equal periods of the parameter time . We can call these systems clocks . Caption: " Astronomical Clock , Cathedral Notre-Dame de Strasbourg, France." An astronomical clock is used to track astronomical motion and not keep daily time only. Credit: User:Sam67fr . Linked source: Wikipedia image http://en.wikipedia.org/wiki/File:Cathedrale_de_Strasbourg_-_Horloge_Astronomique.jpg . Permission: Licensed under the