Re:
Hi, all diallists ! Writing from France (this to please you excusing my under-basic English !:-)), I have the same problem than Alexei Pace : I'm asked to realise a vertical sundial on a circular tower. I don't know exactly (it's a truism !) how to proceed. This sundial will probably be built on stone, but we can't curve the stone. Then I think that it will be possible to realise like a multiface dial. What does the sundial list think about such a challenge ? Completely crazy or just a half ? Best regards (under the falling rain in Alsace :-(( Alain MORY Alexei Pace a écrit: Hello If you design a vertical south dial , and 'wrap' it around a south facing gnomon on a cylinder,,, will it work? I would like to know how one may design a sundial on say a cylindrical buildingyou cannot simply turn the building around like as in a cylindrical/sheperd's dial! The one at Disney was of this form... but I dont knwo what type of dial is that thanks for any help alexei pace
GPS and sundialling
Hello, sundiallers ! One crazy question isn't enough ! A second one came me in brain (or the thing I use as ;-)) How can a diallist use a GPS receiver to mesure the declination of a wall ? I asked me this question just some minutes before the buing of such an apparatus ! It's very practical for walking, but can this machine have an utility on sundialling ? Best regards Alain MORY 47°N 7°E 500 m
Re: Declination Table
Daniel Lee Wenger [EMAIL PROTECTED] writes: The reading of standard time via a sundial may be accomplisted by mearly reading the declination of the sun and using an analemma, determining standard time. At no point is the current date needed to do this. Way, way back I explained why I was not totally satisfied with this method, essentially because there are (almost always) two values of the EoT for each value of declination. At the solstices there are even an infinite number of values (in some technical sense). Consequently, if you are interested in relating the sundial reading to clock time, you always need some knowledge of the current date. Art Carlson
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: Declination Table
Art At the solstices there is no ambiguity. The analemma intersects the tropic of cancer and the tropic of capricorn at one point so there is only one value for the EoT. At all other times of the year, except for one instant when the two paths of the analemma cross, there is only the need to know which leg of the analemma to use. This does not require knowledge of the date, only knowledge of the season. True, the date of the solstices is needed if one is to know if the sun has started north or south yet. In the spring the lower right leg is used (with the analemma viewed projected onto a sphere with the gnomon in front of the analemma, upper left if the analemma is projected onto a surface behind the gnomon), upper left in early summer, upper right in late summer and lower left in the fall. I place a small arrow on each leg of the analemma to indicate the motion of the sun during the year. This identifies to the user which leg to use. One could also label each leg with the season but there does not seem to be reason to know the date. I have attached two views of the analemma, one as projected onto a sphere and one projected onto a plane behind the gnomon. The first shows the arrows that I use, the second I have labeled with the season. Dan Wenger Daniel Lee Wenger [EMAIL PROTECTED] writes: The reading of standard time via a sundial may be accomplisted by mearly reading the declination of the sun and using an analemma, determining standard time. At no point is the current date needed to do this. Way, way back I explained why I was not totally satisfied with this method, essentially because there are (almost always) two values of the EoT for each value of declination. At the solstices there are even an infinite number of values (in some technical sense). Consequently, if you are interested in relating the sundial reading to clock time, you always need some knowledge of the current date. Art Carlson Attachment converted: Macintosh HD:analemma.arrows.gif (GIFf/GKON) (FB7A) Attachment converted: Macintosh HD:analemma.labeled.gif (GIFf/GKON) (FB7B) Daniel Lee Wenger Santa Cruz, CA [EMAIL PROTECTED] http://wengersundial.com http://wengersundial.com/wengerfamily
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
Re: Azimuthal sundials - again
Hi Steve: Your interpretation of the various dial classifications seems to be correct, although I'm certainly not an expert, as you know! Reading your letter, which was written so clearly, gave me confidence that I had interpreted the discussion correctly. Thanks, John C. Gianni wrote: The Monofilar and Bifilar sundials can be built with any kind of Time: Middle Time (Standard), Local Apparent Time, with Italic, Babylonian, Temporary hours, etc. Ah ha! I must have misunderstood the issue being discussed. I can see that in abstract terms that we have dials which are - projection of a point onto a surface (perhaps curved) - projection of a line (perhaps curved) onto a surface (perhaps curved) - projection of two lines (perhaps curved) onto a surface (perhaps curved) - other non-projection types, such as the wonderful CD-diffraction dial. If the third class is already known by common usage as Bifilar, then I accept that it makes sense to call the second type Monofilar (even though for me personally the word filar carries an implication of a wire or thread, rather than being a general term for a line or edge). I assume the first class are called Nodal. The other half of the discussion is what to call a dial with a seasonal time adjustment. I though that someone was suggesting that because the existing examples had already been called monofilar then that name applied to the adjustment feature. So a monofilar dial can be Standard, Local , or other hours Upright, polar axial (axial?), or other principal axis Horizontal, Vertical or other dial face planes So the ordinary garden dial could be called Axial Local Horizontal Monofilar, Mr.Singleton's dial is Axial Standard Horizontal Monofilar. The various forms af azimuthal dials are all Upright monofilars. The Wenger dial is a Local Spherical Nodal dial. Am I getting close? Steve
Thumbs up on Singletons!
Hello John D. In answer to your challenge, I would pick a Singleton type dial over the Swensen dial because: 1) A Swensen dial can only give accurate times on the hour. For times between the hour you need to guesstimate. 2) A Singleton time line is easier to read since the analemma is unfolded. Smaller increment time lines can easily be drawn onto a Singleton, but if more analemmas are added to a Swensen, then they begin to overlap and reading would become even more difficult. 3) Singleton sundials avoid the Swensen's analemma date confusion. 4) The Swenson's nodus has two support rods which create unnecessary and confusing shadows not present in a Singleton. 5 )Even if the Swensen dial were horizontal instead of vertical, It would be impossible to show early morning and late afternoon hours on a Swensen dial because the analemmas would be prohibitively long. A Singleton does not have this considerable limitation. 6) The only drawback I really see with a Singleton is that it suffers from time line compression on the inner rings. This problem might be solved by making the dial larger (which would also increase its precision). Thumbs up on Singletons! John Carmichael Tucson Arizona John Davis wrote: 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]
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.
Re: Declination Table
Hello Arthur, While it is true that at a given solar declination one could mistakenly read the wrong side of an analemma it is quite simple to add guide arrows along the analemma to indicate the direction the sun is currently traveling, e.g., with the sun in the winter months the analemma is read along the portion indicated by the first set of rising arrows (sun). In fact, this is exactly what Dan has incorporated into his dial, quite beautifully too. I chose a somewhat different approach, I use colors to break the analemma into its respective seasonal segments (see my WWW sundial generator). Most people know what season it is (we hope!) and that is enough to make the correct reading, i.e., knowing the date is not necessary. Regarding accurate mean time reading during the period of the solstices (when the rate of solar declination is slowest) I have found that projecting an image of the solar disk onto the analemma is best (moving parts though). Best, Luke Coletti Arthur Carlson wrote: Daniel Lee Wenger [EMAIL PROTECTED] writes: The reading of standard time via a sundial may be accomplisted by mearly reading the declination of the sun and using an analemma, determining standard time. At no point is the current date needed to do this. Way, way back I explained why I was not totally satisfied with this method, essentially because there are (almost always) two values of the EoT for each value of declination. At the solstices there are even an infinite number of values (in some technical sense). Consequently, if you are interested in relating the sundial reading to clock time, you always need some knowledge of the current date. Art Carlson
Re: GPS and sundialling
At 19:36 1-3-00 +0100, you wrote: -Original Message/Oorspronkelijk bericht-- How can a diallist use a GPS receiver to mesure the declination of a wall ? if it is a very long straight wall you can walk along the wall and find out the course of this movement. Then deduct or add 90° (depends on the direction you walked. A civil GPS is not accurate enough I think for a short wall. A DGPS however should do the trick also for a short wall. but can this machine have an utility on sundialling ? You get at least a pretty good position on your screen for the sundial - Thibaud Taudin-Chabot 52°18'19.85 North 04°51'09.45 East home email: [EMAIL PROTECTED] (attachments max. 500kB; for larger attachments contact me first)
