Re: Design challenge
John Davis [EMAIL PROTECTED] writes: I have a question/challenge to all you sundial designers: what is the most accurate design for a Standard Time dial? ... As a starter, the Singleton dial recently discussed here would seem to be a reasonable candidate. It's main limitation, common to all dials which incorporate an EoT correction, is that it is drawn for a some MEAN EoT curve, and no allowance is made for the leap year cycle and the other minor variations. Is there some geometry of dial plate and style which minimises the time error caused by small year-to-year variations in the mean daily declination? If this is achieved, then the small change in the EoT over a single day may be allowed for. The maximum rate of change of the EoT is about 30 sec/day toward the end of December. Averaging over leap years can be done to make the chart wrong by at most half a day, or 15 sec. The diameter of the sun is 0.5 degree, or 120 sec of time. Before you worry about the leap year problem, you first need to find a way to locate the center of the shadow edge 8 times more precisely than the degree to which it is smeared. We (e.g., John Carmichael and I) have discussed here some designs which might be capable of this accuracy, but they tend to be a bit hard to use. If you insist, one possibility is a camera obscura with a slit (ideally oriented parallel to the Earth's axis). This gives a sharp line image of the sun, which can be used to read the time from a series of date lines like we have been discussing. If you're really worried about leap years, you can pile four years' worth of dates on top of each other. The other approach advocated by some, namely determining the EoT directly from the declination, rather than the date, will always suffer near the equinoxes. For example, if you determine that the declination is 23 deg 11 arcmin +/- 15 arcmin, the EoT can vary over a range of 11 minutes! --Art Carlson
Re: Design challenge
John My recent postings relate to this question. The leap year is not relavent in the use of an analemma for reading standard time. The leap year is an adjustment to keep the number of rotations of the earth in synch with the revolution about the sun. The reading of standard time using an analemma should make use of the declination of the sun and that is completely independant of issues relating to the rotation of the earth. The difference in right ascension of the sun and of the mean sun (the EoT) is independant of the rotation of the earth on its axis and is only dependant upon the revolution of the earth about the sun. The EoT has meaning if there were no rotation of the earth or an arbitary rate of rotation for the earth. The mean sun is a construct that can be defined completely independantly of the rotation of the earth. Once the mean sun has been defined it may be used to measure the rate of rotation of the earth and of the position of Greenich meridian as a function of the mean sun time. I would suggest that a spherical dial is the most accurate as the reading of the time is as accurate at noon as at any other hour of the day. If the Singleton dial uses an analemma based upon a mean EoT then it is date related and not declination related. Per my arguments this analemma is not correctly designed to be accurate and invarient over a period of years. If the mean EoT is the same as the declination related analemma then the word mean can be removed and it will be accurate over a period of years. Dan Wenger Hi all, I have a question/challenge to all you sundial designers: what is the most accurate design for a Standard Time dial? The reason behind the question is to find a way to stop members of the public looking at a public dial, inspecting their watches, and concluding that dials never tell the right time! The criteria for the dial are, in my opinion: a) it should tell Standard Time, (or possibly Daylight Saving Time - BST in the UK) b) it should be in a fixed location c) it must have no moving parts (which rules out adjustable equatorials and changeable gnomons etc) and should be as physically robust as possible. d) it must not require reference to a separate table or computer program eg to get an exact declination for the sun. All data must be built into the dial plate. e) the accuracy should be interpreted as the mean error for the hour lines 3 hours either side of noon (or 12:00) for the years 2000 to 2050. As a starter, the Singleton dial recently discussed here would seem to be a reasonable candidate. It's main limitation, common to all dials which incorporate an EoT correction, is that it is drawn for a some MEAN EoT curve, and no allowance is made for the leap year cycle and the other minor variations. Is there some geometry of dial plate and style which minimises the time error caused by small year-to-year variations in the mean daily declination? If this is achieved, then the small change in the EoT over a single day may be allowed for. There is no prize for the competition, but I promise I will build a physical example of the best suggestion, and share it with the list! Happy designing, John -- Dr J R Davis Flowton, UK 52.08N, 1.043E email: [EMAIL PROTECTED] Daniel Lee Wenger Santa Cruz, CA [EMAIL PROTECTED] http://wengersundial.com http://wengersundial.com/wengerfamily
Design challenge
Hi all, I have a question/challenge to all you sundial designers: what is the most accurate design for a Standard Time dial? The reason behind the question is to find a way to stop members of the public looking at a public dial, inspecting their watches, and concluding that dials never tell the right time! The criteria for the dial are, in my opinion: a) it should tell Standard Time, (or possibly Daylight Saving Time - BST in the UK) b) it should be in a fixed location c) it must have no moving parts (which rules out adjustable equatorials and changeable gnomons etc) and should be as physically robust as possible. d) it must not require reference to a separate table or computer program eg to get an exact declination for the sun. All data must be built into the dial plate. e) the accuracy should be interpreted as the mean error for the hour lines 3 hours either side of noon (or 12:00) for the years 2000 to 2050. As a starter, the Singleton dial recently discussed here would seem to be a reasonable candidate. It's main limitation, common to all dials which incorporate an EoT correction, is that it is drawn for a some MEAN EoT curve, and no allowance is made for the leap year cycle and the other minor variations. Is there some geometry of dial plate and style which minimises the time error caused by small year-to-year variations in the mean daily declination? If this is achieved, then the small change in the EoT over a single day may be allowed for. There is no prize for the competition, but I promise I will build a physical example of the best suggestion, and share it with the list! Happy designing, John -- Dr J R Davis Flowton, UK 52.08N, 1.043E email: [EMAIL PROTECTED]
Re: Design challenge
In reply to John Davis: I have a question/challenge to all you sundial designers: what is the most accurate design for a Standard Time dial? My vote is of course for a dial with the EOT built into the hour lines to give the annalema shapes such as used in the Swensen Sun dial at : http://www.uwrf.edu/sundial/welcome.html The criteria for the dial are, in my opinion (John Davis's): a) it should tell Standard Time, (or possibly Daylight Saving Time - BST in the UK) It does. b) it should be in a fixed location It is:-) c) it must have no moving parts (which rules out adjustable equatorials and changeable gnomons etc) and should be as physically robust as possible. It is. d) it must not require reference to a separate table or computer program eg to get an exact declination for the sun. It does not. All data must be built into the dial plate. e) the accuracy should be interpreted as the mean error for the hour lines 3 hours either side of noon (or 12:00) for the years 2000 to 2050. Its less than a minute except for the 2 weeks about the Winter Solstice at which I would put the reading uncertainty to about 3 minutes. As a starter, the Singleton dial recently discussed here would seem to be a reasonable candidate. It's main limitation, common to all dials which incorporate an EoT correction, is that it is drawn for a some MEAN EoT curve, and no allowance is made for the leap year cycle and the other minor variations. This is not so. If the annalemmas are used and the sun's declination incorporated so that the time is read by a fixed point on the gnomon, (the end in the case of the Swensen Dial), no problem occurs at the leap year. It is only when dates are used to determine the correction that there is a jump at the leap year. Cheers, John Professor John P.G.Shepherd Physics Department University of Wisconsin-River Falls 410 S. 3rd. St. River Falls,WI 54022 Phone (715)-425-3196, eve. (715)-425-6203 Fax (715)-425-0652 44.88 degrees N, 92.71 degrees W.
