To All: Thanks to those of you who took time to review and comment on my sundial page, http://netnow.micron.met/~petes/sundial . I appreciated your replies.
I do want to specifically respond to Dr. Carlson's reply... he is absolutely correct in his statement that nearthe summer and winter solstices you can not simultaneously determine both the date and the time from this sundial, if both are unknown. From about 10 June to 1 July, and 10 Dec to 1 Jan, you must know the date in order to accurately tell the time, or you must know the time in order to accurately determine the date. (And actually with this sundial, at any time, you must have a general idea of the season in order to determine whether to use the left or right half of the analemma shadow to indicate the correct date and time.) However, I do disagree with his statement that you would be doing well within these periods to determine the date with an accuracy of 1 week. On the first and smaller brass sundial I built, with an 11.5" radius (an equatorial scale of 20 minutes/inch) the date may be determined to within +- 3 to 4 days during the summer solstice, to within +- 2 to 3 days during the winter solstice, and to within 1 day most other months of the year. On the much larger park sundial, with a 32.75" radius (an equatorial scale of 7 minutes/inch) I can easily determine the date to within 2 days around the summer solstice (if I know the correct time,) to within 1 day around the winter solstice, and to within 1/4 day during most other months of the year. Knowing the date within these two solstice periods, on the park sundial I can still easily read the time within 20 seconds (on my home sundial the time can be read to within the minute.) Today for example, when I checked the park sundial, I could still distinguish today's date exactly (June 7,) but I was a little disappointed to note at 1:57 this nice sunny afternoon that it was 8 seconds slow in indicating the correct time ;) My appologies if I appeared to oversell the result on my web pages. As an engineer, and not a mathematician or a physicist ;) I am usually thrilled if I am able determine a solution to any problem within 40%, and then I multiply the answer by a safety factor of 2x or more. So please forgive me, I have been ecstatic with the results from my park sundial. It will not get people to the moon, but (ignoring acts of God and the environment) it will allow them to correctly set their Timex for the next several thousand years. :) I guess I should stop now before this e-mail becomes even more pompous than I intended. I would like to attend the NASS meeting in Seattle this September, and would like to bring up my brass sundial, hear some of the presentations, see some of the other work out there, and hopefully meet a few of you I have corresponded with. Hope to see you there! Thanks again, Pete Swanstrom