The old Egyptians used a rather simple sundial. As the sun at noon near the equator nearly moves right over the heads of the old Egyptians, they used a horizontal bar, oriented in N-S-direction as "Shadow-Stick" and measured the length of the shadow on a ruler perpendicular to this bar.
http://members.aon.at/sundials/berlin-egypt_e.htmIf the sun is moving over the equator with constant speed at a constant hight, these marks should be equidistant. This can easily be deduced by the geometrical rule of similar triangles. But they are not equidistant.