Planispheres

Planispheres do have an expiration date, but that expiration date as far as most people who are starting out in astronomy don’t really need to worry about for quite awhile. Most planispheres made currently have stars plotted according to equinox 2000.00, meaning stars are plotted according to their celestial coordinates as of 1 January, 2000. Star positions slowly change against the celestial coordinate grid over the centuries, due to the fact that the Earth slowly wobbles on its axis over a 25,800 year period resulting in both the north and south poles tracing out large circles in the sky. The present north star, Polaris, Alpha Ursae Minoris hasn’t always been our north star. Earth’s north pole has pointed reasonably close to Polaris only for the last few hundred years, and will be our north star for only a few more centuries, so by then a whole new crop of planispheres will have to be created to account for the fact that Earth’s north pole in the future will no longer point at Polaris. So someone living in the distant future, say about 14,000 years from now will have to use a planisphere that has Vega, Alpha Lyrae as the north star. Another factor besides precession (wobbling of the Earth’s spin axis), is proper motion, the movement of the stars themselves as they very slowly change position relative to each other, gradually altering the constellation patterns until they are no longer recognizeable, which by that time, future astronomers will have to invent an entirely new crop of constellations.