astronomy

astronomy The study of the space beyond the Earth and its contents; phenomena in the Earth's upper atmosphere that have their origin in space, such as aurorae and meteors, are also included. Before the application of the telescope to astronomy early in the 17th century, astronomy was concerned purely with measuring the positions and movements of celestial bodies, a branch now known as astrometry. The realization in the 17th century that gravitation governed the movements of celestial bodies led to mathematical methods for calculating orbits, now known as celestial mechanics. In the 19th century the development of spectroscopy, by which the composition of bodies could be determined by analysing their light, marked the birth of astrophysics. The recognition in the 20th century that galaxies exist outside our own and the discovery of the expansion of the Universe gave rise to modern cosmology, which deals with the origin and evolution of the Universe.