Re: optical resolution problem
The minimum would be one arc minute for stroke width and a five arc minute height for characters if they are to be legible. This is the basis for 20/20 vision (resolution or visual acuity) on eye charts. I would think that you would want to at least double this. One can detect and determine the location of a shadow that is narrower than this. I seem to recollect that power lines can be detected against the sky when their width is perhaps five or ten arc seconds. Gordon At 06:55 PM 2/10/00 -0700, John Carmichael wrote: Good dialing question Ross Caldwell! (We might need an optomologist for this one.) John Carmichael >You write: > >Do you know of any kind of formula to estimate the proper proportions for >seeing high or distant dial lines, letters etc. clearly? I suspect >architects, engineers and advertisers (billboards) must have some way of >knowing what will be clear to a given eye at a given distance. > >Ross Caldwell >[EMAIL PROTECTED] >119 48 11W >36 46 14N >Fresno, CA USA Gordon Uber [EMAIL PROTECTED] San Diego, California USA Webmaster: Clocks and Time: http://www.ubr.com/clocks
Re: optical resolution problem
>Do you know of any kind of formula to estimate the proper proportions for >seeing high or distant dial lines, letters etc. clearly? I suspect >architects, engineers and advertisers (billboards) must have some way of >knowing what will be clear to a given eye at a given distance. The best the average corrected eye can do is about a minute of arc. That's with very high contrast like truly black lines on a truly white background. For low contrast objects, you'd want more like 3 minutes of arc. I'm talking about spacing of lines here where the line width and the space width are equal. One example which might help would be the Snellen eye test chart. The lines in the letter E for 20-20 vision (when it's the correct distance away - no cheating) are 1 minute and the spaces are 1 minute so this has a 2 minute spacing. With corrected vision, that letter E should be easy. So all in all, I think I would try a minimum of 3 or 4 minute spacing because not everyone's been to his optometrist or ophthalmologist recently. Of course if you have the space, by all means do increase the size for more legibility. Also to make it faster to read - no one has to puzzle over it. As far as isolated lines, that's another thing. Here you just need high contrast and some width. As an example of very high contrast, think of power lines against a bright sky. You can see them for miles even though the angle they subtend is well below what your eye is "diffraction limited" at. So seeing an isolated line is strongly a matter of contrast as well as angle subtended. I'd still go for a minute or two of angle even with high contrast, though. John B
optical resolution problem
Good dialing question Ross Caldwell! (We might need an optomologist for this one.) John Carmichael >You write: > >Do you know of any kind of formula to estimate the proper proportions for >seeing high or distant dial lines, letters etc. clearly? I suspect >architects, engineers and advertisers (billboards) must have some way of >knowing what will be clear to a given eye at a given distance. > >Ross Caldwell >[EMAIL PROTECTED] >119 48 11W >36 46 14N >Fresno, CA USA > >__ >Get Your Private, Free Email at http://www.hotmail.com > >
Re: Sundials at Train Stations
The eye/brain is very good at seeing lines, and can see lines thinner than the usual 1 arcminute given as the resolution of the eye. 20/20 vision is based on reading letters that are 5 arcminutes high that have 1 arcminute features (line width and gaps). So this might give you an idea of how big to make lines and numbers on a sundial, although I would guess that you shouldn't require 20/20 vision among the viewers.
Re: Sundials at Train Stations
Ross Caldwell wrote: > >Do you know of any kind of formula to estimate the proper proportions for >seeing high or distant dial lines, letters etc. clearly? I suspect >architects, engineers and advertisers (billboards) must have some way of >knowing what will be clear to a given eye at a given distance. Ross > I'm sure you are correct and that these exist but I ever was one for personal experiment and simply used a graphics program to print various thicknesses of line on a variety of coloured backgrounds and then hauled them up my house wall for subjective assessment. 'Cut and try' I think is the US expression! Best Tony M. == \ ** ** \\ ** ** \\** *** *\\ ** ** *\\ ****** **\\ ***\\ Tony Moss, Lindisfarne Sundials *\\ 43, Windsor Gardens, Bedlington, ***\\Northumberland, England, NE22 5SY, **\\55° 07' 45" N1° 35' 38" W Tel/FAX +1670 823232 Mobile: 07970 208 540 Website: http://www.lindisun.demon.co.uk == Horizontal, Vertical, Declining, Analemmatic, Equatorial, Polar and Capuchin Sundials individually made in solid engraving brass or phosphor bronze. Professional-quality Dialling Scales, 'engine-divided' Meridian Layout Instruments with software. Analemmatic dial plots - any size for any latitude. Graduation, re-cutting and restoration of scales. 'Lintique' patination of brass. ===
Re: shadow velocity & shadow tracing
On Thu, 10 Feb 2000, John Davis wrote: > Since what you are actually trying to do is to reproduce the method > of a trigon (as Chris pointed out), why not use a real laser trigon. > Bob Terwilliger has one on his webpages at > http://www.shadow.net/~bobt/trigon/trigon.htm (based on a transit > mechanism), and I published a cheap-and-cheerful small version in the > BSS Bulletin October 99. John: Are you at liberty to re-print that design for us, say to put up on the archive sites? Dave Bell
Re: shadow velocity & shadow tracing
On Thu, 10 Feb 2000, John Carmichael wrote: > Hi Chris: > > You're right! I realized as soon as I sent my message that a small diameter > floodlight with a wide beam would be better than a tight beam, because it > must illuminate the entire style and not just a portion of it. > Also, the smaller and more point-like the source, the better. Try a Mini-MagLite flashlight, with the focussing reflector head entirely unscrewed. This leaves you with a tiny bare lamp, almost painful to look at directly. The total light output is fairly low, but should work on a dark night! Dave
Re: shadow velocity & shadow tracing
John Carmichael wrote (replying to Chris Lusby Taylor): >You're right! I realized as soon as I sent my message that a small diameter >floodlight with a wide beam would be better than a tight beam, because it >must illuminate the entire style and not just a portion of it. > A drawback with the "floodlight" scheme is that a very tall ladder will be needed unless you are only going to make use of the small bit of the gnomon near the origin. I know to my cost that big gnomons rarely have the small-scale accuracy required! Since what you are actually trying to do is to reproduce the method of a trigon (as Chris pointed out), why not use a real laser trigon. Bob Terwilliger has one on his webpages at http://www.shadow.net/~bobt/trigon/trigon.htm (based on a transit mechanism), and I published a cheap-and-cheerful small version in the BSS Bulletin October 99. Once the trigon is fixed to the style, and the hour angle set using the incorporated auxilliary dial, all you have to do is swing the laser on its declination axis and the spot will follow the humps and bumps on the ground precisely. If done at night, the laser spot is easy to see. If done in bright sunshine, you might need a red specular-reflector target and some red sunglasses to see the laser spot clearly. It's a shame you are so far away, as I built a "big" version of my trigon for the New Hall Oxford dial, but it didn't get fully used and I'm looking for another application!. By indexing the auxillary dial with 0.25 degree intervals, it gives 1 minute of time resolution. Regards, John Dr J R Davis Flowton, UK 52.08N, 1.043E email: [EMAIL PROTECTED]
Re: Sundials at Train Stations
Ken - Sundials at train stations follows a well established tradition. Rene Rohr mentions " It is not generally known that this instrument was used into the twentieth century by some networks of the French railways for uniformity in the setting of the station clocks." (end of chapter one) Solar noon lines for other cities on a railroad line might be a good emphasis of the longitude difference and the reach of the railroad.
Re: Book
This was announced in October on this list by Mark Gingrich. Although it deals with the use of cathedral meridiana it goes much further in summarizing the history of astronomy leading to the calendar reform and the involvement of the church. Very thoroughly referenced. I was delighted to read so much greater detail of the lesser figures of and astronomy following Galileo. May not be a quick read! Lufkin Brad wrote: > I've just seen an ad for a book called "The Sun in the Church: Cathedrals as > Solar Observatories" by J.L. Heilbron, Professor and Vice Chancellor > Emeritus, UC Berkeley, and Senior Research Fellow, Worcester College, > Oxford. It's gotten pretty good reviews, including a nice blurb by Owen > Gingerich of Harvard. > If anyone has read it, I'd like to hear about it before spending $35.
Re: Sundials at Train Stations
Hi Tony - You write: Don't forget that hourlines become harder to see as the dial gets higher. 3mm wide looks impossibly clumsy at ground level but just right above 4 metres high! Do you know of any kind of formula to estimate the proper proportions for seeing high or distant dial lines, letters etc. clearly? I suspect architects, engineers and advertisers (billboards) must have some way of knowing what will be clear to a given eye at a given distance. Ross Caldwell [EMAIL PROTECTED] 119 48 11W 36 46 14N Fresno, CA USA __ Get Your Private, Free Email at http://www.hotmail.com
R: Sundials at Train Stations
Hi Ken, a friend of mine, that works for the Society of the Italian Railroads, has built some sundials on the walls of the stations of a local railroad that, passing long alpine valleys, joins two small towns : Domodossola (in Italy) and Locarno (in Switzerland). The friend is the arch. Giacomo Bonzani ; he is also the mayor of the little town of Villette (on the Alps) of about 200 inhabitants. The sundials are very simple and the lines are built in steel rod to remember the the railroad : in Italy the railroad is called "ferrovia" that means "iron way" I send attached thee photo of one of these sundials: the image has a low definition because of the excessive reduction. In the photo you can see ( it was a misty day when the photo was taken ) on the wall of the station toward East two sundials: in one there are the hour lines from 7 to 11 and in the other only the line of noon In the opposite wall there is a similar sundial with the haul line from 14 to 17 A regard Gianni Ferrari P.S. If you want , I can send to you the photo with a better definition --- Ing.Gianni Ferrari Via Valdrighi, 135 41100 - MODENA (ITALY) EMail : [EMAIL PROTECTED] http://members.tripod.com/meridiane/index.htm --- Attachment converted: Macintosh HD:VIGEZ2.jpg (JPEG/JVWR) (DF06)
Re: shadow velocity & shadow tracing
Hi Chris: You're right! I realized as soon as I sent my message that a small diameter floodlight with a wide beam would be better than a tight beam, because it must illuminate the entire style and not just a portion of it. Thanks for confirming, John >Good idea John. But when you say "tightly focused" I take it you mean a >flashlight with a wide beam emanating from a small aperture. This is what gives >a >sharp edge to the shadow. Conversely, a flashlight with a narrow beam >from a large reflector would be poor. >The poor man's laser trigon, indeed. > >Chris > >John Carmichael wrote: > >> Hi once more: >> >> Just read all your e-mails on drawing hour lines by shadow tracing. Chris >> Lusby Taylor pointed out that it would be difficult to trace the whole hour >> line because the shadow is constantly moving; so he suggested drawing a >> point on the hour line first and then visually guestimating the entire hour >> line's position on the ground. This seems like it might cause small errors >> in the hour lines because it is based on a guesstimate and not an actual >> shadow tracing. >> >> I have an idea, though, that might facilitate drawing an hour line from the >> face edge to the center: How about first marking only the hour points as >> Chris suggests. Then, at night, fasten a high power, tightly focused >> flashlight to a ladder to simulate the sun. Position the light so that the >> style's shadow falls on the hour point that was marked earlier using the >> sun. The wiggly hour line could then be traced at one's convenience using >> an unmoving "sun". >> >> Whadaya think? >> >> John > >
Re first dial
To the list: Thanks so much for all the suggestions on my first dial and getting started. Enjoy all the posts. Bill Murrill Morehead City, NC USA
Re: shadow velocity & shadow tracing
Good idea John. But when you say "tightly focused" I take it you mean a flashlight with a wide beam emanating from a small aperture. This is what gives a sharp edge to the shadow. Conversely, a flashlight with a narrow beam from a large reflector would be poor. The poor man's laser trigon, indeed. Chris John Carmichael wrote: > Hi once more: > > Just read all your e-mails on drawing hour lines by shadow tracing. Chris > Lusby Taylor pointed out that it would be difficult to trace the whole hour > line because the shadow is constantly moving; so he suggested drawing a > point on the hour line first and then visually guestimating the entire hour > line's position on the ground. This seems like it might cause small errors > in the hour lines because it is based on a guesstimate and not an actual > shadow tracing. > > I have an idea, though, that might facilitate drawing an hour line from the > face edge to the center: How about first marking only the hour points as > Chris suggests. Then, at night, fasten a high power, tightly focused > flashlight to a ladder to simulate the sun. Position the light so that the > style's shadow falls on the hour point that was marked earlier using the > sun. The wiggly hour line could then be traced at one's convenience using > an unmoving "sun". > > Whadaya think? > > John
Re: Sundials at Train Stations
Ken Clark wrote: > My local community is in the planning stages of revitalizing our local >train station. I was thinking, why not include a sundial as part of the >design for the passengers to look at while waiting for the train. I like >vertical declining dials because they are some what vandal proof if high >enough on the wall. > Don't forget that hourlines become harder to see as the dial gets higher. 3mm wide looks impossibly clumsy at ground level but just right above 4 metres high! If it's a metal dial then a light coloured 'fill' will ensure the lines remain visible as the plate patinates in the atmosphere. Good Luck with the project. Tony Moss.
Re: Sundials at Train Stations
There is a wonderful dial designed by Harriet James for a railway enthusiast. It is a west facing dial, and the time lines depict a set of railway lines receding into the distance. An old fashioned signal is used as the gnomon. Harriet has a web site - There is a picture of it there see: http://www.sundials.co.uk/sunfair.htm#hjames There is also a picture of it in BSS Volume 11(i) Page 42 It would make a great design for a railway station Mike [EMAIL PROTECTED] 53.37N 3.02W Wirral, UK
Re: drawing hour lines using gnomon
Arthur Carlson <[EMAIL PROTECTED]> writes: > [EMAIL PROTECTED] (John Carmichael) writes: > > > Let's say ... > > ... Will this technique produce the same shape hour lines at any > > time of the year? > > Yes. The hour lines will always have the same shape. This is even > true if the gnomon is not aligned with the axis of the Earth, as long > as it is straight. Some other respondents have touched upon the question of the orientation of the gnomon. I stand by my answer to the question as stated: John's technique will "produce the same shape hour lines at any time of the year", for any straight gnomon. If you want to label these lines for clock time, the labels will have to change during the year. Or you can put labels on them that are valid for some day of the year and read corrections to these labels from a table. The advantage of a polar gnomon is that these corrections are just a function of the day of the year (the familiar Equation of Time). With, say, a vertical gnomon, the correction will depend both on the day of the year and also the time of day. (Obviously, I've been thinking too much about my Dali dial. I better go cool off my brain.) Art
Re: Sundials at Train Stations
Dear Ken, I recall that there are sundials at train stations around the US. I think that we may have seen 2 in Chicago's near north suburbs during the NASS dial tour there in 1997. One was by Stephen Luecking in Highland Park, IL. (images are at http://www.depaul.edu/~slueckin/founderspage.htm). Another was an analemmatic sundial (exact location eludes me right now, possibly Glencoe?). And I recently heard of one installed by New Jersey Transit in a station in South Orange, New Jersey. (my home town, although I had nothing to do with this, much to my father's chagrin!) Cheers, Sara --- Sara Schechner, Ph.D. Center for History of Physics American Institute of Physics 1 Physics Ellipse College Park, MD 20740 Tel: 301-209-3166 / Fax: 301-209-0841 [EMAIL PROTECTED] Gnomon Research __Curators on Call __Outreach Adventures 1142 Loxford Terrace Silver Spring, MD 20901 Tel/Fax: 301-593-2626 http://www.wam.umd.edu/~sschech [EMAIL PROTECTED]
