Astronomy is the science that deals with the study of celestial bodies. It includes cosmology, which deals with the origin and life of the universe. Most research in astronomy employs ideas and theories from physics, hence it is also known as astrophysics.
In ancient times, astronomy was used to keep track of dates and for astrological purposes. For example, the appearance of a certain star in ancient Egypt marked the onset of the yearly flooding of the Nile. Another practical application of astronomy was in navigation. Modern astronomy is mostly concerned with research on the universe and its composition.