astronomy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The scientific study of matter and phenomena in the universe, especially in outer space, including the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial objects.
• noun A system of knowledge or beliefs about celestial phenomena.

from The Century Dictionary.

• noun Several other branches of the science are recognized: gravitational astronomy or astronomical mechanics, replacing the term physical astronomy, which is now generally discarded because of the danger of confusion with astronomical physics or astrophysics; nautical astronomy, astronomy applied to navigation; sidereal astronomy, the branch of the science which deals with the stars; spheric astronomy, which treats of the apparent position and motions of bodies on the celestial sphere; theoretical astronomy, which deals with the calculation of orbits and perturbations. These various branches interlace in all directions.
• noun The science which describes the heavenly bodies and explains their apparent motions, etc.
• noun Astrological skill.

from the GNU version of the Collaborative International Dictionary of English.

• noun obsolete Astrology.
• noun The science which treats of the celestial bodies, of their magnitudes, motions, distances, periods of revolution, eclipses, constitution, physical condition, and of the causes of their various phenomena.
• noun A treatise on, or text-book of, the science.
• noun See under Physical.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The study of the physical universe beyond the Earth's atmosphere, including the process of mapping locations and properties of the matter and radiation in the universe.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the branch of physics that studies celestial bodies and the universe as a whole