Re: Dodwell Dial
Dear Bill, Luke, Edley, et al., Is there something wrong with the list? It seems that Edley is responding to a message about a Dodwell dial, but I never received it. My apologies. As Edley noted in a later not , I sent a direct copy to Edly, and then inadvertently sent my posting to the list to our tolerant host Daniel Roth. And thanks to Edley for sending the images on--but my apologies to Robert and others for my now having compounded the vice by sending the original with images to the list in the meantime. I tried to compress the jpegs as much as possible to minimize any inconvenience. If anyone wishes to have a larger image with better resolution, I can send it direct. Peter = Peter Mayer [EMAIL PROTECTED] Politics Department tel: +91-8-8303-5606/ +91-8-8303-5610 University of Adelaidefax: +91-8-8303-3446 Adelaide, SA 5005 Australia =
RE: Query about solstices
Surely the whole business of finding the solstice is like a slower analogue of finding noon by observing solar altitude? Just around noon the altitude changes extremely slowly, and it is hardly possible to judge the time of maximum altitude exactly. Here, today, noon is at 12:04:49 but even if we take 20 minutes before and after then the Sun will only be 8 minutes of arc lower. 1/365 of a day is about 4 minutes, and that interval either side of noon will see the Sun lower than its noon altitude by roughly 8 * (4/20)^2 = 0.3 minutes. I am neglecting change in declination, which is insignificant so near the solstice. Hence the benefit of taking altitude sights at known times some significant time either side of noon and calculating the instant of noon from those. Even if the times are only known by a clock which may not be correct, the result is noon as shown by that clock, which can thus be corrected. Conversely, if the clock is trusted, the longitude can be found. In general, one would allow for the rate of declination change for the day and its effect on the two readings. Similarly, by analogy, we see the advantage of taking declination readings (by whatever means) on noted dates a few days either side of the solstice. Andrew James
Attachments, Pictures
Dear Membership, Pardon if this is a repeat. My mail has had a couple of hiccups. Yes, I will send no attachments. Yes, I'll post pictorial or lengthy content on one of my websites for a while, or send it directly to those requesting it without copying the info to the list. Bear in mind that I may keep the data/pictures myself for only a very limited time. Yes, I do start some conversations on my own, hoping to learn things from others more experienced than myself, or to pass on something I believe may be of interest to the membership. Yes, I do appreciate hearing from any or all of you that wish to write to me. If others have no site to post their pictures on, If you send them to me as attachments and ask me to post them on one of my sites I will do so and send you an email of where they are at. I can only keep large files, unconnected to my main files, onsite for a few days though. My chosen email program, Pegasus, under File, selective mail download, lets you download just the headers then choose which files to download and which to erase from your ISP. It has saved me the time and expense of large files since I too have a dial up account. Thank all of you for your patience and help! Edley McKnight [43.126N 123.327W]
accuracy
Message text written by walter.jonckheere I have a feeling 1 second could be obtained The ability of a conventional sundial to show accurate time is usually compromised by the half degree angle subtended at the earth by the sun's disc. This makes shadows fuzzy and it makes it very difficult to judge the position of the shadow of any edge - like one of the two styles of a thick gnomon of a horizontal dial. It is true that sundials of the conventional horizontal type should have a noon gap (and hence show time using two meridians) and indeed all well made conventional horizontal dials do have this. If one is considering the design of a large dial this fuzziness of the shadow has to be considered. It is usually better therefore to use a cylindrical rod or tube as the gnomon since then time may be better judged from the centre of the shadow. It is not very difficult to get a dial set up to read fairly accurately. One of which I have experience is the dial I designed recently for Amble town square (Amble is on the East coast of Northern England) This dial is about 12m across. The gnomon is very close to being in the right position - the (local apparent) noon shadow on a vertical surface being 'out' by about 75 mm at a distance of around 30m from the dial centre (figs from memory so are approx). However the evening before the opening ceremony I photographed the dial showing time to within 15secs. You can see the dial and this particular photo on http://ourworld.compuserve.com/homepages/Patrick_Powers/amble.htm and clicking to see the opening ceremony shots. For those who might doubt that one can see a movement of the shadow in 15 secs I have another photo not on the web site to show what it looked like 15 secs later. I can up load that to the website if there should be any interest.. Despite this euphoria one must remember that his was one measurement at one specific hour and on one specific day of the year. Of course all the other various and inevitable misalignments present in such a large dial will ensure that it will not necessarily be as accurate as that on other days of the year or even at other times on the same day. I am afraid that in the case of the Amble dial I have not been able to make any long term study of its accuracy. In any case, those local vandals who climb the gnomon on occasion may already have made some adjustments of their own! Patrick
Re: attachments
those attachments anyhow. Thibaud Chabot At 16:25 19-12-2001 -0500, you wrote: I think attachments (less than 200K) are fine. The recipient can make the choice whether to download them or not. This is a useful way to share ideas. Bill Gottesman - Th. Taudin Chabot, home email: [EMAIL PROTECTED]
Re: accuracy
Patrick powers is correct in noting that the 0.5 degree width of the sun (corresponding to 2 minutes of time) creates a shadow penumbra that is virtually impossible to read to the second. But, a sharp edge can be achieved by a focusing dial, which creates an crisply defined image of the sun, 2 minutes of time wide, with no penumbra. If the leading, or trailing edge of this image is chosen as the indicator, then it would be possible to read it to a few seconds, and perhaps to 1 second, on a large dial scale. Alignment becomes the limiting factor for such a dial. A focusing dial that is accurate within 60 seconds all day long (and oftern within 30 seconds) can be seen at www.precisionsundials.com/renaissance.htm Bill Gottesman Burlington, VT In a message dated 12/20/2001 5:03:43 AM Eastern Standard Time, [EMAIL PROTECTED] writes: Subj: accuracy Date:12/20/2001 5:03:43 AM Eastern Standard Time Greetings to all We all know that the atomic clock has the highest possible resolution, while for sundials 2 to 1 minute seems to be the best achievement. I have a feeling 1 second could be obtained considering what follows. ( I consider a second ideal as one can feel it, I mean it is a timespan well related to the human body, one second you live, the next you may be dead; it is also somewhat related to the heartbeat) Consider a horizontal sundial whith the 15 degrees hour lines; it is impossible to trace minute lines when keeping usual dimensions for the dial. However, if we go far away from the base point of the gnomon, the portion of the arc between 2 hours, becomes larger; so imagine we go to where 60 lines can be drawn between two adjacent minute lines and we keep only the top part of the line 11h59m59s and try to draw it on the ground related to a very accurate positioning of the local meridian. These data allow also to calculate the height of the gnomon tip. If both dimensions are feasable to realise, we would have the desired accuracy of one second; of course the positioning of the meridian is of paramount importance and probably the most difficult to materialise. ( is this a nice subject for a university ?) Of some important influence is the width of the gnomon. The indication of noon time is never accurate because of this width, while the shadow cast for the earlier hours, may be considered as a hairline and is thus more accurate, which means that the morning part of the dial is more accurate if the shadow hairline is taken into account for determination of the meridian. So in fact, sundials should have a gap at noon corresponding to the width of the gnomon, and the afternoon part should be calculated in function of the right hairline shadow related to a second parallel meridian. Very interested to read your comments Walter
Accuracy - Attachments
Hi Walter, It is not possible to increase the dimension of a sundial to increase the precision of the reading and to reach the possibility to read intervals of time very small. The reason is in the diameter of the Sun, because of which when the style goes away from the (horizontal) plane its shadow becomes more and more fuzzy and uncertain (penumbra) One can find easily that in a horizontal sundial for the latitude of 45 d., at noon, in the equinoxes, the width of the shadow of an infinitely thin style, is equal about to the movement of the same shadow in 1 minute (without optical systems) For hours different from noon, the distance between the extremity of the stylus and the shadow increases, provoking an increase of the dimension of the shadow itself. It is this the reason that prevents the construction of sundials with a precisions superior to around 1 m. With some artifices (shaped holes, shadow sharpeners, etc.) we can reach also a better precision ( maximum 10-20 sec) +++ For the problem of the attachments I agree completely with Mac Oglesby Often it is necessary to show an image or a sketch and we cannot wait, before showing it, to put it in a web site (moreover not all have an URL ! ) and, after to write to the list. If I don't mistake, a message longer then 20 or 24 kb is rejected by the Majordomo: this rule of the 20-24 kb must then be accepted by everybody, also from those people that don't have interest for these attachments. Gianni Ferrari Lat.44o 39' N Long. 10o 55' E
Re: accuracy
I consider a second ideal as one can feel it, I mean it is a timespan well related to the human body, one second you live, the next you may be dead; Walter, This is very strange statement. True that you live one second and are dead the next, but by the time that happens it's a bit late to start measuring and assessing the time span! Moreover you only get one chance, which isn't much good to get a feeling for it. Nonetheless I understand what you mean... Regards peter Tandy
Ceiling Sundial
Hello Sundial Friends: I am planning on making a ceiling sundial (as a project with my children in an actual science/math application). I have the tiny lipstick mirror mounted at my window frame (half way up), and was told that the easiest way (void of all the mathematical calculations that they would not understand) is to mark each hour in the day during the winter and summer solstices (12/21 and 06/21) -- then connect the dots. Granted this process takes six months to complete, but it would ensure that all longitude corrections are incorporated, effortlessly. Any opinion or comments today, from anyone who has done one, would be quite helpful, (as this process is to start tomorrow)! Thanks! J Romano
Giant Sundial for Britain...
Hi all, A report in today's Daily Telegraph newspaper in the UK, says that there are plans to build a 132 feet tall sundial in the English Midlands. It will consist of3 giant sails of stainless steel erected in the shape of a pyramidwith shadows thrown by two of the sails on the third allowing visitors to tell the time...lights and lasers will allow it ...to work in overcast conditions and at night. The artist's impression accompanying the article shows 3 polygonal sloping structures, one in the centre of a circular area like a giant gnomon (though the rear polygonal facets suggest it is not a planar piece of steel), and 2 facing it around the periphery at about 11.00 and 1.00o'clock. I can't really see how it will work, at least not in the sense(s) alluded to above...but maybe we must wait and see. Regards and best wishes for Christmas and the new Year Peter Tandy London
On attached images
I do have a slw connection and an ooold computer but, AS FAR AS I AM CONCERNED, please keep sending SMALL images: they are nice and sometimes MUCH more explanatory than a whole paragraph. But, well, maybe Robert is right and it would be a politer idea to post the URL when writing to the whole list. Best regards, Anselmo [ 41.63 N 4.73 W ] AS FAR AS I AM CONCERNED, PLEASE DO NOT SEND ANY ATTACHMENTS! It is rude and not very considerate to people who have to download their e-mails through a slow telephone connection. If you have images that you feel you have to share with the rest of the world, put them on your website and post the URL's to the list. Then everyone can decide for him- or herself whether they want to download them or not.
accuracy attachments
Hello again, thank you for all for the reactions, but what is wrong with my feeling about a second, when I say you can feel it , I mean of course you can count in seconds not in milli- or nano- seconds. I had thought about the sharpness of the shadow, but forgotten to mention it. Considering the center of a shadow of a thick gnomon I do not like, it is to subjective - your eyesight angle of view may be different as to another person. But what about the reverse, instead of a shadow use the light. This was used by clockmakers of the past for adjustment of their watches. They used a horizontal dial the gnomon was a small disc with a pierced small hole positioned according the local latitude looked only at noon to the spot thrown on the dial. (as you maybe discovered my interest in sundials is in relation with mechanical clocks or watches). So, why not with the aid of modern optics, obtain this needelpoint of light, the sun is needed in either case, shadow or light;Again, very interested in your comments. ( and also, as said, a university for this study would be nice, no?) Now on attachments, I am a bit surprised by the comments I read, what is prohibitive about pictures ? If it is the price of the connection-time, my opinion is, forget your PC use the conventional method offered by the postal services, you will spend money in either case as you know, the speed is uncompatible between the two. I started with a 56K modem, after that idsn, now I have ADSL, fast indifferent to your connection time which may be 24/on 24, the price remains the same, in my country all providers are constantly lowering their prices. As to the danger of a virus enclosed in an attachment, you have to live with it trust the anti-virus programs, which you have to update often. I personally like pictures in a mail as insertions, use the insertion facility often for drawings taken by my digital camera. So long, Walter 50.42.1 north4.33.46 east
MC
A very merry Christmas to all the diallists! John Schilke
RE: accuracy
Dear Walter, Greetings. There is another factor which limits sundial accuracy to about +/-22 seconds. This is the variation in the value of Equation of Time from year to year within a leap cycle. See www.sunlitdesign.com/infosearch/sundialaccuracy.htm David www.sunlitdesign.com -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of walter.jonckheere Sent: Thursday, 20 December 2001 8:47 PM To: sundial@rrz.uni-koeln.de Subject: accuracy Greetings to all We all know that the atomic clock has the highest possible resolution, while for sundials 2 to 1 minute seems to be the best achievement. I have a feeling 1 second could be obtained considering what follows. ( I consider a second ideal as one can feel it, I mean it is a timespan well related to the human body, one second you live, the next you may be dead; it is also somewhat related to the heartbeat) Consider a horizontal sundial whith the 15 degrees hour lines; it is impossible to trace minute lines when keeping usual dimensions for the dial. However, if we go far away from the base point of the gnomon, the portion of the arc between 2 hours, becomes larger; so imagine we go to where 60 lines can be drawn between two adjacent minute lines and we keep only the top part of the line 11h59m59s and try to draw it on the ground related to a very accurate positioning of the local meridian. These data allow also to calculate the height of the gnomon tip. If both dimensions are feasable to realise, we would have the desired accuracy of one second; of course the positioning of the meridian is of paramount importance and probably the most difficult to materialise. ( is this a nice subject for a university ?) Of some important influence is the width of the gnomon. The indication of noon time is never accurate because of this width, while the shadow cast for the earlier hours, may be considered as a hairline and is thus more accurate, which means that the morning part of the dial is more accurate if the shadow hairline is taken into account for determination of the meridian. So in fact, sundials should have a gap at noon corresponding to the width of the gnomon, and the afternoon part should be calculated in function of the right hairline shadow related to a second parallel meridian. Very interested to read your comments Walter
RE: accuracy
At 08:25 AM 12/21/01 +1100, David Pratten wrote: Dear Walter, Greetings. There is another factor which limits sundial accuracy to about +/-22 seconds. This is the variation in the value of Equation of Time from year to year within a leap cycle. See www.sunlitdesign.com/infosearch/sundialaccuracy.htm David and others, This might be true if the equation-of-time correction is obtained from a look-up table by date. But if the sundial includes the EoT correction in the shapes of its curves, or in the shape of the gnomon, the declination of the Sun is what controls the value of the correction being applied. In that case, I think the error would be *much* smaller than +/- 22 seconds. -- Roger
RE: Query about solstices
John's question is a good one. Accurate determination of the solstice would have been difficult for ancient cultures due to the low rate of change of the declination and sunrise azimuth. The techniques suggested will work but they require repeated observations under difficult conditions. Clear skies to the horizon are rare these days. But they did it! With stone circles, tunnels, and observation monuments, our forbearers fixed the day of the winter solstice. Determining that the sun had stopped its descent and had started to return with its life giving light, heat and energy was a critical event in most cultures. It is unfortunate that this has been lost in our modern cultures although solstice festivals still exist. How many realize that the lights, stars, trees, greenery, etc are all cultural residues of solstice celebrations. . We have gotten away from the real and significant event, start of the new solar year and now focus on festivals that have co-opted the theme. The constants in all these festivals are light, renewal, salvation, rebirth, whether it is Christmas, New Years, Epiphany, Saturnalia, Sol Invictus, St Lucia, Chanukah, Mother Earth etc. All celebrate the return of the sun. The current dates for these festivals reflect the difficulty in accurately determining the date and the calendar in vogue when the date was set. Saturnalia was a 10 day festival, Chanukah 7 and Christmas 12. Let's enjoy the whole solstice season and celebrate the return of the sun. Happy Solstice Roger Bailey Walking Shadow Designs -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of John Schilke Sent: December 18, 2001 3:26 PM To: Sundial Mail List; Judy Young Subject: Fw: Query about solstices Greetings! It occurred to me today that I probably do not fully understand something that would be of interest to the List. Granted the difficulty of determining the equinox, using only a dial or a Stonehenge-like ring (mainly because of the rapid change in solar declination), I thought about the opposite problem. How does one, using either of these tools only, determine the time (i.e., epoch) of the solstice, since the declination of the sun changes very little over several days -- only a couple of minutes of arc daily. I've pencilled out some intriguing possibilities, but they all depend on 19th and 20th century math and astronomy. How did the ancients do it? I'm probably looking right at the answer and shall feel quite silly when it is shown to me, but I still will ask. Thanks for the help. John F. Schilke, MD Oregon City, Oregon, USA 122W36, 45N21
