Hi Anselmo, Fer, Ceiling Dialists, This is a repeat of a message I sent earlier that bounced. My ISP sends my mail with one of three addresses depending on which site I'm logged into, so I changed my sundial address to this one: [EMAIL PROTECTED] Sorry if you receive more than one copy of this.
It seems that one solution to a ceiling sundial, within the tropics, would be to make a mesh room, with mesh ceiling and walls, Enough sun could come through to strike the mirror and project a spot which could be seen on the mesh surface. colored twists of wire could be hung to mark these. Perhaps the mirror could be made somewhat larger with an X marked from corner to corner, thus allowing us to find the center of the light spot even if partly blocked. Since we, ourselves might create this room, the surface of the ceiling could be quite level, perhaps using the hose with water in it technique or a laser level. As a note: Perhaps false ceilings were invented to give us a flat ceiling for dials when the old ceiling was tilted or lumpy. <g> Or, as you suggest, we could mount different mirrors in multiple windows or at multiple tiltings, coloring them different colors by a dip in colored lacquer and marking their spots with colored paint or thumbtacks. It would surely be a colorful way to tell time. :-) Couldn't one mount a playhouse in a window and construct a ceiling dial on it's small ceiling? What a deal for miniaturists! ( It could solve the flatness problem ) Yes, Fer's analysis is really splendid!! Edley Anselmo wrote: > Wow! > > The analysis of the limitations of polar ceiling sundials made by > Fer > de Vries is just splendid. Essentialy the problem is: if we move the > mirror inwards it could fall under the ceiling shadow and thus produce > no dot of light, and if we move it outwards the ray of light goes back > outside when the Sun is high, so, once again, we are in one of these > equilibrium-of-forces problem. In my opinion the worst limitation is the > one imposed by the size of the window, and I guess there are no general > solutions and we are bound to a trial-and-error balance analysis for > each case. > > Maybe we could have several non-overlapping mirrors, each one for every > case, but then our problem looses a lot of its simplicity. > > Anselmo Pérez Serrada > > > Dialists, > > > > In my mind I wanted to make a try for a polar ceiling dial at latitude > > 52 degrees, mid Netherlands. > > > > In the attached picture you see the mirror M at a distance g from the > > ceiling. The polar pattern for this configuration is drawn below left > > and I want to have the hours 8 am - 4 pm on the dial, if possible. ( > > no longitude correction > in > > this story ) > > > > The line BC then is about the limit of the space I need on the > > ceiling, or with other words, the mirror should be inward the room > > over a distance AB. > > > > Let's now have a look if the sun will shine on the mirror all year at > noon. > > It's obvious that I need to look for the limits at summer solstice. > > > > It is seen that during the summer solstice the sun can't shine on the > mirror > > at all. > > > > The first change I need to make is to cut of the ceiling at the line > > PCQ > in > > which C is the intersection point of the summer ray and the ceilng. > > > > In the hourline pattern the hours before 10 am and after 2 pm will be > > cut > of > > too and the first conclusion is that at summer solstice the dial is > useable > > from about 10 am to 2 pm. > > > > However, there are more things to concider. > > > > In this configuration the mirror will catch the sun at summer solstice > only > > at noon. > > Shortly before and after noon the mirror is in the shade. > > > > To see this we need to imagine a vertical south facing dial through AM > with > > a (pin)gnomon of length AC. > > That pattern is drawn below at the right side. > > > > At noon the edge of the ceiling gives a line of shadow KL and the > > mirror just catches the sun. Before noon and after noon this line of > > shadow moves down ( to the right > in > > the drawing ) and the mirror won't catch the sun. > > It sounds strange, after noon the altitude of the sun decreases and > > also > the > > shadow on a south facing dial decreases. > > Yes, this is true and can be seen with the shape of the dateline for > > the summer solstice. Before and after noon this date line in going > > down. So at the summer solstice my dial only gives a "flash" at noon > > and no more....... > > > > The third problem to concider is the width of the window. > > Because the mirror is inside the room the azimut of the lines from > > mirror through the edges of the window also will give limitations For > > this an analemmatic dial or Oughtred dial could be of use to see what > > the limitations are. > > > > Who will show us the first real polar-mirror-ceiling dial? > > Who has an inclined ceiling, higher at the south end? > > Won't that be better? > > > > Fer. > > > > Fer J. de Vries > > [EMAIL PROTECTED] > > http://www.iae.nl/users/ferdv/ > > Eindhoven, Netherlands > > lat. 51:30 N long. 5:30 E > > > > > > > > > > > > > -- Edley McKnight [43.126N 123.357W] _______________________________________________ 1 cent a minute calls anywhere in the U.S.! http://www.getpennytalk.com/cgi-bin/adforward.cgi?p_key=RG9853KJ&url=http://www.getpennytalk.com
