Sergio Doret wrote: > Suppose a horizontal sundial at latitude 4º S. His gnomom is a very acute >square triangle. What happens with the shadow when sun's declination get the >other side of celestial equator? What happens at the july solstice, does >the shadow >make redable, correct indication?
A sundial placed between the Tropics will, at certain times of the year, experience the inconvenience of having the sun pass overhead from the hemisphere south of the dial into the hemisphere north of the dial. I think I had first better explain what I mean by this, as what follows will rely heavily upon it. Suppose you have a dial set up as normal at about 15°N, for sake of arguement. A plane going through the centre of the earth and through the 6 o'clock lines on the dial will divide the celestial sphere into a hemisphere to the north of the dial and one to its south. For a great part of the year the sun will rise, reach its greatest altitude and set, all south of the dial. However, as Summer approaches there will come a date when the sun has its greatest altitude directly above the dial. i.e. at the local noon, the sun will be just touching the above mentioned plane. In the days to come, the sun will pass from the hemisphere south of the dial into that to its north in the morning and back again in the afternoon. Initially the times of these transits will be close to noon, but will get further away until they are at a maximum at mid-summer. However, at the time the sun crosses from one hemisphere to the other it will be due east or west of the dial, by the definition of the plane above. In this case it will cast its shadow due west or east, thereby indicating it to be 6 o'clock! I've considered this as a thought experiment, not really being a gnomonist by profession, but it looks to me as though a sun dial will be in real trouble in the Tropics. If I'm wrong I'd be happy to learn where my mistake is... Best wishes David Robert Higgon 51°27' N 0°15' W
