Astronomy is a branch of science that deals with the physical universe outside of Earth's atmosphere. It includes the study of celestial objects such as stars, planets, comets, nebulae, star clusters and galaxies as well as phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). Historically, astronomy has included disciplines as diverse as celestial navigation, observational astronomy, the making of calendars, and even astrology, but professional astronomy is nowadays often considered to be synonymous with astrophysics..