Friday, March 12, 2010

The sundial strikes again

Once you start making sundials, it seems hard to stop. I have made over a dozen now. Unfortunately, I made a mistake programming up the formulas for the angles, taken from Meeus's book. I went to make a vertical sundial and discovered my mistake. The markings are a function of your latitude, the hour angle (angle made between the sun's meridian and yours) and the declination (or, for our purposes, latitude) of the sun. I put the sun on the equator. There are adjustable sundials, but they are complex.

Here's this week's crop of sundials:

The sundial on the right is a vertical sundial. The one closest to you in the picture is a horizontal sundial, the most common kind. The other two are equatorials, one fixed, the other in a trunnion mount, so it can actually be adjusted for latitude. In order to cut the markings on these, I made a burin, an engraver's tool, out of a broken drill bit. It has made the marking process much simpler. Much better than my bicycle-spoke burin. That is because it does not flex. Burins are relatively cheap on the internet, but why buy anything you can make?

The equatorials all have to point south - true south, not magnetic. Sure enough, I set my sundials with a compass and applied the magnetic declination the wrong way (blush), I am not the only navigator ever to do that, however.

There is a sure-fire way to find true south. If you go to the U.S. Naval Observatory, and click on "One day's ..." you will find a form that allows you to get sunrise, sunset, and transit times for the sun, at your location. The transit time is when the sun is on your meridian; so wait for that time, and then turn the sundial till the shadow falls on the "12" mark. It's aligned.

Sundials are fascinating. I have deviated from my original intention of building a sun compass; but this is all good practice. Now that I have my new burin, I can even carve latin mottoes on my sundials: carpe diem, nihil novum sub sole, tempus fugit, and the rest!

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