Gaia hypothesis galactic centre galactic cluster galactic coordinates galactic corona galactic halo galactic plane galactic poles galactic year Galatea galaxy Galaxy galaxy cluster Galilean satellites Galilean telescope Galileo Galileo National Telescope gamma-ray astronomy gamma-ray burster Gamma Ray Observatory Ganymed Ganymede gardening Garnet Star gaseous nebula Gaspra Gassendi gas tail gegenschein GEM Geminga Gemini(1) Gemini(2) Gemini 8-meter Telescopes Geminids Gemma General Relativity geocentric model geocorona geodesic geodesy Geographos geoid geomagnetic field geomagnetic storm geometric albedo geospace geostationary orbit geosynchronous orbit German mounting Ghost of Jupiter Giacobinids Giacobini-Zinner, Comet giant branch Giant Metrewave Radio Telescope giant molecular cloud giant planet giant star gibbous Ginga Giotto Giraffe glitch Global Oscillation Network Group globular cluster globule GMAT GMC GMRT GMST GMT gnomon Goat Goddard Space Flight Center Goldstone GONG Gossamer Ring Gould's Belt graben GRANAT Grand Unified Theory granulation granule graticule grating spectrograph Graving Tool gravitation gravitational collapse gravitational instability gravitational lens gravitational radiation gravitational redshift gravitational waves gravity gravity assist grazing occultation Great Attractor Great Bear great circle Great Dark Spot Greater Dog Great Nebula in Orion Great Red Spot Great Rift Green Bank Green Bank Telescope green flash greenhouse effect Greenwich Mean Astronomical Time Greenwich Mean Solar Time Greenwich Mean Time Greenwich Observatory Greenwich sidereal date Greenwich Sidereal Time Gregorian calendar Gregorian telescope Griffith Observatory Grimaldi GRO Groombridge Catalogue Grus GST G star G-type asteroid Guardians guest star guide star Guide Star Catalog guide telescope Gum Nebula GUT