The easiest way to define a star's celestial position is in relation to the horizont line and the point of vernal equinox. Due to the rotation of the earth these position data are valid only for a certain observer's location, time and date. Observation of periodically recurring events in the sky (the sun's orbit, lunar phases, venus' rise and set as morning and evening star, comets) allows to determine the seasons up to developping a precise calendar, but results are fixed to a certain location. With every change of location new observation is neccesary. This is why the horizont system cannot be used for navigation purposes.