On northern vs. southern dials

[Anselmo P�rez Serrada]

Hi everyone,      Imagine an oriented wall (gnomonic declination equals to 0 deg in one side and 180 deg in the other), all of us know that the southern side is exposed more time to sunlight than the northern one, but where is this difference bigger?  It is clear that in the equator and in the North Pole the rate is 50% to 50%, ie., both sides receive the same amount of sunlight through a whole year,  so there must be in the middle some latitude where the difference becomes maximal. Which is that latitude?      OK, I know we can solve it easily by 'brute force' but is there a simpler method to determine that value? I didn't find it.      Now that I remember, a kind of touristical question: in this month's issue of the spanish version of Scientific American there is an article by D. Savoie about sundials an in it he says that in the Place de la Concorde in Paris it was drawn a sundial (an Oughtred sundial I suppose) based on the obelisk erected there. Does anybody know if the lines are still drawn or were they erased?     Anselmo Perez Serrada     PS: My apologizes for Southern diallists (you all know about our Northern egoccentrism!). Just swap the words North and South.