Message text written by INTERNET:sundial@rrz.uni-koeln.de >Can any of our Mailing-List experts suggest some horizontal sundial design suitable for locations almost on the Equator, (actually 2 Degrees North) ?<
Hi Douglas. Not quite what you wanted (because it's not horizontal) but faced with these sorts of problems either of the following ideas might just be acceptable! One would have the merit of still being analemmatic and all at 'ground' level.. I recently designed an analemmatic dial for a school here and our problem was finding flat ground! So it occurs to me that there may be some sloping ground at the school. In such a case you could go for an analemmatic version of sun dial if the school has any slope where the land rises to the South at greater than (say) 21.5 degrees - to avoid any conflict of time and date scales . Then you would calculate an analemmatic dial as if for a location at a latitude equal to the slope plus the 2 degs latitude. Now such a dial would strictly need to have a gnomon 'stand' at right angles to the slope and you could of course suggest they use a stick on a tripod set for that angle or you could easily make a reasonable guess about the height of children and the lengths of their arms so that they could stand on the slope at some revised date point and say facing East or West such that when their arm is stretched out down the slope their hand is at the right point 'above' the true date point. Then all the children have to do is to mentally draw a line from the true date point to the shadow of their hand on the slope. That line will then cross the elliptical time scale at a point allowing local apparent time to be interpreted. The two points would actually be very close together so I doubt much error would be introduced if you ignored it and got the child to mentally draw a line from their feet to the hand's shadow. I haven't done the arithmetic but a child of shoulder height of 1250mm with an outstretched arm length of 600mm would allow an angle correction of some 25 degrees so if you were able to choose that angle of slope then the true date point and the point on which to stand would really be the same. Alternatively, and still using a slope, you could design an equiangular dial rather like the one at Herstmonceux. The circular time scale has equal angles between the hours - like a clock but the gnomon is a vertical rod that moves up and down the slope of the dial face. Gordon Taylor published his calculations for this sort of dial in "Taylor GE J Brit astr Ass 86 7 ; Dec 1975". That would be very elegant though more work for you. Ah well, just two more ideas. Do let us know what you decide. Patrick -
