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Astronomy & Telescopes Glossary - R
Radiant - The point in the sky from which a meteor shower seems to originate.
Reflection nebula - A nebula that glows from the reflected light of nearby stars.
Reflecting or reflector telescope - A telescope that uses mirrors, instead of lenses, to magnify and focus images.
Refracting or refractor telescope - A telescope that uses lenses, instead of mirrors, to magnify and focus images.
Resolution - The ability of a telescope to resolve, or reveal, details.
Resolving Power - The ability of a telescope to separate, or split, objects that are close together.
Retrograde motion - When a planet seems to slow down, stop, and move backwards from east to west for a short time. Mercury and Venus exhibit retrograde motion.
Retrograde orbit - An object that orbits in a clockwise, instead of a counter-clockwise, direction when viewed from the north pole of the ecliptic.
Retrograde rotation- All the planets in the Solar System rotate in the same direction except for Venus, Uranus and Pluto, which rotate in the opposite (retrograde) direction.
Right ascension (RA) - Part of a coordinate system similar to latitude on the Earth's surface. Right-ascension (RA) is measured in hours, minutes and seconds. Zero degrees of right ascension is the position of the Sun during the Vernal (Spring) equinox. RA is used along with Declination to find objects in the night sky.