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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.


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 »

 Epoch (Chronology)

Epoch $eBp$oBk in chronology (timekeeping) a point in time such as 302 B.C. or October 30 1936 or 7:34 A.M. See more »


Equinox, During the course of a year, the sun appears to move northward for about six months and southward for about six months. The times when the sun appears to pass over the earth's equator (on or about March 21 and September 23) are called the eq ... See more »

 Evening Star

Evening Star, the name given to any of the five bright planets in the sky at sunset. It is thus not actually a star and there may be several at one time. The evening stars continually change as the planets move in relation to the sun. A planet may be ... See more »


Hour, a unit for measuring time, defined as either 1/24 of a day or 3600 seconds (60 minutes). Used alone, the word hour refers to clock time, a composite of mean solar time and atomic time. There are other types of hours, such as sidereal hours (9.8 ... See more »