  An equation, generally used for solar energy applications, can be applied to
archaeoastronomical calculations to determine the day of foundation of ancient
towns. The equation is that giving the sunrise amplitude, and, the direction of
the sunrise on the day of foundation was sometimes used to determine the main
street of the town. Here we discuss the case of Timgad, a town founded by the
Roman emperor Trajan. Probably, Timgad was founded during the first decade of
September.
