Dear Frank, Your story reminds me of Eratosthenes, the father of geography. He invented latitude and longitude and, in about 250 BCE, measured the circumference of the earth. Eratosthenes had heard that at noon at the summer solstice at Syene (present-day Aswan) in Southern Egypt you could look down a deep well and see a reflection of the sun. Eratosthenes, who lived in Alexandria, measured the shadow of a local obelisk when the shadow was shortest. Knowing that it took a camel fifty days to travel from Syene to Alexandria, he was able to deduce the circumference of the earth. Aswan is actually about forty miles north of the Tropic, and is not precisely on the same meridian as Alexandria. Still, like you, Eratosthenes was a genius! John Lynes
Dear Jackie, You are right... > I seem to remember hearing about a "sun well" > ... The sun only shone right down it at midday > when it was directly overhead. I once made a hunt-the-sub-solar-point trip to Hong Kong. I had been invited to give a course of lectures but I insisted on visiting at a time of year when the solar declination suited my ulterior motive! I also visited a friend who was chief engineer on a site where a massive pumping station was being built in a deep-water reservoir. At this stage the structure consisted of six huge vertical tubes, perhaps 3m in diameter and standing 40m high. When you looked down a tube you could normally see nothing beyond the top few metres. This site was five miles north of where I wanted to be but the sun's altitude would still be about 89 deg. 55'. While I was hunting the true sub-solar point my friend called the entire work-force together and told them all to look down these tubes. Spot on cue, the sun shone down the tubes and for a minute or so there was a blinding reflection from the pools of water at the bottoms of the tubes. Fast forward 10 years and I met another structural engineer at a wedding reception and he told me that he had worked on a number of projects in Hong Kong. It then turned out he had worked on this pumping station. One incident he remembered clearly... One day when it was fearfully hot, the chief engineer called everyone on site together and told us all that he had a mad friend who had calculated when the sun would shine directly down the tubes. We all had to lean over and look down and, sure enough, the holes suddenly went from deeply gloomy to blindingly bright. Hmmm. You never know where an understanding of the ways of the sun might lead you! All the best FrankFrom: Frank King <[email protected]> To: Jackie Jones <[email protected]> Cc: 'Sundial List' <[email protected]> Sent: Thursday, 28 July 2011, 10:51 Subject: Re: special events
--------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
