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Science | Dictionary | Astronomy Terms

Astronomy terms are used to describe the various phenomena in space. In this section you can learn what every astronomy term means and how it helps us to better understand the cosmos.

Planetesimal Hypothesis

Planetesimal Hypothesis, a theory of the origin of the solar system. It was proposed by Forrest R.


Chronology, the science of measuring time. Chronology divides time into regular divisions or periods, and assigns events their proper place and sequence by giving them dates. Some striking event or change is chosen as the starting point in measuring ... See more »


Chronometer, a timepiece that is exceptionally accurate. Traditionally, the term refers to the marine chronometer, a rugged mechanical instrument used at sea to keep time for navigational purposes. By measuring the position of a celestial body (with ... See more »


Cosmogony, the study of the origin and development of the universe as a whole and of the individual bodies that compose it. Since cosmogony attempts to deal with creation, cosmogonies of the past have been a part of religion or mythology. Modern cosm ... See more »


Cosmology, the study of the universe. It is both a scientific subject and a philosophical one. See more »


Day, in astronomy, the average length of time between successive noons. Noon is defined as the instant when the sun is highest in the sky. Since apparent motion of the sun is caused by the rotation of the earth, a day is therefore the length of time ... See more »

 Double Star

Double Star, a pair of closely-spaced stars that to the unaided eye usually appear as a single star. The pair is called an optical double if the two stars seem to be close together only because of their alignment as seen from earth. A true double sta ... See more »


Ecliptic, the apparent annual path of the sun among the stars. The sun appears to follow a path through the stars because the earth revolves around the sun. The apparent motion of the sun along the ecliptic each day amounts to about one degree (about ... See more »