Astronomy Dictionary

Word Definition Absolute magnitude How bright a star would look if it were 32.6 light years away from the Earth. Absolute zero The lowest possible temperature, -273.16 degrees C. Acceleration Change in velocity (speed, or direction). Airglow The natural glow of the night sky due to reactions that take place in the Earth's upper atmosphere. Albedo The albedo of an object is how much light it reflects. A perfect reflector such as a mirror would have an albedo of 100, the moon has an albedo of 7, and the Earth has an albedo of 36. Angstrom unit Unit used to measure the wavelength of light, and other electromagnetic radiation. Annular Shaped like or forming a ring. Apastron When two stars that orbit each other are as far away from each other as they can get. Aphelion The point in an object's orbit around the Sun when it is furthest from the Sun. Apogee The point in an object's orbit around the earth when it is furthest from the Earth. Aerolite A meteorite which is stoney. Asteroid A rock, or Minor Planet orbiting the Sun. Astrology A belief that links the positions of the stars and planets to human destinies. It has no scientific background. Astronomical Unit The distance from the Earth to the Sun. Usually written AU. Astrophysics The use of physics and chemistry in the study of Astronomy. Atmosphere The gaseous area surrounding a planet or other body. Atom The smallest particle of any element. Aurora Beautiful lights seen over the polar regions which are caused when energized particles from the Sun react with the Earth's magnetic field.  Axis An imaginary straight line on which an object rotates. Background radiation Weak microwave radiation coming from space in all directions. It is believed to be the remnant of the Big Bang. Barycentre The center of gravity of the Earth and moon. Binary star A star which is actually made up of two stars orbiting each other. Black Hole A region of space around a very small and extremely massive object within which the gravitational field is so strong that not even light can escape. Bolide A brilliant meteor, which may explode during its descent through the Earth's atmosphere. Bolometer A sensitive radiation detector. Celestial sphere An imaginary sphere surrounding the Earth. It is used to help astronomers explain where objects are found in the sky.  Cepheid A variable star that scientists can use to determine how distant a galaxy or star cluster is. Charge-Coupled Device (CCD) A sensitive imaging device which is replacing photography in most branches of Astronomy. Chromosphere Part of the Sun's atmosphere, it is visible during a total solar eclipse. Circumpolar star A star which never sets, but can be viewed year round. Clusters A group or stars or galaxies which are held together by their common gravity. Color index A measure of a star's color, which tells scientists how hot the star's surface is. Coma The hazy-looking patch surrounding the nucleus of a comet. Comet A small, frozen mass of dust and gas revolving around the sun. Conjunction When a planet appears to come close to another planet or star. It only appears to come close because it moves in between the other object and the Earth. Constellation A grouping of stars which have been given names by ancient astronomers because of the way they look. Corona The outermost part of the Sun's atmosphere. Coronagraph A type of telescope designed to view the Sun's Corona. Cosmic rays High-speed particles that reach the Earth from Outer Space. Cosmology The study of the universe. Day Amount of time it takes the Earth to spin once on its axis. Density The compactness of matter. Direct motion Objects moving around the Sun in the same direction as the Earth are moving in direct motion, objects moving in the opposite direction are moving in retrograde motion. Diurnal motion The apparent motion of the sky from East to West caused by the Earth moving from West to East. Earthshine The faint glow of the moon when the side facing Earth is dark. Caused by light reflecting off the Earth. Eclipse When our view of one object in the sky is blocked by either another object or the Earth's shadow. Ecliptic The path the Sun, Moon, and planets all follow in the sky. Ecosphere The area around a star where it is just the right temperature for life to exist. Electron Negative particle which orbits an atom. Element Substance which cannot be broken down any further. There are 92 known elements. Equinox March 21st and September 22nd. Twice a year when the day and night are the same amount of time all around the world. Escape velocity The speed an object must have in order to escape from another object's gravity. Exosphere The outermost part of the Earth's atmosphere. Flares (Solar Flares) Beautiful eruptions in the outer part of the Sun's atmosphere. Galaxy A group of stars, gas and dust held together by gravity. Gamma ray Extremely short-wavelength and energetic electromagnetic radiation. Geocentric Simply means the Earth in the Center. People used to believe the Universe was geocentric, or that the Earth was in the center of the Universe. Geophysics Study of the Earth using Physics. Gibbous When the Moon is more than half full, but less than completely full. HI region Cloud of neutral hydrogen. HII region Cloud of ionized hydrogen. Hertzsprung-Russell Diagram A diagram which helps scientists understand different kinds of stars. click here to learn more. Hubble Constant The relationship between the distance of an object and the speed at which it is traveling away from us. The further away an object is the faster away from us it is traveling. Inferior planets Mercury and Venus which lie closer to the Sun than the Earth are called inferior planets. Ionosphere A region of the Earth's atmosphere. Kelvin A measurement of temperature often used in astronomy. 0 degrees Kelvin equals -273 degrees Celsius and -459.4 degrees Fahrenheit. Kepler's Laws of Planetary Motion 1. The planets move in elliptical orbits with the Sun at one focus. 2. An imaginary line joining the center of a planet to the center of the Sun sweeps the same amount of space all the time. 3. The time it takes a planet to orbit the Sun is related to how far away from the Sun an object is. Kirkwood gaps Regions in the asteroid belt where almost no asteroids can be found. This is due to the fact that the giant planet Jupiter changes the orbits of any object which enters these areas. Light Year The distance which a ray of light would travel in one year. This is about 6,000,000,000,000 (6 trillion) miles. Limb The edge of any object in Outer Space. The edge of the Moon for example. Local Group A group of around two dozen galaxies. It is the group to which our galaxy belongs. Lunation Period between new moons. 29 days 12 hours 44 minutes. Magnetosphere Region around an object where the influence of the object's magnetic field can be felt. Mass How much matter an object contains. It is not the same as weight, although an object's mass does help determine how much it will weigh. Meteor A shooting star, observed when a particle of dust enters into the Earth's atmosphere. Meteorite An object from Outer Space, such as a rock, that falls into the Earth and lands on its surface. Meteoroids Any small object in Outer Space, such as dust or a rock. Micrometeorites An extremely small object. They are so small that when they hit the Earth's atmosphere they do not create a shooting star effect. Milky Way Our Galaxy. (The word "Galaxy" actually means milky way in Greek). Minor planet Asteroid Molecule A group of atoms linked together. Multiple star A group of stars that orbit each other. Nadir That point on the celestial sphere directly below the observer. Nebula A cloud of gas and dust. Neutrino A very small particle with no mass or charge. Neutron star The remnants of a dead star. They are incredibly compact and spin very quickly, some spin 100 times a second. Nova A star which suddenly flares up to many times its original brightness before fading again. Oblate Spheroid A planet that is not perfectly round because it is wider in the middle and shorter from top to bottom. Occultation The covering up of one celestial body by another. Opposition When a planet is exactly opposite the Sun so that the Earth is between them. Orbit The path one object takes around another. Ozone An area in the Earth's upper atmosphere which absorbs many of the lethal radiations coming from space. Parallax The shift of an object when it is viewed from two different places. For example if you close one eye and look at your thumb nail, and then switch eyes, you will see everything in the background move back and forth. Scientists use this to measure the distance to stars. Parsec 3.26 light years Penumbra The lighter part of a shadow found on the shadow's edge. Periastron When two stars that orbit each other are at their closest point. Perigee The point in an object's orbit around the Earth when it is closest to the Earth. Perihelion When an object which revolves around the Sun is at the closest point it gets to the Sun. Perturbations The disturbances in the orbit of a celestial object caused by the gravitational pull of another object. Phases The apparent change in the shape of the Moon, Mercury, and Venus due to how much of the sunlit side is facing the Earth. Photosphere The bright surface of the Sun. Planet An object moving around a star. Planetary nebula A nebula of gas surrounding a star. Precession The Earth behaves like a spinning top. Its poles are spinning in circles causing the poles to point in different directions over time. It takes 25,800 years for the Earth to complete one precession. Proper motion The motion of the stars across the sky as seen from Earth. Closer stars have a higher proper motion than more distant ones, just as in your car closer objects such as road signs seem to move faster than distant mountains and trees. Proton The center of an atom is made up of one or more protons. It has a positive charge. Quasar A very distant, immensely bright object. Radiant The area in the sky where during a meteor shower the meteors appear to radiate from. Radio galaxies Galaxies which are extremely powerful emitters of radio radiation. Red shift When an object is traveling away from the Earth, the light from this object is stretched out, making it look redder. Revolve When something is moving in a circle around another object, such as the way the Moon Circles the Earth, it is said to revolve around that object. Rotate When an object spins it is said to be rotating. Saros cycle A period of 18 years 11.3 days in which eclipses repeat themselves. Satellite A small object orbiting a larger one. There are many electronic objects that orbit the Earth. Scintillation Twinkling of stars. Due to the Earth's atmosphere. Seeing The condition of the Earth's atmosphere at a particular time. If the sky is clear astronomers say there is good seeing. Selenography The study of the Moon's surface. Seyfert galaxies Galaxies with small bright centers. Many Seyfert galaxies are good sources of radio waves. Shooting star A light in the atmosphere caused by a meteor falling towards the Earth. Sidereal Period The period of time that it takes an object in space to complete one full orbit in relation to the stars. Solar System The system of planets and other objects orbiting the star Sol, which happens to be our Sun. Solar wind A steady flow of particles streaming out from the Sun in all directions. Solstice 22 June, and 22 December. Time of the year when the day is either shortest, or longest depending on where you are. Spicules Jets up to 16,000 kilometers in diameter in the Sun's atmosphere. Stratosphere Level of the Earth's atmosphere from about 11-64 kilometers (7-11 miles) above sea level. Star A self-luminous object that shines through the release of energy produced by nuclear reactions at its core. Supernova A super bright explosion of a star. A supernova can produce the same amount of energy in one second as an entire galaxy. Sundial Ancient instrument used to tell time. Sun spots Dark patches on the Sun's surface. Superior planets The planets which lie further from the Sun than the Earth. Synchronous satellite An artificial satellite which moves around the Earth at the same speed that the Earth rotates, so that it is always above the same part of Earth. Synodic Period The time it takes an object in space to reappear at the same point in relation to two other objects, e.g., the Earth and Sun. Syzygy The position of the Moon in its orbit when at new or full phase. Terminator The line between day and night on any celestial object. Thermocouple An instrument used for measuring very small quantities of heat. Time dilation The idea that as you approach the speed of light time slows down and mass increases. Trojans asteroids Asteroids that circle the Sun following Jupiter's orbit. Troposphere The lowest part of the Earth's atmosphere. Umbra The dark inner part of a sunspot, or shadow. Variable stars Stars which fluctuate in brightness. Zenith Point directly above your head in the night sky.