Parallax The degree of angle between the two lines of a triangle arising from the baseline; in astronomy, used to determine the distance of stars. Parsec (pc) A measure of extremely great distance within the universe, equal to 3.26 light-years or 19 trillion miles (30.4 trillion km). Penumbra Partly-shaded convergent ring around the dark center of a shadow, such as the shadow cast by the Earth on the Moon during a lunar eclipse. The lighter outer part of a sunspot is also called the penumbra. Periastron The smallest distance apart between two stars in a binary star system, as they orbit each other. Perigee Point on a satellite's orbit where it is closest to the planet. In the case of the Moon, it is about 32,000 miles (51,000 km) closer at perigee than at apogee. Perihelion The point in a celestial body's orbit when it is closest to the Sun. Phases The periods when an object in space is partly or fully lit by the Sun. Like Earth's Moon, Mercury and Venus pass through phases as we watch them from Earth. Phobos Satellite of Mars, only 17 miles (27 km) in diameter; orbits Mars in 8 hours. Phoebe Satellite of Saturn, 140 miles (220 km) in diameter; takes 550 days to orbit once, traveling around the planet in the opposite direction from most other bodies in the Solar System. Photons Sub-Atomic particles that make up electromagnetic radiation; they have no mass, but are conveyors of energy in both particle and wave form. Photosphere The layer of the Sun that we see as its surface. Planet One of the bodies that revolve around a star. Our Earth is one of the Sun's planets. Planet X Original name given to Pluto before it was discovered. Some astronomers believe that there may be another planet-sized body in the outer Solar System, beyond Pluto's orbit. Planetary nebula The cloud of an old-age star that has thrown off its outer shell of hydrogen. Originally named because they looked like planets, but there is no connection between planets and planetary nebulae. Planetesimals Original matter from the Sun's maturing into a star that did not become part of a full-fledged planet. Polestar The star designated as the North Star, that approximately identifies the location of the celestial north pole. Precession The circular movement of an axis. As an example, the Earth's rotation precesses very slowly in a circle about 46 degrees in diameter, over the course of thousands of years. This causes a different star to become the pole star every few thousand years. Primary mirror The mirror in a reflecting telescope that reflects light to the focus, the spot where incoming light rays converge. Plates Sections of Earth's crust that are created by the movement of rock in Earth's mantle. Pleiades Cluster of stars in the constellation Taurus, about 250 light-years away, often refereed to as the Seven Sisters. The stars were formed less than a hundred million years ago, making the cluster one of the youngest in the sky. Pluto Ninth planet, 1,370 miles (2,200 km in diameter. Pluto takes over 248 years to orbit the Sun. In 2113, at its farthest point from the Sun, it will be 4.6 billion miles (7.4 billion km) away. Polaris, the North Star Yellow giant star, about 300 light-years away, 1,500 times as luminous as the Sun. To someone standing at the North Pole, Polaris would be directly over head. Although called the Pole Star, Polaris is slightly less than 1 degree from the true celestial pole. Pole Either end of the axis around which a planet, moon, or star rotates. Probe A craft that travels in space, photographing celestial bodies, taking measurements, and even landing on some of them. Prominence Eruption of gas from the Sun's surface. Sometimes visible during a Total Solar Eclipse. Proper motion Changes in the relative position of stars over many years, caused by their motions in the galaxy. Protoplanet The mass of dust, gases, and debris ejected during the explosion of a new star, which are coalescing to become a planet. Protostar An early stage of a star just prior to gaining enough mass to spark thermonuclear fusion, thereby becoming a full-fledged star. Proxima Centauri Closest star to the Sun, 4.2 light-years away. A red dwarf about 10,000 times dimmer than the Sun. It belongs to a triple-star system which includes the bright binary Alpha Centauri, also known as Rigel Kentaurus. Pulsars High-density celestial objects that emit regular bursts of radiation (mostly radio waves), which many scientists believe are neutron stars. Quasars Originally QSO or QSR, meaning Quasi-stellar Objects or Quasi-stellar Radio Source. The most distant observable objects in the universe. Thought to be the centers of highly active, young galaxies. Quasar 3C 273 The first quasar to be discovered and one of the brightest from Earth. Radio dishes The name given to instruments that gather radio waves as a telescope collects light rays. Radio waves relatively long wavelength and slow frequency energy emission, useful in detecting cooler objects in space, such as nebula. Radio telescope An instrument that uses a radio receiver and antenna to both see into space and listen for messages from space. Rays (Crater) Visible streaks emanating from a crater, created by material that was thrown out beyond the crater's edge after a meteorite impact. Red dwarfs Main sequence stars at the cooler, fainter end of the Hertzsprung-Russell diagram. Red giants (Super Giants) Cool, but bright and luminous stars. Red shift A shift towards lower energies in the light waves from galaxies that are receding from our galaxy. Refracting telescope An optical telescope that uses ground glass lenses to gather and enlarge light. Retrograde In the opposite direction from the norm. Retrograde motion The planets appear to move slowly in front of the stars, from west to east. However, when they are near, the Earth's opposition speed makes them seem to move backward, or "retrograde". Rhea Crater-covered satellite of Saturn, 950 miles (1,530 km) in diameter; takes 4.5 days to orbit the planet. Rigel Brilliant super giant star at the South end of Orion, 850 light-years away. About 40,000 times as luminous as the Sun; the seventh brightest star in the sky. Right ascension (RA) The imaginary vertical lines on the celestial sphere used with declination to locate the positions of celestial objects; corresponds to longitude lines on Earth. Rotate To turn or spin on an axis. Rocky planets Mercury, Venus, Earth, and Mars; the planets closest to the Sun. They all have rock and metal at their centers. Saros Ancient astronomer who found that Solar and Lunar eclipses repeat after an interval of 18 years, 10 days and 8 hours. Saturn Sixth planet from the Sun, with a diameter of 75,000 miles (120,000 km), making it second to Jupiter in size. Saturn's bright rings are visible from Earth with virtually any telescope. Saturn has over 25 satellites, the largest of which is Titan. Saturn orbits the Sun in about 29.5 years. Therefore it takes almost 2.5 years to move through each Zodiacal constellation on the average. Satellite A natural or manufactured object in space that orbits a host planet. There are more than 70 natural satellites found in our solar system. They are referred to as Moons. Scintillation Star's twinkling appearance caused by the atmosphere. Secondary mirror A mirror in a reflecting telescope that reflects incoming light from the primary mirror toward an eyepiece, through which the light can be viewed. Seyfert galaxies Highly active galaxies that emit 100 times the electromagnetic radiation of ordinary galaxies. Sidereal time The system of keeping time based on the stars, or a 'day' of 23 hours, 56 minutes, and 4 seconds, the time it takes Earth to complete one rotation, as viewed against the fixed stars. Sirius The brightest star in the sky, only 9 light-years away and about 25 times more luminous than the Sun. Has a dim white dwarf companion that was once much brighter than Sirius, but has already passed through the red giant stage. SN 1987A Bright supernova that erupted in the large Magellanic Cloud on February 24, 1987. Reached peak brightness of about 50 million Suns. Solar eclipse The obscuring of the Sun from the view of the Earth when the Moon lines up directly between the Sun and Earth, e.g. a total solar eclipse took place on Aug. 11, 1999 in southwestern England, northern France, Hungary, Germany, Romania, and Austria. Solar system The organization of celestial bodies orbiting the Sun and the space they inhabit. Solar time The system of keeping time based on a 24-hour day, the average time it takes the Sun to reappear in the same spot at high noon. Solar wind Tiny energized particles that travel from the Sun's surface at a speed of about 250 miles (402 km) a second. Space shuttle A reusable spacecraft originally designed to travel between Earth and an orbiting spacecraft. Space station A permanent or semi-permanent orbiting satellite intended to house laboratories, equipment, and crews that are in space for extended periods of time. Spectral classes Categories of stars based on their temperatures. From hottest to coolest they are traditionally referred to as O, B, A, F, G, K, M. Spiral galaxy A type of galaxy with a round nucleus and arms that spiral out like a pinwheel. Star clusters Groups of stars held in relative formation by mutual gravitational force, ranging from a few stars to many thousands. Steady state theory The theory that the universe is balanced by the creation of new mass as it expands, thereby escaping the fate of both the closed and open-universe models. Stratosphere The second layer of atmosphere from Earth's surface. Sulfuric acid A corrosive liquid able to dissolve solid rock. It is found in Venus's atmosphere, making Venus one place where there is truly "acid rain". Sun Yellow main sequence star at the center of the Solar System, 900,000 miles (1,392,000 km) in diameter and about 4.6 billion years old. It is 8 light-minutes away from the Earth. Sunspots Cooler areas on the Sun's surface indicating magnetic disturbances. Super clusters A group of galaxy clusters held together by mutual gravitational force. Superior conjunction When the Sun is in direct alignment between an inferior planet and Earth. Superior planet Any planet that orbits the Sun further than the Earth, e.g., Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are Earth's superior planets. Supernova The explosion of a star that has used up its nuclear fuel. Terraforming Making another world like Earth by giving it qualities that are, as far as we know, special to Earth, such as oxygen and water. Terra is Latin for Earth. Terrestrial planets Mercury, Venus, Earth, Mars, and perhaps Pluto; so called because they are primarily made of rocky material similar to Earth. Tethys Satellite of Saturn, 660 miles (1,060 km) in diameter. Orbits the planet in 1.9 days. Shares its orbit with two other tiny moons, Telesto and Calypso. Thermal energy The energy of a system contained in the random motions of its constituents; measured by the system's temperature. Thermonuclear fusion The process by which elements join together to form other elements, e.g., 4 hydrogen atoms combine and interact to form 1 atom of helium. The basic source of a star's energy. Thermosphere The fourth, and final, layer of atmosphere from Earth's surface. Titan Largest satellite of Saturn, 3,200 miles (5,150 km) in diameter; orbits Saturn in 15.9 days. Second-largest satellite in the Solar System. Has a methane atmosphere 120 mi. (200 km) deep. There is currently a space probe, Cassini, headed towards Titan. Titania Largest satellite of Uranus, 728 miles (1,172 km) in diameter; orbits the planet in 4.1 days. Titania has many craters and wide valleys, as though the hard surface split open in its youth. Total solar eclipse The eclipse of the Sun during which the Moon completely hides the Sun from the viewer on Earth, in a narrow band from 0 to 150 miles wide. Such an eclipse can last from 0 to 7 minutes. Trace chemicals Chemicals that appear in very small amounts, or traces. Transit When a smaller celestial object crosses in front of a larger one. Mercury transited the Sun in the year 2000. Triton Satellite of Neptune, 1,680 miles (2,705 km) in diameter; orbits the planet in 5.9 days, traveling backward like Phoebe in Saturn's family.    Troposphere The layer of atmosphere nearest Earth's surface.   Twilight The time at sunset when the Sun is below the horizon but there is still a little light left in the sky. There are 3 different definitions of twilight: Civil twilight - when the sun is from 0 to 6 degrees below the horizon. Nautical twilight - when the sun is from 6 to 12 degrees below the horizon. Astronomical twilight - when the sun is from 12 to 18 degrees below the horizon. Generally, at the end of astronomical twilight, the sky is as dark as it can be.