Astronomical Dictionary
Letter: R


Dictionary of Astronomical Definitions and Terms.
Astronomical | R

 

Definition


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Radiant
The point on the Celestial Sphere where the meteors of a meteor shower appear to originate. Meteor showers are usually named after the constellation in which the radiant originates.
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Red Giant
A type of star with high luminosity and low temperature, eg Betelgeuse in Orion...
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Red Shift
The lengthening of the wavelength of electromagnetic radiation caused by relative motion between source and observer. Spectral lines are red-shifted from distant galaxies, indicating that the galaxies are moving away from us due to the expansion of the Universe. This is one of the central facts in the Expanding Universe Theory.
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Resolution
The amount of small detail visible in an image (usually telescopic); low resolution shows only large features, high resolution shows many small details.
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Retrograde
Rotation of a planet, or orbit, opposite to that normally seen.
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Right Ascension
The angular distance, measured Eastwards, from the Vernal Equinox. It is one of the ordinates used to reference objects on the celestial sphere. It is the equivalent to a longitude reference on the Earth. There are 24 hours of right ascension within 360 degrees, so one hour is equivalent to 15 degrees. Together with declination, it represents the most commonly used co-ordinate system in modern astronomy.
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Relief
The maximum regional difference in elevation.
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Rhea
In Greek mythology, mother of Zeus and wife of Cronos the Titan .
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Rift
A valley formed at a divergence zone or other area of extension.
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Rille
One of several trenchlike, or cracklike valleys up to several hundred km long and 1-2 km wide commonly occurring on the Moon's surface.
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