Astronomy, which literally means “the study of the stars,” involves the study of the universe and every type of object that exists in the universe, including our own planet. Astronomers study the universe by investigating the origins, evolution, composition, motions, relative positions, and sizes of celestial bodies.
Unlike astrology, which alleges that the positions of the Sun, Moon, and stars affect human affairs, astronomy is a scientific discipline based upon an understanding of physical principles (the so‐called Laws of Nature) and how their operation produces phenomena that may be observed. These principles can often be summarized in simple mathematical equations. Learning some of the simpler and more fundamental principles allows you both insight into how astronomers learn about the universe as well as an understanding of why the objects we observe have the properties that they do.
The number of physical factors that astronomers can measure is small. These factors include masses, sizes, densities, temperatures, and brightnesses, as well as the time over which changes may occur. Often, these factors are measured in the metric system based on kilograms, meters, and seconds. Many quantities can be numerically large; therefore, astronomers also use other units specific to the discipline of astronomy, such as astronomical units for distances within the solar system, parsecs for distances to stars, solar masses (the mass of the Sun) for masses of stars, and solar luminosities (the luminosities of the Sun) for the energy‐per‐second emitted by stars.
Astronomy has had a long, rich history, from ancient peoples' interpretations of celestial phenomena to modern studies of the universe. Many individuals have made significant contributions to the development of astronomy throughout human history.