How is the tilt of earth's axis calculated?
From the length of the shadow at noon, on the equator, at summer (about June 21) or (about December 21) winter solstice. If h is your height, l is the length of the shadow, l/h = tan (tilt angle)..
The light rays from the Sun are very nearly parallel to the ecliptic (the Earth's orbital plane), for all latitudes.
At a Solstice' noon, for locations on the equator (like some in Brazil), the angle of incidence of the solar rays is the angle they make with the vertical there.upon. This angle is the tilt angle