I would like to forward the following enquiry to all those dialiasts who know more than I do about celestial mechanics. It seems to me that the answer has something to do with the fact that the sun appears as a disk rather than a point in space, but I don't know enough to think this through logically. The question was posed by Rory Sellers ([EMAIL PROTECTED]). What exactly does he have to do to subscribe to the sundial list?
>Many years ago a golf-playing uncle in Los Angeles asked me why it was >that the shortest day of the year was the winter solstice, but the sun >actually set earliest on about Dec. 13 (he had noticed that he could get >in an extra hole of golf on Dec. 21 -- I'm not making this up.) > >After a lot of research (there was no Web then!) I realized the answer >had to do with the equation of time, the difference between solar and >sideral days, and mean solar time (and no doubt you know more about this >than I do.) > >So I told my uncle this, and the family got down to accomplishing other >life-tasks. Unfortunately, I recently realized that this explanation >must be false. This came about by looking more closely at an analemma on >a globe. If I am not mistaken, the analemma describes not only the >difference between mean and clock-time (due to the eccentricity of the >earth's orbit being non-zero) but also the (I guess arbitrary) dates on >which our clock is "set." I.e. when the official timekeepers say that >the annual clock is zeroed. And here, I was shocked to see a) that this >date seems to be the winter solstice; and b) that I had never noticed >this before! > >You see, I always knew that the earth was moving fastest around >Christmas time (near perihelion) and so I figured it made sense that the >clock was furthest "out of whack" around this time. And sure enough, >consulting either an almanac or a St. Joseph's Aspirin calendar, one can >see that the sun indeed sets earliest on Dec. 13, not Dec. 21. BUT HOW >IS THIS POSSIBLE, IF THE CLOCK IS "ZEROED" ON THE SOLSTICE? Shouldn't >the equation of time being equal to zero on Dec. 21 mean that the sun >should set the earliest on that date, too? > >Please help! A family conundrum that I thought we had settled twenty >years ago is now bothering me! > >Thanks, > >Rory Sellers > >P.S. Interesting sidelights: > >1. When I first tried to answer my Uncle's question, I tried calling up >the chief of the Griffith Park Planetarium in Los Angeles. He not only >did not know the answer to my question, he didn't even know it was true! >(I.e. that the shortest day and the day on which the sun sets the >earliest are not the same.) > >2. The year before he died, I wrote a letter to Richard Feynmann trying >to get him interested in a project I was undertaking involving >interactive video and physics teaching. After reading "Surely You're >Joking, Mr. Feynmann" I knew the only way I could get his attention and >possibly get an answer to my letter was to pique his interest by >announcing on the outside of the envelope that there were "puzzles >enclosed." It worked! Feynmann wrote back, regretted he couldn't take >part in my project, and solved one of the two puzzled I had posed. >Unfortunately, he chose the one OTHER THAN the one about the equation of >time! (In fact, our correspondence was noticed by Gleick researching his >book "Genius" about Feynmann who then wrote me for an explanation.) It >was all very gratifying to my ego, but didn't help with the time >question!! > > Jack Aubert E-Mail: [EMAIL PROTECTED] Homepage: http://www.cpcug.org/user/jaubert
