Quasar;0; 
Radar;149; 
Radial velocity;267; 
Radian;355; 
Radiant;393; 
Radiation;495; 
Radiative zone;572; 
Radio astronomy;679; 
Radio telescope;802; 
Radio waves;895; 
Recoil;945; 
Red dwarf;1002; 
Red giant;1088; 
Red shift;1115; 
Reflecting nebula;1230; 
Reflecting telescope;1317; 
Reflector;1515; 
Refractor;1581; 
Relativity;1652; 
Resolution;1718; 
Right ascension;1831; 
Rockets;1917; 
Rotation;1985; 
Satellite;2051; 
Saturn V rocket;2092; 
Schiaparelli;2167; 
Schmidt telescope;2245; 
Scintillation;2382; 
Seasons;2514; 
Seyfert galaxies;2568; 
Signs of the zodiac;2667; 
Silicate rock;2769; 
Silicates;2869; 
Singularity;2966; 
Solar eclipse;3044; 
Solar spectrum;3277; 
Solar system;3385; 
Solar wind;3479; 
Solstice;3536; 
Spacecraft;3675; 
Spectroscope;3727; 
Spectrum;3818; 
Speed of light;3951; 
Spicules;4017; 
Spiral galaxy;4102; 
Standard candle;4182; 
Star motion;4390; 
Star rotation;4457; 
Star;4265; 
Static mass;4527; 
Sumerians;4580; 
Sun;4668; 
Sunspot cycle;4852; 
Sunspot;4752; 
Supergiant;5001; 
Supernova;5072; 
Synodic period;5205;