Polaris, the North Star, is often suggested for sundial alignment. This fairly bright star is pretty close to north, perhaps a half a degree off the polar axis, generally available in the northern hemisphere and easy to find in the sky. The altitude of Polaris also gives a pretty good indication of your latitude. All things considered then, Polaris is a useful celestial object. It comes highly recommended for Boy Scouts finding their way home in the dark, sailors navigating lifeboats, downed pilots trying to survive and subscribers to this list for aligning sundials.
Having tried polar alignment a few times, I would like to offer some cautions on the utility, precision and accuracy of this technique. In practice, it does not work very well for sundial alignment. For the typical horizontal garden variety dial, polar alignment can be accomplished by sighting Polaris along the gnomon. The accuracy is usually sufficient if you can see the Polaris. Generally you can't. Since the dial is on the sunny side of the garden, there is usually a building or tree in the way. For vertical dials, the building is always in the way. For vertical dials, you have to determine the wall declination long before you come to mount the dial. For laying out the north axis of analemmatic dials, accuracy is important. The larger size magnifies the errors. Over the typical 2 meter semi diameter of an analemmatic dial, an error of half a degree is an offset of almost a couple of centimetres on the hour circle. To do better that this, you have to catch Polaris when it transits the zenith, either the upper or lower culmination, when it is true north. This occurs at some time every night. Right. These nights Polaris transits at about 4 AM. I don't hang around public parks or school yards at 4 AM. Other times of the year are better but dark, convenient times to do this are surprisingly rare. Precision is also important and hard to achieve. You have to reduce the azimuth of sight to a line on the ground. The best way to do this is with a transit or theodolite but unless Fernando is willing to lend his prize possession, we have to use other techniques. A pair of plumb bobs can be used to reduce the sight to the horizontal plan. The vertical string of one is lined up with Polaris, the other with your eye to bring your eye position to the ground. Lying on the ground helps. Mark on the horizontal plane the position of the plumb bobs when your eye and the two strings are aligned with Polaris an culmination. This can give you a precise and accurate polar alignment. In researching this topic I was surprised to see how outdated my reference materials were. Precession has a pronounced effect on the Right Ascension of Polaris. In 50 years the RA has increased by 45 minutes of time or 12 degrees of longitude! Polaris is so close to the pole, a small change in position causes a large change RA. The old rules of thumb, such as drawing a line from the big dipper to Cassiopeia don't work any more. Even a line though the end of the handle of the big dipper and the last star of the W is outdated. A better line these days is through Kochab, the bright star in the bowl of the little dipper. When this line is vertical, Polaris is true north. The Nautical and Astronomical Almanac have detailed work sheets to reduce Polaris sights but who has these at hand these days. A planetarium program like "Earth Centred Universe" will give you the transit time as well as the RA and Declination. It is shareware and the demo is readily available to download from David Lane's site http://www.nova-astro.com/ I used ECU to calculate the effects of precession on Polaris. The results are shown on the attached pdf file. Note the pronounced change in the RA with time and see how the rate increases as the polar distance diminishes. This calculation also shows that things are getting better. Polaris is getting closer to the polar axis, from ~44 arc minutes now to 28 in about 2140. I cannot vouch for these calculations or my interpretation of them but they do have a good source. The "Earth Centred Universe" by David Lane uses the "Astronomical Algorithms" by Jean Meeus, in particular the optional software by Jeffrey Sax that accompanies the book. My other references are "Explanatory Supplement to the Astronomical Almanac" by the USNO, "American Practical Navigator" originally by Nathaniel Bowditch, and "Survival" Air Force Manual 64-5. The latter should be subtitled "101 Uses for Parachute Cord. I hope this is useful for any of you interested in polar alignment of sundials. Personally I prefer to use solar noon and a shadow stick. Roger Bailey Walking Shadow Designs N 51 W 115 Attachment converted: Macintosh HD:Polaris.pdf (PDF /CARO) (0003F1EF)
