Anton Reynecke <[EMAIL PROTECTED]> writes: > When I was a young boy it amazed me that it was actually possible, but > now realise it is just a form of sundial. > > It is situated in Pretoria and can only be seen in action less than an > hour every year (annum), and is is a special feature of the > "Voortrekker Monument" (roughly translated as Pioneers Monument) from > South-Africa's controversial past. > > A spot of sunlight (about a foot in diameter) shining through a hole in > the roof, onto the middle of an epitaph in the centre of this monument, > at exactly 12 'o clock (Standard time) on December 16, every year, to > commemorate a certain event in history. On the other days of the year, > the sunlight does not enter the monument at all.
This is not possible. In the first place, the sun is in exactly the same place in the sky at 11:55 on Dec 26 as it is at 12:00 on Dec 16, so these times cannot be distinguished in principle. The finite size of the sun's image makes this much worse, so some sunlight is bound to get through the hole on at least 10 days no matter how you arrange it. If the date you want to commemorate happens to be an equinox, then you could in principle create a monument which lets in a ray of light at a particular time on that day, but you can only use a sliver of about 0.0002 the area of the sun's image. Did primitive peoples determine the solstices from direct observation near the solstices or by halving the days between the equinoxes, which can be determined accurately? Art Carlson
