Astronomy is the scientific study of the objects visible in the night sky by means of telescopes and associated instruments that analyze the radiation received from these objects. Using such instruments, astronomers determine their positions, apparent motions, distances, sizes, and total radiation emitted. From their spectra (the decomposition of light received from them into wavelengths) astronomers determine their chemical composition and radial motion. Astronomy distinguishes planets from stars, and identifies the way stars are spatially associated in star clusters, galaxies, and clusters of galaxies. Astronomy has ancient roots arising from peoples' attempts to relate the annual change of seasons to positions of stars in the sky. Astronomy is to be distinguished from astrology, which purports to relate the events in human lives to positions of the planets at the time of one's birth.

See also COSMOLOGY, PHYSICAL ASPECTS

GEORGE F. R. ELLIS