Fault A break in the Earth's crust, often found where tectonic plates come together. Frequently the site of earthquakes. Favorable opposition When a superior planet (to Earth) is at its perihelion while in alignment with Earth and the Sun. Fireball A very bright meteor.  Fissure A long, narrow crack, as in a rock or cliff face. Flat universe The theory that the rate of expansion of the universe will even out the rate of gravitational contraction until the universe reaches a point of rest. Focal distance The distance incoming light travels within an optical telescope before reaching the focal point, where it becomes focused for viewing; also known as focal length. Focal point The spot within a refracting telescope where incoming light is focused. Galactic clusters Groups of galaxies held in formation by mutual gravitational force. Galactic year The time it takes our solar system to rotate around the nucleus of the Milky Way. About 230 million years. Galaxy Red giant and the brightest star in the constellation of Bootes, the Herdsman. One of the closest stars, at 27 light years. Galileo Galilei The Italian astronomer who is credited with inventing the first refracting telescope. He discovered the 4 large moons of Jupiter, (they are still called the Galilean satellites), and many other astronomical bodies. One of the first scientists to use the so-called 'scientific method'. Galileo held that the Earth revolved around the Sun, and came into sharp dispute with the Catholic church because of his beliefs. Gamma rays Deadly radiation at the shortest wavelength and highest frequency; they occur during thermonuclear fusion and are detectable in supernovae. Geocentric model A theory of the organization of the solar system that places Earth at the center. Ganymede Satellite of Jupiter, 3,270 miles (5,262 km) in diameter; orbits Jupiter in 7.2 days. The largest moon in our solar system. Globular clusters Densely packed groups of 10,000 to 1 Million stars appearing in the spherical region around a galaxy, called the galactic halo. Globules Protostars, dark clouds of dust and gas about one light-year across. Each globule is heating up at the center, and after many thousands of years will start to shine as a new star. Granules Undulating waves of gas on the Sun's surface. Gravity The force of attraction exerted by any object with mass. Greenhouse effect The phenomenon whereby heat entering a planet's atmosphere becomes trapped and continues to build up until the surface temperature of the planet is raised. It is thought to be responsible for Venus being the hottest planet in the Solar system. Halley's Comet Orbiting the Sun in 76 years, on average. Last passed close to the Sun in 1985-1986 and will be return in 2061. Heliocentric model A theory of the organization of the solar system that places the Sun at its center. Hertzsprung-Russell diagram A chart that plots stars according to luminosity, absolute magnitude, and temperature. Helium A light, colorless gas that makes up part of every star. About 23% of the Universe is Helium. Hydrogen A colorless, odorless gas that is the simplest and lightest of the elements. Stars are three-quarters hydrogen. Hyperion Satellite of Saturn, irregular body about 185 miles (300 km) in diameter, orbits the planet in 21.3 days. Lapetus Satellite of Saturn, 930 miles (1,500 km) in diameter; It takes 79 days to complete an orbit. One of its hemispheres is coal-black. Icarus An asteroid that approaches the Sun even more closely than Mercury does. Named after a mythological boy whose father made him wings of wax and feathers. He flew too close to the Sun and his wings melted, and he tumbled to the sea below. Ice Ages Periods in the history of Earth that saw the movement of great ice glaciers across parts of the land surface of the planet. Ice cap A cover of permanent ice at either or both poles of a planet. Mars has ice caps at both poles. Incline See orbital tilt. Inferior conjunction When an inferior planet is in alignment between the Sun and Earth. Inferior planet Any planet that orbits the Sun at a closer distance, e.g., Mercury and Venus are Earth's only inferior planets. Infrared radiation Beneath the red radiation. Infrared wavelengths are longer than red light wavelengths. Infrared radiation is invisible to the naked eye, but you can feel it as heat. Inner planets Those planers on the inner side (closer to the Sun) of the asteroid belt: Mercury, Venus, Earth, and Mars. Interstellar medium The gas and dust that occupies space between stars. Io Satellite of Jupiter. 2,256miles (3,630 km) diameter and orbiting in 1.8 days. The hottest and most active body in the Solar System. Its surface is a waste of deadly sulfur deposits and live volcanoes. Ionosphere A layer of Earth's atmosphere carrying electrically charged particles; it overlaps the mesosphere and thermosphere. Irregular galaxy A galaxy with a shape that is neither spiral nor elliptical, such as M82. Ishtar Terra The northernmost of Venus's two continents; about the same size as the United States, named for the Babylonian goddess of love. All planarian features on Venus have female names. (See also Aphrodite Terra). Jupiter Largest planet in the Solar System. Its diameter is 88,730 miles (142,800 km); its day lasts only about 10 hours. Has no solid surface and is made principally of hydrogen. Most famous surface feature is the cyclone known as the Great Red Spot, a storm that is much large than the Earth and has been active since at least the early 1600s.