RA radar astronomy radial velocity radial-velocity curve radian radiant radiation belt radiation era radiation pressure radiative transfer radio astronomy radio brightness distribution radio galaxy radiograph radioheliograph radio interferometer radiometer radio source radio star radio telescope radius vector Ram Ramsden disc Ramsden eyepiece Ranger RAS Ras Algethi Ra-Shalom RATAN-600 ray R Coronae Borealis recombination epoch recombination line recurrent nova reddening red giant Red Planet Red Rectangle redshift Red Spot reflecting telescope reflection nebula reflectivity reflector(1) reflector(2) refracting telescope refractor refractory regio regolith regression of nodes regular satellite Regulus relativistic Relativity remote sensing réseau residual resolution resolving power resonance resonance gap retardation reticle Reticulum retrograde reversing layer RGO Rhea Rho Ophiuchi cloud Rigel right ascension Rigil Kentaurus rille rima ring antenna ring galaxy Ring Nebula ring system Ritchey-Chrétien telescope River Robertson-Walker metric Roche limit Roche lobes ROE Ronchi test Roque de los Muchachos Observatory Rosalind ROSAT Rosetta Rosette Nebula Rossi X-ray Timing Explorer (RXTE) rotation curve rotation measure Royal Astronomical Society Royal Greenwich Observatory Royal Observatory, Edinburgh r-process RR Lyrae star RS Canum Venaticorum star R star R-type asteroid Rudolphine tables Rule runaway star runoff channel rupes RV Tauri star RXTE Ryle Telescope