> > [EMAIL PROTECTED] wrote: > > > I saw "Cast Away" last week, and had a question about sundials and the > > movie. > > After Tom Hanks has spent several years on a desolate island, he constructs > > an impressive analemma from a thin beam of light that enters his cave, > > complete with days of the months. This would not be hard, if he had a > > working watch, but I don't think he did. So, my question is, is this just > > Hollywood chicanery, or is it really a possible thing to do? Bill G. (I > > need to know, in case my plane goes down in the south Pacific some day).
I haven't seen the film, but if I had a watch which was accurate to within, say, a few seconds per day (not too ambitious), I would use the beam of light as a transit to sight a star every night. I would use this to set the watch, having calculated the length of a siderial day as 24 hours times 365.25/366.25. To ensure my eye was always in the same position, I would place a stone as a backsight or a headrest. I could use the same star for several months, then choose another. Thus, I would always know the time of midday to within a fraction of a minute, and could use this to create a pretty good analemma. or.... if I knew the eccentricity of the earth's orbit, date of perihelion, inclination of the ecliptic (I could measure this from the sun's altitude changes) and date of the vernal equinox, which I might know was March 21st, I could do the whole thing graphically or mathematically. That way, I could get to sleep at night. Without all that information, and lacking a watch, I know of no way to draw an analemma. By the way, on what surface did he draw the analemma? Happy New Year / Millennium to all Chris Lusby Taylor Newbury, England 51.4N, 1.3W
