Science Encyclopedia

Science Encyclopedia

If you would like to prepare for school subjects or simply increase your general knowledge, then enjoy our science encyclopedia. We tried to focus only on very important terms and definitions. We also kept our terminology very brief so that you absorb the concept more quickly and easily.

Science Glossary (Page 1)


ABERRATION: Property of an optical system that causes an image to have certain easily recognisable flaws. Aberrations are caused by geometrical factors such as the shapes of surfaces, their spacing, and alignments. Image problems caused by factors such as scratches or contamination are not called aberrations.
ABSOLUTE TEMPERATURE: Temperature in degrees centigrade (also known in this case as "degrees Kelvin" K°) measured from the absolute zero of -273.1° C, the temperature at which all atomic and molecular motions are expected to cease.
ABSOLUTE ZERO: The lowest temperature ever reachable in the Universe: 0 Kelvin (0K), equivalent to minus 273 degrees Celsius (-273 oc). In laboratories on Earth physicists can get very close to that temperature, but it is impossible to achieve the absolute zero.
ABSORPTION: Decrease in intensity of radiation, when it crosses a material medium, as a consequence of an interaction between the radiation and the material medium.
ABSORPTION SPECTRA: See Line (absorption).
ABUNDANCE: Relative number of atoms of a particular element, or isotope of an element, in the chemical composition of a single substance or object.
ACAROLOGY: Branch of Zoology dealing with ticks & mites.
ACCELERATION: Rate at which velocity changes (negative acceleration.
ACCELEROMETER: Oscillatory mechanical system measuring the acceleration of the body to which it is attached.
ACCRETION (DISK, ZONE): Process whereby small particles of matter accumulate and create larger bodies under the influence of their mutual gravitational attraction or as a result of chance collisions.
ACOUSTICS: The study of sound or the science of sound.
ACROBATICS: The art of performing acrobatic feats (gymnastics).
ACTIVE GALACTIC NUCLEUS (AGN): Central region of a galaxy in which considerable energy is generated by processes other than those present in normal stars. The energy generated by the nucleus may outshine all the other stars in the galaxy. Most astronomers believe that at the centre of an AGN lies a supermassive black hole.
ACTIVE GALAXY: A galaxy which releases large amounts of energy from its centre, the active galactic nucleus. The central engine of an active galaxy probably is a supermassive black hole. Seyfert galaxies, quasars and blazars are active galaxies.
ADIABATIC INVARIANT: A quantity conserved in periodic motion a bit the way energy is conserved, but here it is just approximate. Adiabatic invariants of electrons orbiting nuclei were once thought to be subject to quantum rules, but that turned to be a false lead. Later however adiabatic invariants (on a larger scale, not atomic) turned out to be important in radiation belt physics.
AERODYNAMICS: 1. The branch of mechanics that deals with the motion of air and other gases. 2. The study of the motion and control of solid bodies like aircraft,missiles,etc in air.
AERONAUTICS: The science or art of flight.
AERONOMY: The study of the atmosphere of a planet, with particular attention to the composition, properties and motion of atmosphere constituents.
AEROSOL: A gaseous suspension of ultramicroscopic particles of a liquid or a solid.
AEROSOL COLLECTOR: An instrument that collects aerosols and analyzes their composition.
AEROSTATICS: The branch of statics that deals with gases in equilibrium and with gases and bodies in them.
AESTHETICS: The philosophy of fine arts.
AETIOLOGY: The science of causation.
AGROBIOLOGY: The science of plant life and plant nutrition.
AGRONOMIC: The science of managing land or crops.
AGRONOMY: The science of soil management & production of field crops.
AGROSTOLOGY: The study of grasses.
ALCHEMY: Chemistry in ancient times.
ALGOL: Best known variable star, varying in brightness from about 2.2 to 3.5 magnitudes over a period of approximately 69 hours. It is in fact a binary system in which the two stars regularly cross in front of each other as viewed from Earth. Also known as the Demon Star.
ALIGNMENT: Process of mounting optical elements and adjusting their positions and orientations so that light follows exactly the desired path through the instrument and each optical element performs its function as planned.
ALTITUDE: Height in space of an object or point relative to sea level or ground level.
ANATOMY: The science dealing with the structure of animals, plants or human body.
ANEMOLOGY: The science of wind.
ANGIOLOGY: The science of blood & lymph vessels.
ANGLE OF ATTACK: In the theory of airplane wings, the angle between the wing profile (roughly, measured along its bottom) and the wing's motion relative to the surrounding air.
ANGSTROM: Unit to measure length, sometimes used to measure the wavelength of light. (See also Electromagnetic spectrum). 10 000 Angstroms correspond to 1 micron.
ANISOTROPY, INHOMOGENEITY: (IN THE COSMIC MICROWAVE BACKGROUND) Very small patches in the sky where the temperature of the cosmic microwave background is slightly different to the average; these temperature variations are of the order of microkelvin.
ANOMALY: In orbital motion, one of the angles which gauges the motion of a planet or satellite around its orbit, increasing by 360o every revolution. The true anomaly f equals the polar angle  in polar coordinates with origin at the center of the motion (e.g. Sun or Earth). The mean anomaly is a related angle which increases in direct proportion to the time elapsed (the true anomaly does not.
ANTENNA (HIGH GAIN, LOW GAIN): An aerial for receiving or transmitting radio signals. A high gain antenna is highly focused, whereas a low gain antenna receives or transmits over a wide angle.
ANTHROPOLOGY: The science that deals with the origin and physical and cultural development of mankind.
ANTIMATTER: The 'opposite' to ordinary matter. For every particle of ordinary matter there is an almost identical antiparticle of antimatter: protons and antiprotons; electrons and positrons... The particle's mass is exactly the same as its antiparticle's mass, but their electrical charges - and other fundamental properties - are opposite. When a particle meets its antiparticle, they annihilate each other.
APERTURE: Opening that allows light to fall onto an instrument's optics.
APHELION: The point on a planet's elliptical orbit at which it is furthest from the Sun.
APOCENTRE: The point on a spacecraft's orbit at which it is furthest away from the body it is orbiting.
APOGEE: The most distant point from Earth on a satellite's orbit.
APOLLO (PROJECT): The US mission to land humans on the Moon and bring them back safely.
APPARENT MOTION: The observed motion of a heavenly body across the celestial sphere, assuming the Earth is at the sphere's center and is standing still.
ARBORICULTURE: Cultivation of trees & vegetables.
ARCHAEOLOGY: The study of antiquities.
ARCMIN, ARCSEC: The size of an object in the sky can be measured by the angle that it covers when viewed from Earth. The full circle has 360 degrees. An arcmin is 1/60 of a degree; an arcsec is 1/60 of an arcmin or 1/3600 of a degree. The diameter of the full Moon is about one-half of a degree or 30 arcmin.
ARGON: A chemical element, (symbol Ar, atomic number 18).
ARIANE (4, 5) ROCKETS: European launcher family (Ariane 4 and Ariane 5) developed by the European Space Agency. Launched from Kourou, French Guiana, flights are commercialised and operated by the Arianespace company.
ASTEROID: One of billions of rocky objects, less than 1000 km in diameter, which orbit the Sun. Also known as minor planets. Thought to be planetesimals leftover from the formation of the planets. The first asteroid (Ceres) was discovered by Giuseppe Piazzi in 1801. More than 10 000 asteroids have so far been discovered and given permanent identification numbers. The largest asteroid is 2001 KX76 with a diameter of at least 1200 km.
ASTEROID BELT: Region between the orbits of Mars and Jupiter which is populated by billions of asteroids.
ASTIGMATISM: Failure of an optical system, such as a lens or a mirror, to image a point source of light as a single point.
ASTROLOGY: The ancient art of predicting the course of human destinies with the help of indications deduced from the position and movement of heavenly bodies.
ASTROMETRY: The branch of astronomy concerned with measuring the positions of celestial bodies, such as stars and galaxies, and their real and apparent motions.
ASTRONAUTICS: The science of space travel.
ASTRONOMICAL UNIT (AU): 1 Astronomical Unit corresponds to the distance separating the Earth from the Sun. 1AU=150 million km.
ASTRONOMY: Study of the space beyond the Earth and of its contents.
ASTROPHYSICS: Study of the physical nature of the Universe, its objects and the composition of the space between them.
ATLAS: An early liquid-fueled rocket, used by US astronauts and still in use for unmanned launches. Because of its lightweight construction it needs no staging to achieve Earth orbit, but only drops two of its engines.
ATMOSPHERE: Layer of gases surrounding a star or planet.
ATTITUDE: Orientation of the spacecraft's axes relative to Earth.
AURORA: Illumination of the night sky, caused when electrons and protons from space collide with atoms and molecules of air in the Earth's upper atmosphere. Satellite observations usually show them as rings centred on the planet's magnetic poles. Popularly known as the Northern and Southern Lights. Various types of aurora are also found on Jupiter, its moon Ganymede, and Saturn.
AZIMUTH AND ELEVATION: Two angles which give the direction of a surveyor's telescope (theodolite). Azimuth is the rotation angle of the telescope around a vertical axis, measured from due north, clockwise from above; that is, the directions (north, east, south, west) have azimuth (0°, 90°, 180°, 270°). Elevation is the angle the telescope is lifted above the horizontal plane. [In 3-dimensional polar coordinates centered on the instrument, azimuth is 360°-φ (since φ is measured counterclockwise), elevation is 90°-θ, the direction of straight up has elevation 90° but θ = 0].
BACTERIOLOGY: The study of bacteria.
BAFFLE: A device used to prevent stray light.
BALLISTIC PENDULUM: A device often used for measuring the energy of motion of a bullet, adapted by Goddard to measure the thrust of small rockets with various nozzles. For a bullet is is a heavy block of wood or sand-filled box, hanging by a string; the bullet is weighed, then fired into the pendulum, and the distance the pendulum rises allows the bullet's velocity to be deduced.
BARYONS: The class of subatomic particles in which protons and neutrons are included. The baryons form the atomic nucleus, along with another class of particles - the mesons. We, and everything we can see around us, are made of baryonic matter.
BEPICOLOMBO MISSION: ESA's cornerstone mission to the planet Mercury due to be launched in 2008.
BEPPOSAX SATELLITE: An Italian-Dutch X-ray astronomy satellite launched in 1996.
BIG BANG: The Big Bang theory is the most accepted theory so far to describe the origin and evolution of the Universe. According to it, all the matter and energy in the Universe was originally contained in a very small 'point' - technically called a 'singularity' - at an almost infinite temperature and density. About 10 to 20 billion years ago this tiny Universe started to expand, and has not stopped expanding since. This theory was first drafted by Russian physicist George Gamow in the late forties, although only when the cosmic microwave background was discovered in 1964 did the astronomical community start to take it seriously. As of today, apart from the cosmic microwave background, two other pillars support the Big Bang theory: the current expansion of the Universe and the measured abundance of light elements.
BIG CRUNCH: See Critical density.
BILLION: One billion equals one thousand million.
BINARY STAR: Pair of stars bound together by mutual gravitation and orbiting their common centre of mass.
BINOMIAL THEOREM: A formula first derived by Newton, giving (1+z) a, the result of raising 1 + z to an arbitrary power a, as a sequence of form (1+z) a = 1 + A1z + A2z 2 + A3z 3 + ....where the terms Ai (i = 1,2,3...) Are given by the formula and where a can be positive, negative, fractional or whole. When the magnitude of z is less than 1, the higher powers get smaller and smaller and the formula can be made as precise as one wishes by including enough of them (for z of small magnitude, 1-2 terms are sufficient), although the result is never exact. For magnitudes of z equal to 1 or more, the formula only holds for values of a which are positive whole numbers. In that case, for any z, the result is exact and the sum of terms with powers of z does not go on arbitrarily but ends with z a.
BIOCHEMISTRY: The study of chemical processes of living things.
BIOMETRY: The application of mathematics to the study of living things.
BIONICS: The study of functions, characteristics and phenomena observed in the living world and the application of this knowledge to the world of machines.
BIONOMICS: The study of the relation of an organism to its environment.
BIONOMY: The science of the laws of life.
BIOPHYSICS: The physics of vital processes (living things).
BLACK BODY RADIATION: Light or other electromagnetic radiation emitted due to heat by a solid, liquid or dense gas, with no color of its own (hence "black"). Distinguished by a continuous distribution of spectral color, with its peak of emission shifting towards shorter wavelengths as the temperature increases.
BLACK HOLE: An object with so much mass concentrated in it, and therefore such a strong gravitational pull that nothing, not even light can escape from it. One way in which black holes are believed to form is when massive stars collapse at the end of their lives.
BLAZAR: Type of active galaxy named after an object in the constellation of Lacerta, the BL Lacertae object. They form a subset of the quasar population. The emission of blazars is highly variable. The activity may be caused by jets of gas being expelled from the central region of the active galaxy, i.e. The supermassive black hole in the active galactic nucleus.
BLUESHIFT: When a distant object moves toward the observer the lines in its spectrum shift to shorter (bluer) wavelengths. This is because of the apparent compression of the wave of light. As a result of this compression the wavelength shortens and thus shifts towards the blue side of the electromagnetic spectrum. The blueshift of an astronomical object is an indication of the speed at which this object is approaching the observer.
BOLOMETER: A kind of detector mainly used to measure infrared radiation. A bolometer works by heating up as it absorbs the radiation that reaches it. The increase in temperature is measured by an internal electrical resistance, and is a measure of the amount of radiation absorbed.
BOTANY: The study of plants.
BREMSSTRAHLUNG: The German word `Bremsstrahlung' means `braking radiation'. A fast, charged particle, for example an electron, is slowed down when it passes through matter. The energy lost by the particle is emitted as electromagnetic radiation, or Bremsstrahlung.
BROWN DWARF: A kind of 'failed' star: a small and opaque object whose mass is not sufficient to start, in its core, the nuclear reaction to transform hydrogen into helium. A brown dwarf cannot therefore produce enough energy to shine as a star. A brown dwarf's mass is not more than 0.08 solar masses.
BUOYANCY: The lifting force acting in a fluid on bodies and regions less dense than their surroundings. The buoyancy of hot air.
CALENDAR: A system of marking days of the year, usually devised in a way to give each date a fixed place in the cycle of seasons.
CALISTHENICS: The systematic exercises for attaining strength & gracefulness.
CALORIE: Unit used in measuring the energy of heat or chemical energy. A "small" calorie is the heat needed to warm up one gram of water by 1 degree centigrade and equals about 4.18 joule. A "kilocalorie" or "big calorie" equals 1000 calories and is the unit usually used in describing the energy content of food.
CAPTURE ORBIT: The first orbit of a spacecraft after it has been captured by the gravitational attraction of a celestial body. The capture is achieved by reducing the speed of the spacecraft. See also Orbit acquisition.
CARBONACEOUS COMPOUNDS: Material containing carbon or carbon compounds.
CARDIOLOGY: The science that deals with heart functions and diseases.
CARPOLOGY: The science of fruits & seeds.
CARTESIAN COORDINATES: A system of uniquely marking the position of a point on a plane [or in 3-dimensional space].

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