Astronomy Vocabulary

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Universe

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All existing matter and space considered as a whole; the cosmos. Believed to be at least 10 billion light years in diameter and contains a vast number of galaxies; expanding since the Big Bang about 13 billion years ago.

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Galaxy

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A system of millions or billions of stars, together with gas and dust, held together by gravitational attraction.

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Solar system

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The collection of planets and their moons in orbit around a sun, together with smaller bodies such as asteroids, meteoroids, and comets.

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Nutation

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A small, cyclic variation of the Earth's axis of rotation with a period of 18.6 years, caused by tidal forces (mostly due to the gravity of the Moon).

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Precession

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Earth's wobbling on axis over a period of 26,000 years.

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Barycenter

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The center of mass of two or more bodies, usually bodies orbiting around each other, such as the Earth and the Moon.

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Rotation

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The spinning motion of a planet about its axis.

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Revolution

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The movement of one object around another.

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Fusion

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The process in which light nuclei combine at extremely high temperatures, forming heavier nuclei and releasing energy.

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Fission

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A nuclear reaction in which a massive nucleus splits into smaller nuclei with the simultaneous release of energy.