Astronomical Dictionary on Letter #R

Radiant

The point on the Celestial Sphere where the meteors of a meteor shower appear to originate. Meteor s...

Red Giant

A type of star with high luminosity and low temperature, eg Betelgeuse in Orion... ...

Red Shift

The lengthening of the wavelength of electromagnetic radiation caused by relative motion between sou...

Resolution

The amount of small detail visible in an image (usually telescopic); low resolution shows only large...

Retrograde

Rotation of a planet, or orbit, opposite to that normally seen. ...

Right Ascension

The angular distance, measured Eastwards, from the Vernal Equinox. It is one of the ordinates used t...

Relief

The maximum regional difference in elevation....

Rhea

In Greek mythology, mother of Zeus and wife of Cronos the Titan ....

Rift

A valley formed at a divergence zone or other area of extension....

Rille

One of several trenchlike, or cracklike valleys up to several hundred km long and 1-2 km wide common...