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Astronomy & Telescopes Glossary - G
Galaxy - A large group of solar systems, stars, nebulae and other celestial bodies bound together by gravitational forces. Our solar system is part of the Milky Way galaxy.
Galactic coordinates - A system of latitude and longitude coordinated defined by the plane of our galaxy rather than the equatorial system (RA and Declination based on the celestial equator.
Galilean moons - The four largest moons of Jupiter discovered by Galileo: Ganymede, Io, Callisto and Europa.
Gas giants - The large outer planets of our Solar System, including Jupiter, Saturn, Uranus, and Neptune.
German equatorial mounting - A telescope mount consisting of two rotating axes at right angles. The axis, called the Right Ascension (RA) axis or polar axis, is parallel to the Earth's rotational axis. The second axis, called declination axis, points toward the celestial equator. The end of the declination axis carries a counterweight.
Gibbous moon - A moon that is between a full moon and a half moon.
Globular cluster - Dense clusters of stars bound by gravity. Many globular clusters surround galaxies, including our own Milky Way galaxy.
Great Red Spot - A large, reddish storm in the atmosphere of Jupiter that is shaped like an oval.
Great Dark Spot - A dark, Earth-sized hole in the methane cloud layer of Neptune.
Green flash - Sometimes, at sunrise or sunset, there are flashes of green light visible near the sun. This effect only happens when there are no clouds nearb the sun. The green flash (also called green ray) is caused mostly by refraction (blue light from the sun bends more than other wavelengths) of light from the sun by the Earth's atmosphere.