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.

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Eclipse. An eclipse of the sun, or solar eclipse, occurs when the moon's shadow sweeps across the earth.


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.

Epoch (Chronology)

Epoch, in chronology (timekeeping), a point in time, such as 302 B.C. or October 30, 1936, or 7:34 A.M.


Equinox, During the course of a year, the sun appears to move northward for about six months and southward for about six months.

Evening Star

Evening Star, the name given to any of the five bright planets in the sky at sunset.


Hour, a unit for measuring time, defined as either 1/24 of a day or 3600 seconds (60 minutes).


Hourglass, a device for measuring time. In its usual form it consists of two cone-shaped or oval glass receptacles joined by a narrow neck.


Interferometer, an instrument that uses the interference patterns formed by waves (usually light, radio, or sound waves) to measure certain characteristics of the waves themselves or of materials that reflect, refract, or transmit the waves.


Magnitude, in astronomy, a unit of measurement of the brightness of stars. The scale of magnitude extends from negative numbers (for example, the minus first magnitude) for very bright stars to positive numbers (for example, the fourth magnitude) for dimmer ones.


Meteor. As the term is commonly used, a meteor is a small celestial body that enters the atmosphere of the earth.

Midnight Sun

Midnight Sun, a name given the sun when it can be seen at midnight during the Arctic or Antarctic summer.


Millennium, in a common Christian belief, the period of 1,000 years before the Last Judgment, during which time Christ and the saints will reign on earth.


Minute, a unit for measuring both time and space. As a unit of time, a minute is 60 seconds, or 1/60 of an hour.


Month, a unit of time. The calendar month is approximately 1/12 of the calendar year.

Nebular Hypothesis

Nebular Hypothesis, an explanation of how the solar system was formed, proposed by Pierre Simon de Laplace in 1796.


Night, the period of darkness caused by the disappearance of the sun below the horizon.


Nova, (plural: Novae), a star that rapidly increases in brightness and then fades again.


Occultation, in astronomy, the passing of the moon or some other object of the solar system in front of a planet, star, or other celestial body, hiding it from view.


Orbit, the path of a celestial body or other object in space, governed by the gravitational attraction of other bodies.


Orrery, a mechanical device used to show the motions of bodies within the solar system.


Parallax, the apparent change in position of an object when it is viewed from two different locations.

Perihelion and Aphelion

Perihelion and Aphelion, The perihelion is the point on the orbit of a planet or comet that is closest to the sun.


Perturbation, in astronomy, is a disturbance in the orbit or motion of a heavenly body.


Planetarium, is an educational device for showing the locations and movements of the planets and other objects in the universe.

Planetesimal Hypothesis

Planetesimal hypothesis is a theory of the origin of the solar system. It was proposed by Forrest R.