Shadow Sharpener
Message text written by INTERNET:[EMAIL PROTECTED] >As a test, you could make use of a device I read about called a "Shadow Sharpener", supposedly used by Chinese astronomers centuries ago. The shadow sharpener is simply a stiff sheet of opaque material with a clean edged round hole in the middle.< This works because it forms a pin hole camera and it focuses the light. As a consequence (and within reason!) it would not matter what shape the hole is since that does not materially affect the result. It just needs to be cleanly cut and not too big. At last weekend's BSS Meeting at Dunchurch UK there was a very interesting talk about the forthcoming solar eclipse and it emerged that during the hours of partial eclipse before and after totality (and indeed when observing from outside the totality zone) the effect of the moon's cut off will ensure that sundial shadows will be sharper - though dials will show a time error first one way and then the other because any time measurement is affected by using only the light from one or other of the sun's limbs. I'm not sure that this is a very useful piece of information but it will be interesting to see if the effect can be seen! Patrick
Re: (Off topic) Left or Right?
Message text written by INTERNET:[EMAIL PROTECTED] >I've heard of at least one European country that made the switch from left to right (though I forget which one), the switch took a 10 year plan. Troy< It was Sweden some time ago. They changed over at midnight. However, it was sensible for them to change; not only was it easier - relatively small and sparse population - but previously they drove mostly lefthand drive cars on the left !!
Re: Center of penumbra....or not?
Hello all: I was just thinking that on a horizontal sundial that the true shadow point would ONLY be at the center of the penumbra at high (apparent) noon. In the late afternoon or early morning, the sun would be to the side of the style, causing the shadow to strike the dial face at an angle. Wouldn't this shift the true shadow point towards the center of the sundial, away from the center of the penumbra? (wish I could make you a little drawing of this as a picture is worth a thousand words). John Carmichael >
Re: (Off topic) Left or Right?
As far as the tounge-in-cheek comment that the more powerful hand should be availible to hold the wheel--in the days before power steering this may have been relevant, but the shifter requires more manual dexterity, of course automatic transmission now makes shifting easier, so whatever you like stick with it! I've heard of at least one European country that made the switch from left to right (though I forget which one), the switch took a 10 year plan. Troy
Re: Center of penumbra....or not?
Hi John, You believe that you are right. The attached sketch (Shadow.jpg) shows this fairly clearly. At 21 kb it is too big to post on the mailing list unless there is a strong demand. Roger Bailey At 06:45 AM 5/4/99 -0700, you wrote: >Hello all: > >I was just thinking that on a horizontal sundial that the true shadow point >would ONLY be at the center of the penumbra at high (apparent) noon. In the >late afternoon or early morning, the sun would be to the side of the style, >causing the shadow to strike the dial face at an angle. Wouldn't this shift >the true shadow point towards the center of the sundial, away from the >center of the penumbra? (wish I could make you a little drawing of this as >a picture is worth a thousand words). > >John Carmichael > >> > >
Re: (Off topic) Left or Right?
We (UK) keep to the left side of the road, so that our (normally) strongest hand is available to fight an enemy with a sword - a throwback to the days of King Arthur and his Knights I suppose. There was a rumour going round that the governmment were considering changing us to driving on the right - but they would introduce the system gradually: Bicycles and motorbikes would change on Monday Cars would change on Wednesday Lorries and Buses on Friday Mike Shaw [EMAIL PROTECTED] 53.37N 3.02W
Re: (Off topic) Left or Right?
Mike, It would be fair to tell us which week, so that tourists would know when to avoid the "aulde countrie" when scheduling visits. Somehow I've lost the connection to the moving shadow edge. Except for... The only vehicular direction that seems to relate to the edge of the moving shadow is "traffic flow" around a roundabout. In those countries where the steering wheel is on the sinister side folks would round around the roundabout in a clockwise or dialwise direction. I.e., the left hand rule applies so that the thumb is up! On the other hand, right side steering wheeled carriages usually turn counterdialwise or antidialwise around the roundabout and the right hand rule rules. As in Britannia. I think that I have the hands, rotations and rules right, right? Knighty Knight, t The Shaws wrote: > We (UK) keep to the left side of the road, so that our > (normally) strongest hand is available to fight an enemy > with a sword - a throwback to the days of King Arthur and > his Knights I suppose. > > There was a rumour going round that the governmment were > considering changing us to driving on the right - but they > would introduce the system gradually: > > Bicycles and motorbikes would change on Monday > Cars would change on Wednesday > Lorries and Buses on Friday > > Mike Shaw > > [EMAIL PROTECTED] > 53.37N 3.02W -- Tom Semadeni O [EMAIL PROTECTED] o aka I (Ned) Ames . Britthome Bounty ><<*> Box 176 Britt ON P0G 1A0 'Phone 705 383 0195 fax 2920 45.768* North 80.600* West
Re: Shadow Sharpener
Gordon, >Roger, thank you for your post. The Shadow Sharpener being a pinhole camera, >why not replace the gnomon with a pinhole? One then could center a circle on >the image and determine the time from its position. Some years ago, when thinking about heliochronometers, I realized that the sun- sighting element of such a device could be a pinhole compound telescope. (I was avoiding the use of refractive and/or reflective "glass" elements, as part of the game.) This would be placed in an equatorial mounting with graduated hour-circle and declination sector. The symmetry of this sighting system removes a number of possible sources of error. The whole rig could be much more compact than the "giant" concept that triggered this thread. I'll attach a GIF image of such a scope. --- 1627 bytes. In the sketch: O, the objective, is a plate with central pinhole sized to diametrically subtend a couple of arc minutes at distance Lo, the primary "focal length." T is a translucent target plate, (frosted glass, even a taut sheet of fairly thin white paper,) with concentric circles drawn on it that span the approximately 1/4 degree radius of the sun's disk image, Is. With the limb viewed against the drawn rings, eccentricity of its position would be manifested by width differences in the narrow gaps between limb and circles. The eyepiece, or ocular, at e.p. is also a pinhole. This permits viewing Is at a distance Lep, that is much shorter than normal accomodation for most eyes. Even with my 7th-decade presbyopia, this can be 2 to 3 inches for an aperture of a little less than a mm. A builder had best experiment by viewing a printed page through various diameter holes, to find the optimal aperture to suit him. The 'scope could be modified to use the primary image cast on an opaque white target. This would be viewed from an eyepiece looking into the tube at an angle from in front of the target. E.g. a slit objective-aperture projected image on a 22.5° tilted target could be viewed from 45° off the primary tube's axis. The target would be a series of parallel lines with separations to cover a range +/- the 0.5 degrees of image width. The plane of the slit, target-lines and tilts would all be in a common radius-plane of the sun's arc. The sun's image from the slit would be a bright bar. (These modifications are what I would have tried first, had I gotten around to making a prototype.) Since the whole thing would be enclosed in a tube, light dilution would have little effect. A 60 inch long tube with 2 to 3 inch eyepiece length would be equivalent to a style-to-dial distance of 100 to 150 feet. (A primary image about 0.5 inch across.) The use of the pinholes/slit and diffuse image plane would minimize the danger in viewing the too bright sun. Even a somewhat fuzzy image can, if symmetrical, be placed very precisely on a symmetrical target. If anyone should try this, I hope that they'll let me know how it works out for them. Sciametrically, Bill Maddux
Re: Shadow Sharpener-2
To all: As soon as I launched my previous note, re: Shadow Sharpener, I realized I had failed to attach the GIF image file ... 1627 bytes. Here it is: (pinhole compound solar telescope) Bill Attachment converted: MAC Hard Disk:PIN-HTEL.GIF (GIFf/JVWR) (00015069)
Re: Shadow Sharpener
Hello: I used what is being called a shadow sharpner in a noon mark made by drilling a hole in an aluminum plate located at the bottom of a south facing sky light. To determine the hole size I first tried various holes in cardboard. A starting point is that a pin hole camera would have an "f" number of 100. That is the hole diameter would be 1/100 the focal length (or distance to the dial). If the hole is smaller the image is dimmer, as the hole gets larger the sun's image gets more fuzzy. The image was an ellipse since it was projected on a wood floor. I had a stiff piece of paper with a number of ellipsis and a small hole in the center. Using a watch synchronized to WWV and the beeper turned on, I tracked the sun's image and stopped when the watch sounded. Then just drive in a brass tack. Have Fun, Brooke Clarke Gordon Uber wrote: Roger, thank you for your post. The Shadow Sharpener being a pinhole camera, why not replace the gnomon with a pinhole? One then could center a circle on the image and determine the time from its position. My rule of thumb is that the angular resolution of a pinhole is one radian (57 degrees) divided by the diameter of the pinhole in wavelengths of light. Since the wavelength of green light is about 0.0005 mm, the resolution for a 2 mm pinhole would be about 0.0005/2.0 = 1/4000 radian or about 0.8 arcminute or 3 seconds of time. This seems consistent with your observations. To achieve this resolution the pinhole would have to be greater than 4000 * 2 mm = 8 meters from the image. Using a slit perpendicular to the direction of motion of the image would increase the brightness of the image without decreasing the time resolution. A lens, such as one from a pair of reading glasses, would provide still greater brightness. As a rough approximation, the brightness (technically the illuminance) of the sun's image relative to that of a sunlit surface is proportional to the area of the pinhole divided by the area of the sun's image. Since the angular diameter of the sun is about 1/100 of a radian, the brightnesses are about equal when the pinhole diameter is 1/100 of the distance from the pinhole to the image. Gordon At 07:52 PM 5/3/99 , Roger Bailey wrote: >By observing the shape of the image of the sun, the middle and the two >edges of the penumbra were easily determined to a precision better than one >inch over the width of the penumbra (about 2 ft). This gives a precision >equivalent to about 5 seconds per day for this size of shadow. The bisected >hemispherical image would be the appropriate position for the sun as a >point source with no semi-diameter correction required. Fixing the hole and >screen and timing the movement of the shadow would give even more precise >results for these events (but if I had a watch, why would I need a sundial). Gordon Uber [EMAIL PROTECTED] Reynen & Uber Web Design http://www.ubr.com/rey&ubr/ Webmaster: Clocks and Time http://www.ubr.com/clocks/
Source article for Shadow Sharpener
A couple of you asked about where I read of the shadow sharpener. I have the article, but it is packed away in storage. It would probably take several hours to find it. The best I can remember, it was in Sky & Telescope magazine (or possibly Astronomy magazine) in one of the issues between 1995 and 1997. The same magazine had an article about building a brass heliochronometer. I know that isn't much help, sorry. If I every find the article, I'll post the source. Charles
Re: Shadow Sharpener
Roger, thank you for your post. The Shadow Sharpener being a pinhole camera, why not replace the gnomon with a pinhole? One then could center a circle on the image and determine the time from its position. My rule of thumb is that the angular resolution of a pinhole is one radian (57 degrees) divided by the diameter of the pinhole in wavelengths of light. Since the wavelength of green light is about 0.0005 mm, the resolution for a 2 mm pinhole would be about 0.0005/2.0 = 1/4000 radian or about 0.8 arcminute or 3 seconds of time. This seems consistent with your observations. To achieve this resolution the pinhole would have to be greater than 4000 * 2 mm = 8 meters from the image. Using a slit perpendicular to the direction of motion of the image would increase the brightness of the image without decreasing the time resolution. A lens, such as one from a pair of reading glasses, would provide still greater brightness. As a rough approximation, the brightness (technically the illuminance) of the sun's image relative to that of a sunlit surface is proportional to the area of the pinhole divided by the area of the sun's image. Since the angular diameter of the sun is about 1/100 of a radian, the brightnesses are about equal when the pinhole diameter is 1/100 of the distance from the pinhole to the image. Gordon At 07:52 PM 5/3/99 , Roger Bailey wrote: >By observing the shape of the image of the sun, the middle and the two >edges of the penumbra were easily determined to a precision better than one >inch over the width of the penumbra (about 2 ft). This gives a precision >equivalent to about 5 seconds per day for this size of shadow. The bisected >hemispherical image would be the appropriate position for the sun as a >point source with no semi-diameter correction required. Fixing the hole and >screen and timing the movement of the shadow would give even more precise >results for these events (but if I had a watch, why would I need a sundial). Gordon Uber [EMAIL PROTECTED] Reynen & Uber Web Design http://www.ubr.com/rey&ubr/ Webmaster: Clocks and Time http://www.ubr.com/clocks/
Re: Shadow Sharpener 2
I was just looking at the section on medieval noon marks (illustrated in its frontispiece) in Waugh's book "Sundials." The one shown had an opaque disk with a central 1/4" hole supported out from a south-facing wall. He suggests timing the passage of the two edges of the solar image and averaging these times to get the time of noon. He estimates an accuracy of 20-30 seconds. He also has a section on ceiling dials, in which there was some interest a few days ago. Gordon Gordon Uber [EMAIL PROTECTED] Reynen & Uber Web Design http://www.ubr.com/rey&ubr/ Webmaster: Clocks and Time http://www.ubr.com/clocks/
Re: (Off topic) Left or Right?
At 22:48 3/05/1999 EDT, [EMAIL PROTECTED] wrote: >It would seem to make more sense to drive on the right side, since the >handedness of a supermajority of people is right handed, allowing the >stronger hand in close proximity to the shifter. That and being an American >that's what I'm used to ha ha. >Troy But shouldn't the stronger hand be on the wheel ;-) Cheers Hank de Wit Regional Computer Manager Bureau of Meteorology South Australia mailto:[EMAIL PROTECTED] ph: 08 8366 2674 http://www.sa.bom.gov.au/~hdewit (INTRANET access only)
Re: (Off topic) Left or Right?
At 08:53 PM 5/4/99 -0400, Patrick Powers wrote: > > >It was Sweden some time ago. They changed over at midnight. However, it >was sensible for them to change; not only was it easier - relatively small >and sparse population - but previously they drove mostly lefthand drive >cars on the left !! and being Swedes, they are mostly sensible.
Re: Shadow Sharpener
Gordon Uber <[EMAIL PROTECTED]> wrote: > Roger, thank you for your post. The Shadow Sharpener being a > pinhole camera, why not replace the gnomon with a pinhole? One then > could center a circle on the image and determine the time from its > position. To form a good solar image the plane of the pinhole needs to be transverse to the direction to the sun's rays. In fact, I believe near-perpendicularity may be desired. To accomplish this with a classic pinhole, the plane would need to rotate during the course of the day. Perhaps there is some clever scheme that would achieve this pinhole behavior without the need for mechanical intervention, but I don't know what it would be. That's the beauty of using a sphere to form an umbra--it needs no such rotation (although the distance between the sphere and the point where the shadow impinges upon the dial changes as the day progresses). Jim --- -- | Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] | | Parametric| Salt Lake City, UT | (801)-588-4632 | | Technology Corp. | 84108-1202 | Fax (801)-588-4650 | --- -- It is better to suffer wrong than to do it, and happier to be sometimes cheated than not to trust.-- Samuel Johnson
