The study of celestial objects and phenomena outside of the Earth's atmosphere.
Astronomy is the study of celestial objects (stars, planets, moons, etc) and phenomena (eg. supernovae explosions) outside of the Earth's atmosphere.
Many ancient civilizations (eg. Babylonians, Chinese, and Maya) observed the night sky and as such astronomy is claimed to be one of the oldest sciences. Ancient civilizations often used astronomical observations to define their calendars (eg. Ancient Egypt used Sirius to predict the seasonal Nile floods).