From the earliest times until today the vernal equinox was a reference point for position data. It can be dertermined in a relatively easy way at any place due to the point of sunrise on the eastern horizont and the point of sunset on the western horizont define a line straight through the oberserver's position on that day. Prehistoric observatories (e.g. Stonehenge) are orientated towards the vernal equinox. Depending on the world picture the vernal equinox defines a point on the horizont, on the sun's orbit or on the earth's orbit. The peculiarities of the coordinate systems are caused by these differences in the point of view.