How do you calculate obliquity of ecliptic?
Imagine that you are an equatorian (in Brazil), standing at noon on summer solstice (about June 21). Then, tan (obliquity) = h/l, where h is your height and l is the length of your shadow.
At a solstice noon, the angle of incidence of light rays from the Sun upon the equator is (almost) obliquity =
If Y is your head, U your foot and US is shadow,