Changes of the celestial coordinates

Because of the movements in the long term of the Earth (first of all that one known with the precession name of the equinoxes), the stars do not have celestial coordinates of all the fixed ones but they are moved with the time. This movement is independent from the motion just of the stars, because draft of a movement of the observer rather than of the star. To eye knot and on scales of little years he is imperceptible, but for the astronomical observations the problem is placed to specify which moment one co-ordinate refers. It has been therefore invented the age concept : all the coordinates specify respect to an age, and exist algorithms in order to pass from an age to the other.

In spite of the entity of the movement it can appear paltry on short periods, on a scale of time of millenia it can carry to remarkable variations in the positions of the stars; as an example, between approximately 13000 years the celestial North Pole it will be indicated from Vega, and not more from the Polar Star.