Roger What a great letter! Lots of practical information. John
John L. Carmichael Jr. Sundial Sculptures 925 E. Foothills Dr. Tucson Arizona 85718 USA Tel: 520-696-1709 Email: [EMAIL PROTECTED] Website: <http://www.sundialsculptures.com> ----- Original Message ----- From: "Roger Bailey" <[EMAIL PROTECTED]> To: <sundial@rrz.uni-koeln.de> Sent: Tuesday, March 12, 2002 10:54 PM Subject: Polar Alignment > 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 > > -
