Astronomy is commonly referred to as the study of the stars, planets and their position within a Solar System, Galaxy and the known Universe
Astronomy is also considered the study of the motion, and nature of "Celestial Bodies" typically, stars, planets and galaxies. But to many it may actually have more than one meaning including all objects and matter outside the Earth's atmosphere including their physical and chemical properties, or perhaps they may feel that it is really the study of everything given that it is the study of the Universe and everything is part of the universe...
But no matter how you consider the definition of Astronomy it does have many sub-sciences or aspects to it as well, including:
Cosmology which studies the known Universe (Cosmos)as a whole including it's origins or believed origins.
Astrometry which is defined as the study and measurement of great distances
Mathematical Astronomy uses math (numbers, calculations and known statistics) to explain the Universe and it's movements
Planetology which is the study of planets that are, within our own Solar system and those orbiting distant stars within our system
Radio Astronomy which includes the use of radio-telescopes to study and listen to the sounds of the Universe
Positional Astronomy studies the positions of the planets within a galaxy and relies heavily on the studies of Astrometry and Mathematical Astronomy to explain planetary positioning
Astronomy is considered the oldest science. Since time began Man has looked to the "Heavens" and tried to contemplate the Universe. Perhaps, it is this primal fascination with the nighttime sky was the spark for the rudimentary beginnings of Astronomy. The Ancient Greeks were the first known humans to develop the science of Astronomy by developing rudimentary astronomical theories about the design and origins of the universe.
There have been many great Astronomers throughout history who have played dramatic roles in developing this science, although many were actually proved wrong as our technology and knowledge of the universe improved. If it were not for their early work however, Science and Astronomers would have had nothing to base early research on. Many of these early Astronomers are known to just about everyone including Copernicus, Galileo, Isaac Newton, Edmond Halley (known for Halley's comet), William Herschel, Albert Einstein and others. Most recent notable Astronomers include Carl Sagan and Stephen Hawking.