Hi John & All, While the rainbow contributions flashed in, I was pondering how to retrieve a Scientific American article on the theory of the rainbow I vaguely remembered to have seen long ago. Most easily, I suddenly realized, through the web.
So I found this wonderful page: http://www.unidata.ucar.edu/staff/blynds/rnbw.html which included in the references section the one I was looking for: Nussenzveig, H. Moyses, "The Theory of the Rainbow", Scientific American 236 (4), 116-127, April 1977 Regards, Frans --------------------------------------------------------- Date sent: Wed, 10 Jan 2001 10:42:03 -0700 (MST) To: sundial@rrz.uni-koeln.de From: [EMAIL PROTECTED] (John Carmichael) Subject: telling time by rainbows > Hello dialists, > > Yesterday, I was able to tell the time by noting the position of a rare > Tucson rainbow! > > I was washing the dishes and looking out the north facing kitchen window > around midday (I didn't know the time exactly though), and saw the > beautiful arch of a rainbow low near the horizon. I thought that's odd, > because whenever I've seen a rainbow, it's been in the afternoon or > morning when the sun is low in the sky. > > (The reason you only see rainbows when the sun is low is because when you > view a rainbow, the sun is located directly behind your head. A line > drawn from the sun through your head leads to the center of the circle of > which the rainbow is just a segment. If the sun is too high in the sky, > the rainbow disappears below the horizon. This is why you can often see a > complete circular rainbow from an airplane because there are raindrops > below the plane. On the ground there are only raindrops above you). The > reason I could see a rainbow at midday is because the sun is low in the > sky at midday only near the winter solstice! This pushes the visible part > of the rainbow up above the horizon. > > This rainbow geometry means that the center of the rainbow lies directly > below the crest or the top of the rainbow. Therefore the crest of the > rainbow is located at the same azimuth as the sun plus 180 degrees. > Anotherwords, if the sun is due south then the rainbow's crest is at due > north. > > Now, I already knew the exact location of north on the horizon from > previous experience and could therefore guesstimate the position of the > north/south meridian. Using my hand as a degree measurement tool, I saw > that the rainbow crest was located 15 degrees west of due north. That > meant that the sun was located 15 degrees east of due south. If the sun > moves towards the west at 15 degrees per hour, then it must be an hour > before high (apparent) noon. In Tucson, high noon is at 12:24 because of > longitude correction. The rainbow said that it was an hour before high > noon, or 11:24 (more or less because I didn't correct for the Equation of > Time). > > And guess what, I turned around and looked at the kitchen clock and it > said 11:30!!! > > Makes washing those dishes a little more fun! > > John Carmichael > Tucson AZ > > p.s. Does this mean that earth is itself a sundial, and the raindrops are > the gnomon? > ------------------------------------------------------------ Dr. Frans W. Maes Dept. of Animal Physiology University of Groningen P.O. Box 14 Tel. : +31-50-3632357 9750 AA Haren Fax : +31-50-3635205 The Netherlands E-mail: [EMAIL PROTECTED] Visit my homepage at: http://www.biol.rug.nl/maes/ ============================================================
