The cosmic calendar

Astrophysical phenomena which are observable and which contribute to the attempt to determine the age of the universe are reviewed. Focusing on the Big Bang theory of the universe, the evolution of stars with reference to the Hertzsprung-Russell diagram has limited the interval of the existence of the universe to the range 8-18 billion years. Observational measurements of the expansion rate of the universe using the apparent luminosity of stellar objects and the Doppler shift of received light yielded an estimated age of 15-20 billion years. Finally, measurements of the expansion rates among the local superclusters of galaxies has produced age estimates of 8-11 billion years.