crafts, like the ones children make.

The brown stain on the sundial is something that has me perplexed. My old molding sand lost its bond, so I had to make new molding sand. I used the same recipe, but everything I cast has this discoloration. I've been able to remove it with a stainless steel wire brush, so it's not a big problem.

After I made this sundial I thought that the other couple (the Porters) would probably want to give one to the first couple (the Foors), and here it is:

So far I've had several people ask me to make a sundial for them and the triangular one is the most popular. I still plan to make one for myself, but I need to find an interesting shape. Feel free to make suggestions using the e-mail link below.

##### Updated January 23, 2003

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