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Scientific Terms and Definitions

A continually growing, personally collected and assembled list of varying scientific terms and definitions in alphabetical order. 

A

  • Absolute Magnitude - “A scale for measuring the actual brightness of a celestial object without accounting for the distance of the object. Absolute magnitude measures how bright an object would appear if it were exactly 10 parsecs (about 33 light-years) away from Earth. On this scale, the Sun has an absolute magnitude of +4.8 while it has an apparent magnitude of -26.7 because it is so close.”
  • Absolute Zero - “The temperature at which the motion of all atoms and molecules stops and no heat is given off. Absolute zero is reached at 0 degrees Kelvin or -273.16 degrees Celsius.”
  • Absorption Line - “A more or less narrow range of wavelengths in aspectrum that is darker than neighboring wavelengths. Absorption lines are seen in stars.”
  • Ablation - “A process by where the atmosphere melts away and removes the surface material of an incoming meteorite.”
  • Accretion - “The process by where dust and gas accumulated into larger bodies such as stars and planets.”
  • Accretion Disk - “A disk of gas that accumulates around a center of gravitational attraction, such as a white dwarfneutron star, or black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation.”
  • Achondrite - “A stone meteorite that contains no chondrules.”
  • Albedo - “The reflective property of a non-luminous object. A perfect mirror would have an albedo of 100% while a black hole would have an albedo of 0%.”
  • Albedo Feature - “A dark or light marking on the surface of an object that may or may not be a geological or topographical feature.”
  • Altitude - “The angular distance of an object above the horizon.”
  • Angles - “Are measured in degrees or arcminutes (denoted by a single quote) or arcseconds (denoted by a double quote) or radians. 1 radian = 180/pi = 57.2958 degrees, 1 degree = 1o = 60 arcminutes = 60’ = 3600 arcseconds = 3600”.”
  • Antimatter - “Matter consisting of particles with charges opposite that of ordinary matter.  In antimatter, protons have a negative charge while electrons have a positive charge.”
  • Antipodal Point - “A point that is on the direct opposite side of a planet.”
  • Apastron - “The point of greatest separation of two stars, such as in a binary star system.”
  • Aperture - “The size of the opening through which light passes in an optical instrument such as a camera or telescope. A higher number represents a smaller opening while a lower number represents a larger opening.”
  • Aphelion - “The point in the orbit of a planet or other celestial body where it is farthest from the Sun.”
  • Apogee - “The point in the orbit of the Moon or other satellite where it is farthest from the Earth.”
  • Apparent Magnitude - “The apparent brightness of an object in the sky as it appears to an observer on Earth. Bright objects have a low apparent magnitude while dim objects will have a higher apparent magnitude.”
  • Arc minutes - “There are 60 minutes (denoted as 60’) of arc in 1 degree. In the sky, with an unobstructed horizon (as on the ocean), one can see about 180 degrees of sky at once, and there are 90 degrees from the true horizon to the zenith. The full moon is about 30’ (30 arc minutes) across, or half a degree. There are 60 seconds (denoted 60”) of arc in one minute of arc.”
  • Asteroid - “A small planetary body in orbit around the Sun, larger than a meteoroid but smaller than a planet. Most asteroids can be found in a belt between the orbits of Mars and Jupiter. The orbits of some asteroids take them close to the Sun, which also takes them across the paths of the planets.”
  • Astrochemistry - “The branch of science that explores the chemical interactions between dust and gas interspersed between the stars.”
  • Astronomical Unit (AU) - “A unit of measure equal to the average distance between the Earth and the Sun, approximately 93 million miles.”
  • Astronomy - “The branch of science that deals with celestial objects, space, and the physical universe as a whole.”
  • Astrometry - “The careful, precise measurement of astronomical objects, usually made with respect to standard catalogues of star positions. For comet orbit computations, astrometry good to 1” or 2” (1 or 2 arc seconds), or better, is the standard nowadays.”
  • Atmosphere - “A layer of gases surrounding a planet, moon, or star. The Earth’s atmosphere is 120 miles thick and is composed mainly of nitrogen, oxygen, carbon dioxide, and a few other trace gases.”
  • Aurora - “A glow in a planet’s ionosphere caused by the interaction between the planet’s magnetic field and charged particles from the Sun. This phenomenon is known as the Aurora Borealis in the Earth’s northern hemisphere and the Aurora Australis in the Earth’s Southern Hemisphere.”
  • Aurora Australis - “Also known as the southern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the southern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth’s magnetic field. Known as the Aurora Borealis in the northern hemisphere.”
  • Aurora Borealis - “Also known as the northern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the northern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth’s magnetic field. Known as the Aurora Australis in the southern hemisphere.”
  • Axis - “Also known as the poles, this is an imaginary line through the center of rotation of an object.”
  • Azimuth - “The angular distance of an object around or parallel to the horizon from a predefined zero point.”

B

  • Bar - “A unit of measure of atmospheric pressure. One bar is equal to 0.987 atmospheres, 1.02 kg/cm2, 100 kilopascal, and 14.5 lbs/square inch.”
  • Barycenter - “The center of mass of a system of bodies, such as the solar system. When a comet, for example, is well outside the orbit of Neptune (the farthest major planet), it sees the sun and major planets essentially as a single object of summed mass, and the center of this mass (called the barycenter of the solar system) is offset somewhat from the sun; “original” and “future” orbits of long-period comets are computed for this barycenter, while perturbed, osculating orbits of currently-observed objects in the inner solar system are computed for heliocentric orbits.”
  • Barycentric Dynamical Time (TDB) - “Differing from TDT only via periodic variations, TDB is used in ephemerides and equations of motion that refer to the barycenter of the solar system.”
  • Baryon - “A  massive elementary particle made up of three quarks. Neutrons and protons are baryons.”
  • Besselian year - “A quantity introduced by F. W. Bessel in the nineteenth century that has been used into the twentieth century. Bessel introduced a system whereby it would be convenient to identify any instant of time by giving the year and the decimal fraction of the year to a few places, but the starting time of the year was not convenient for dynamical studies that utilize Julian dates (see definition for Julian date), differing by 0.5 day, and the Besselian year varies slowly. The recent change to Julian year usage in dynamical astronomy (and the J2000.0 equinox) took effect in solar-system ephemerides of the Minor Planet Center and Central Bureau for Astronomical Telegrams on Jan. 1, 1992. (See Julian year.)”
  • Big Bang - “The theory that suggests that the universe was formed from a single point in space during a cataclysmic explosion about 13.7 billion years ago. This is the current accepted theory for the origin of the universe and is supported by measurements of background radiation and the observed expansion of space.”
  • Binary - “A system of two stars that revolve around a common center of gravity.”
  • Blackbody - “An object with a constant temperature that absorbs all radiation that hits it.”
  • Black Hole - “The collapsed core of a massive star. Stars that are very massive will collapse under their own gravity when their fuel is exhausted. The collapse continues until all matter is crushed out of existence into what is known as a singularity. The gravitational pull is so strong that not even light can escape.”
  • Blueshift - “A shift in the lines of an object’s spectrum toward the blue end. Blueshift indicates that an object is moving toward the observer. The larger the blueshift, the faster the object is moving.”
  • Bolide - “A term used to describe an exceptionally bright meteor. Bolides typically will produce a sonic boom.”

 

C

  • Caldera - “A type of volcanic crater that is extremely large, usually formed by the collapse of a volcanic cone or by a violent volcanic explosion. Crater Lake is one example of a caldera on Earth.”
  • Catena - “A series or chain of craters.”
  • Cavus - “A hollow, irregular depression.”
  • CCD - “Charge-coupled device, a very sensitive electronic device that is revolutionizing astronomy in the 1990s. CCD cameras are composed of silicon chips that are sensitive to light, changing detected photons of light into electronic signals that can then be used to make images of astronomical objects or to analyze how much light is being received from such objects. CCDs require computers for reduction of data, so the expense can be much greater than for, say, photography —- but CCDs can detect much fainter objects than can photographs. Unfiltered CCDs tend to be more red-sensitive than the human eye.”
  • Celestial Equator - “An imaginary line that divides the celestial sphere into a northern and southern hemisphere.”
  • Celestial Poles - “The North and South poles of the celestial sphere.”
  • Celestial Sphere - “An imaginary sphere around the Earth on which the stars and planets appear to be positioned.”
  • Cepheid - “A type of pulsating variable star with a luminosity that can be determined from the period of its variation: Cepheids with long pulsation periods are bigger and thus more luminous than short period Cepheids.”
  • Cepheid Variable - “This is a variable star whose light pulsates in a regular cycle. The period of fluctuation is linked to the brightness of the star. Brighter Cepheids will have a longer period.”
  • Chaos - “A distinctive area of broken terrain.”
  • Chasma - “Another name used to describe a canyon.”
  • Chondrite - “A meteorite that contains chondrules.”
  • Chondrule - “Small, glassy spheres commonly found in meteorites.”
  • Chromosphere - “The part of the Sun’s atmosphere just above the surface.”
  • Circumpolar Star - “A star that never sets but always stays above the horizon. This depends on the location of the observer.  The further South you go the fewer stars will be circumpolar. Polaris, the North Star, is circumpolar in most of the northern hemisphere.”
  • Circumstellar Disk - “A torus or ring-shaped accumulation of gas, dust, or other debris in orbit around a star in different phases of its life cycle.”
  • CMB - “Cosmic Microwave Background radiation, also CMBR, CBR and the “3 K blackbody radiation”. Radiation left over from the hot Big Bang which has cooled by expansion to a temperature slightly less than 3 degrees above absolute zero.”
  • Cold Dark Matter - “A type of dark matter that was moving at much less than the speed of light 10,000 years after the Big Bang.”
  • Coma - “An area of dust or gas surrounding the nucleus of a comet.”
  • Comet - “A gigantic ball of ice and rock that orbit the Sun in a highly eccentric orbit. Some comets have an orbit that brings them close to the Sun where they form a long tail of gas and dust as they are heated by the Sun’s rays.”
  • Conjunction - “An event that occurs when two or more celestial objects appear close close together in the sky.”
  • Constellation - “A grouping of stars that make an imaginary picture in the sky.”
  • Continuum - “A smooth spectrum without emission or absorption lines. Some sources like tungsten lamps or blackbodies are purely continuum sources, while in sources with lines the continuum is a smooth spectrum drawn through the points between the lines.”
  • Corona - “The outer part of the Sun’s atmosphere. The corona is visible from Earth during a total solar eclipse. It is the bright glow seen in most solar eclipse photos.”
  • Cosmic Ray - “Atomic nuclei (mostly protons) that are observed to strike the Earth’s atmosphere with extremely high amounts of energy.”
  • Cosmic String - “A tube-like configuration of energy that is believed to have existed in the early universe. A cosmic string would have a thickness smaller than a trillionth of an inch but its length would extend from one end of the visible universe to the other.”
  • Cosmogony - “The study of celestial systems, including the Solar System, starsgalaxies, and galactic clusters.”
  • Cosmology - “A branch of science that deals with studying the origin, structure, and nature of the universe.”
  • Cosmological Constant - “A  term in Einstein’s general relativity equations that leads to an acceleration of the expansion of the Universe. Usually denoted by Λ, the capital Greek letter Lambda when expressed with units of inverse length squared, or by ΩΛ or the lower-case Greek lambda (λ) when normalized to the critical density like Ω.”
  • Crater - “A bowl-shaped depression formed by the impact of an asteroid or meteoroid. Also the depression around the opening of a volcano.”
  • Critical Density - “The density of the Universe necessary so the expansion rate of the Universe is just barely sufficient to prevent a recollapse. Numerically the critical density is 3Ho2/[8*pi*G] or 19*[Ho/100]2*10-30 gm/cc.”

D

  • Dark Energy - “A more general form of the vacuum energy density than the cosmological constant.”
  • Dark Matter - “A term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies.”
  • Debris Disk - “A ring-shaped circumstellar disk of dust and debris in orbit around a star. Debris disks can be created as the next phase in planetary system development following the protoplanetary disk phase. They can also be formed by collisions between planetesimals.”
  • Declination - “The angular distance of an object in the sky from the celestial equator.”
  • Degree - “A unit used in the measurement of angles, heavily used particularly in astronomy. Due to ancient Babylonian mathematics, we still divide a circle into 360 even units of arc and call each of these units one degree. The entire sky, therefore, spans 360 degrees. Up to about 180 degrees of sky is visible from any given point on earth with an unobstructed horizon (as measured from, say, east to west, or north to south). The degree is used to make measurements of distance, or position (as with declination) in astronomy. In turn, a degree is composed of 60 minutes of arc, and also of 3600 seconds of arc.”
  • Delta - “The upper-case Greek letter used to denote an object’s geocentric distance in ephemeris tables; see “ephemeris”. (Note that lower-case delta is used to denote declination.)”
  • Density - “The amount of matter contained within a given volume. Density is measured in grams per cubic centimeter (or kilograms per liter). The density of water is 1.0, iron is 7.9, and lead is 11.3.”
  • Disk - “The surface of the Sun or other celestial body projected against the sky.”
  • Distance Modulus - “The difference between the magnitude and the absolute magnitude, so DM = 5 log10(D/[10 pc]).”
  • Dipole - “A pattern with one hot side of the sky and one cold side of the sky.”
  • Double Asteroid - “Two asteroids that revolve around each other and are held together by the gravity between them. Also called a binary asteroid.”
  • Doppler Effect - “The apparent change in wavelength of sound or light emitted by an object in relation to an observer’s position. An object approaching the observer will have a shorter wavelength (blue) while an object moving away will have a longer (red) wavelength. The Doppler effect can be used to estimate an object’s speed and direction.”
  • Double Star - “A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system.”
  • Dwarf Planet - “A celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. It has to have sufficient mass to overcome rigid body forces and achieve hydrostatic equilibrium. Pluto is considered to be a dwarf planet.”

E

  • Eccentricity - “The measure of how an object’s orbit differs from a perfect circle. Eccentricity defines the shape of an object’s orbit.”
  • Eclipse - “The total or partial blocking of one celestial body by another.”
  • Eclipsing Binary - “A binary system where one object passes in front of the other, cutting off some or all of its light.”
  • Ecliptic - “An imaginary line in the sky traced by the Sun as it moves in its yearly path through the sky.”
  • Ejecta - “Material from beneath the surface of a body such as a moon or planet that is ejected by an impact such as a meteor and distributed around the surface. Ejecta usually appear as a lighter color than the surrounding surface.”
  • Electromagnetic Force - “One of the four forces of nature. Electromagnetic interactions hold electrons in atoms, hold atoms in molecules, and are used in all electronic devices.”
  • Electromagnetic Radiation - “Another term for light. Light waves created by fluctuations of electric and magnetic fields in space.”
  • Electromagnetic Spectrum - “The full range of frequencies, from radio waves to gamma waves, that characterizes light.”
  • Electroweak - “Unified force that combines the electromagnetic and weak nuclear interactions. Predicted by Weinberg and Salam, experimentally verified by Rubbia and van der Meer.”
  • Ellipse - “An ellipse is an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical in shape rather than circular.”
  • Elliptical Galaxy - “A galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.”
  • Elongation - “The angular distance of a planetary body from the Sun as seen from Earth. A planet at greatest eastern elongation is seen in the evening sky and a planet at greatest western elongation will be seen in the morning sky.”
  • Emission Line - “A more or less narrow range of wavelengths in aspectrum that is brighter than neighboring wavelengths. Emission lines are seen in quasars.”
  • Energy - “The ability to do work, with units of ergs or Joules. One Joule is 10,000,000 ergs. One erg is the kinetic energy of a 2 gram mass moving at one cm/sec. Energy per unit time is power, and 1 Watt of power is 1 Joule per second.”
  • Ephemeris - “A table of data arranged by date. Ephemeris tables are typically to list the positions of the Sun, Moon, planets and other solar system objects.”
  • Ephemeris Time (ET) - “Determined in principle from the sun’s apparent annual motion, ET is the numerical measure of uniform time, which is the independent variable in the gravitational theory of the earth’s orbital motion, coming from Simon Newcomb’s Tables of the Sun. In practice, ET was obtained by comparing observing positions of the Moon with gravitational ephemerides calculated from theories. In 1992, standard (apparent geocentric) ephemerides of comets and minor planets changed from using Ephemeris Time to Terrestrial Dynamical Time (TDT, or TT).”
  • Equation of State - “The ratio of the pressure to the energy density in the dark energy or vacuum energy. Usually denoted by w. For thecosmological constant w = -1.”
  • Equinox - “The two points at which the Sun crosses the celestial equator in its yearly path in the sky. The equinoxes occur on or near March 21 and September 22.  The equinoxes signal the start of the Spring and Autumn seasons.”
  • Escape Velocity - “The speed required for an object to escape the gravitational pull of a planet or other body.”
  • Event Horizon - “The invisible boundary around a black hole past which nothing can escape the gravitational pull - not even light.”
  • Evolved Star - “A star that is near the end of its life cycle where most of its fuel has been used up. At this point the star begins to loosemass in the form of stellar wind.”
  • Extinction - “The apparent dimming of star or planet when low on the horizon due to absorption by the Earth’s atmosphere.”
  • Extragalactic - “A term that means outside of or beyond our own galaxy.”
  • Extraterrestrial - “A term used to describe anything that does not originate on Earth.”
  • Eyepiece - “The lens at the viewing end of a telescope. The eyepiece is responsible for enlarging the image captured by the instrument.  Eyepieces are available in different powers, yielding differing amounts of magnification.”

 

F

  • Faculae - “Bright patches that are visible on the Sun’s surface, or photosphere.”
  • Filament - “A strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun.”
  • Finder - “A small, wide-field telescope attached to a larger telescope. The finder is used to help point the larger telescope to the desired viewing location.”
  • Fine Structure Constant - “Usually denoted as α, the lower case Greek alpha, is the dimensionless ratio e2/(hbar*c) = 1/137.03599976… [in cgs units, or e2/(4*pi*epsilono*hbar*c) in MKS units], which gives the strength of the electromagnetic interaction. Here e is the electron charge, hbar is Planck’s constant divided by 2*pi, and c is the speed of light.”
  • Fireball - “An extremely bright meteor. Also known as bolides, fireballs can be several times brighter than the full Moon. Some can even be accompanied by a sonic boom.”
  • Flare Star - “A faint red star that appears to change in brightness due to explosions on its surface.”
  • Fluence - “Energy per unit area. Fluence is time times flux.”
  • Flux - “Power per unit area. The flux from the Sun at the Earth is 1367 Watts per square meter. This total power is often divided up into different frequency or wavelength bands, giving for example Watts per square meter per Hertz or ergs per square cm per second per micron. 1 Jansky is 10-26 Watts per square meter per Hertz.”

 

G

  • Galactic Halo - “The name given to the spherical region surrounding the center, or nucleus of a galaxy.”
  • Galactic Nucleus - “A tight concentration of stars and gas found at the innermost regions of a galaxy. Astronomers now believe that massiveblack holes may exist in the center of many galaxies.”
  • Galaxy - “A large grouping of stars. Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and contains several billion stars. Some galaxies are so distant the their light takes millions of years to reach the Earth.”
  • Galilean Moons - “The name given to Jupiter’s four largest moons, Io, Europa, Callisto & Ganymede. They were discovered independently by Galileo Galilei and Simon Marius.”
  • Gamma-ray - “The highest energy, shortest wavelength form of electromagnetic radiation.”
  • Geosynchronous Orbit - “An orbit in which a satellite’s orbital velocity is matched to the rotational velocity of the planet. A spacecraft in geosynchronous orbit appears to hang motionless above one position of a planet’s surface.”
  • Giant Molecular Cloud (GMC) - “Massive clouds of gas in interstellar space composed primarily of hydrogen molecules. These clouds have enough mass to produce thousands of stars and are frequently the sites of new star formation.”
  • Globular Cluster - “A tight, spherical grouping of hundreds of thousands of stars. Globular clusters are composed of older stars, and are usually found around the central regions of a galaxy.”
  • Grand Unified Theory - “A model for unifying the strong nuclear force, the weak nuclear force, and the electromagnetic force into a single interaction. Several such GUTs have been proposed, but not yet experimentally verified.”
  • Granulation - “A pattern of small cells that can be seen on the surface of the Sun. They are caused by the convective motions of the hot gases inside the Sun.”
  • Gravitational Lens - “A concentration of matter such as a galaxy or cluster of galaxies that bends light rays from a background object. Gravitational lensing results in duplicate images of distant objects.”
  • Gravitational Potential - “The gravitational energy per unit mass of a particle, equivalent to the acceleration of gravity times the height in ordinary circumstances near the surface of the Earth.”
  • Gravity - “A mutual physical force of nature that causes two bodies to attract each other.”
  • Greenhouse Effect - “An increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by theatmosphere. Carbon dioxide and water vapor are two of the major gases responsible for this effect.”
  • GYR - “Gigayear, or one billion years. See for a table of all the metric prefixes from yocto (10-24) to yotta (1024).”

 

H

  • Heliocentric - “Referring to the sun. A heliocentric orbit is one based on the sun as one of the two foci of the (elliptical) orbit (or as the center of a circular orbit); a heliocentric magnitude is the brightness of an object as would be seen from a heliocentric distance of 1 AU (which means a distance of 1 AU from the sun).”
  • Heliopause - “The point in space at which the solar wind meets the interstellar medium or solar wind from other stars.”
  • Heliosphere - “The space within the boundary of the heliopause containing the Sun and the Solar System.”
  • Homogeneous - “The same at all locations. Homogenized milk is not separated into cream and milk.”
  • Hot Big Bang - “A model of the Universe beginning at very high density and temperature, which expands and cools to become like the Universe we observe now.”
  • Hot Dark Matter - “A type of dark matter that was moving at close to the speed of light 10,000 years after the Big Bang.”
  • Horizon - “The edge of the visible Universe, but not the edge of the Universe since the Universe has no edge.”
  • Hydrogen - “An element consisting of one electron and one proton. Hydrogen is the lightest of the elements and is the building block of the universe. Stars form from massive clouds of hydrogen gas.”
  • Hubble Constant - “or Ho, the ratio of velocity to distance in the expansion of the Universe, so v = HD. The “o” [pronounced “naught”] on Ho means the current value, since the Hubble “constant” changes with time (but it is the same everywhere in the Universe at a given time). The measured value of Ho has also changed dramatically since even before Hubble’s work, as shown in Huchra’s Ho history.
  • Hubble’s Law - “The law of physics that states that the farther a galaxy is from us, the faster it is moving away from us.”
  • Hydrostatic equilibrium - “A state that occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. Hydrostatic equillibrium is responsible for keeping stars from imploding and for giving planets their spherical shape.”
  • Hypergalaxy - “A system consisting of a spiral galaxy surrounded by several dwarf white galaxies, often ellipticals. Our galaxy and the Andromeda galaxy are examples of hypergalaxies.”

 

I

  • Ice - “A term used to describe water or a number of gases such as methane or ammonia when in a solid state.”
  • Inclination - “A measure of the tilt of a planet’s orbital plane in relation to that of the Earth.”
  • Inferior Conjunction - “A conjunction of an inferior planet that occurs when the planet is lined up directly between the Earth and the Sun.”
  • Inferior Planet - “A planet that orbits between the Earth and the Sun. Mercury and Venus are the only two inferior planets in our solar system.”
  • Inflationary Scenario  - “A modification of the Big Bang model in which a large cosmological constant exists temporarily early in the history of the Big Bang, leading to a rapid accelerating expansion of the Universe, which is then followed by the normal Big Bang model with a decelerating expansion.”
  • International Astronomical Union (IAU) - “An international organization that unites national astronomical societies from around the world and acts as the internationally recognized authority for assigning designations to celestial bodies and their surface features.”
  • Interplanetary Magnetic Field - “The magnetic field carried along with the solar wind.”
  • Interstellar Medium - “The gas and dust that exists in open space between the stars.”
  • Ionosphere - “A region of charged particles in a planet’s upper atmosphere. In Earth’s atmosphere, the ionosphere begins at an altitude of about 25 miles and extends outward about 250.”
  • Iron Meteorite - “A meteorite that is composed mainly of iron mixed with smaller amounts of nickel.”
  • Irregular Galaxy - “A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape.”
  • Irregular Satellite - “A satellite that orbits a planet far away with an orbit that is eccentric and inclined. They also tend to have retrograde orbits. Irregular satellites are believed to have been captured by the planet’s gravity rather than being formed along with the planet.”
  • Isotropic  - “The same in all directions. Anisotropic - not isotropic. Anisotropy - difference between different directions. In the standard color scheme for CMB anisotropy maps measured by the COBE DMR, red shows areas of the sky that are warmer, while blue shows the cooler regions.”

 

J

  • Jansky - “A unit used in radio astronomy to indicate the flux density (the rate of flow of radio waves) of electromagnetic radiation received from outer space. A typical radio source has a spectral flux density of roughly 1 Jy. The jansky was named to honor Karl Gothe Jansky who developed radio astronomy in 1932.”
  • Jet - “A narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole.”
  • Julian date (JD) - “The interval of time in days (and fraction of a day) since Greenwich noon on Jan. 1, 4713 BC. The JD is always half a day off from Universal Time, because the current definition of JD was introduced when the astronomical day was defined to start at noon (prior to 1925) instead of midnight. Thus, 1995 Oct. 10.0 UT = JD 2450000.5.”
  • Julian year - “Exactly 365.25 days, in which a century (100 years) is exactly 36525 days and in which 1900.0 corresponds exactly to 1900 January 0.5 (from the Julian-date system, which is half a day different from civil time or UT). The standard epoch J2000.0, now used for new star-position catalogues and in solar-system-orbital calculations, means 2000 Jan. 1.5 Barycentric Dynamical Time (TDB) = Julian Date 2451545.0 TDB. When this dynamical, artificial “Julian year” is employed, a letter “J” prefixes the year.”

 

K

  • Kelvin - “A temperature scale used in sciences such as astronomy to measure extremely cold temperatures. The Kelvin temperature scale is just like the Celsius scale except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees Kelvin. Absolute zero, the coldest known temperature, is reached at 0 degrees Kelvin or -273.16 degrees Celsius.”
  • Kepler’s First Law - “A planet orbits the Sun in an ellipse with the Sun at one focus.”
  • Kepler’s Second Law - “A ray directed from the Sun to a planet sweeps out equal areas in equal times.”
  • Kepler’s Third Law - “The square of the period of a planet’s orbit is proportional to the cube of that planet’s semi major axis; the constant of proportionality is the same for all planets.”
  • Kiloparsec - “A distance equal to 1000 parsecs.”
  • Kirkwood Gaps - “Regions in the main belt of asteroids where few or no asteroids are found. They were named after the scientist who first noticed them.”
  • Kuiper Belt - “A large ring of icy, primitive objects beyond the orbit of Neptune. Kuiper Belt objects are believed to be remnants of the original material that formed the Solar System. Some astronomers believe Pluto and Charon are Kuiper Belt objects.”

 

L

  • Lagrange Point - “French mathematician and astronomer Joseph Louis Lagrange showed that three bodies could lie at the apexes of an equilateral triangle which rotates in its plane. If one of the bodies is sufficiently massive compared with the other two, then the triangular configuration is apparently stable. Such bodies are sometimes referred to as Trojans. The leading apex of the triangle is known as the leading Lagrange point or L4; the trailing apex is the trailing Lagrange point or L5.”
  • Lambda - “The upper case Greek Lambda is usually used to denote Einstein’s cosmological constant. A non-zero Lambda indicates a non-zero vacuum energy density and causes a long-range repulsive effect which leads to theaccelerating expansion of the Universe.”
  • Lenticular Galaxy - “A disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular galaxies tend to look more like elliptical galaxies than spiral galaxies.”
  • Libration - “An effect caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon’s surface can be seen over a period of time.”
  • Light pollution - “The emission of stray light or glare from lighting fixtures in manners that counter the purpose of the light (which is to light what is below); also known as the waste of money and energy in the form of electric light, usually meant in the form of outdoor night lighting. Such light trespass causes severe safety problems for motorists, pedestrians, and cyclists at night from lighting that shines onto streets and highways and sidewalks from poorly-designed or poorly-mounted lighting. Such glare also imposes on privacy, by shining brightly into bedroom windows at night and into backyards where adults and children are trying to observe the night sky. While most people have accepted such bad, glare lighting without question and assumed that nothing could be done about it, dedicated groups of volunteers around the world are now showing that effective laws and guidelines can be instated at the local and regional levels of government (and in planning and engineering offices), which mean that proper outdoor night lighting can be a norm so that everybody benefits —- auto drivers, sleeping residents, government budgets, and skygazers alike. Laws mandating full-cutoff light fixtures are already in place in states such as Maine and Connecticut and are pending elsewhere.”
  • Light Year - “An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.”
  • Limb - “The outer edge or border of a planet or other celestial body.”
  • Local Group - “A small group of about two dozen galaxies of which our own Milky Way galaxy is a member.”
  • Luminosity - “The amount of light emitted by a star.”
  • Lunar Eclipse - “A phenomenon that occurs when the Moon passes into the shadow of the Earth.  A partial lunar eclipse occurs when the Moon passes into the penumbra, or partial shadow.  In a total lunar eclipse, the Moon passes into the Earth’s umbra, or total shadow.”
  • Lunar Month - “The average time between successive new or full moons. A lunar month is equal to 29 days 12 hours 44 minutes. Also called a synodic month.”
  • Lunation - “The interval of a complete lunar cycle, between one new Moon and the next.  A lunation is equal to 29 days, 12 hours, and 44 minutes.”
  • Lyman Alpha Line  - “The strongest line in the spectrum of the most common atom in the Universe, hydrogen. It is emitted at a wavelength of 122 nm. The general formula for the wavelength of hydrogen lines is 1/wavelength = [n-2 - m-2]/[91.2 nm] where n = 1, 2, 3, … is the lower state quantum number and m = n+1, n+2, … is the upper state quantum number. When n = 1, these lines with m = 2, 3, … are called Lyman alpha, Lyman beta, … and form the Lyman series. Whenn = 2, these lines with m = 3, 4, … are called H-alpha, H-beta, … and form the Balmer series.”

 

M

  • MACHO - “MAssive Compact Halo Object, and also one of projectssearching for MACHOs by looking for gravitational microlensing: the other projects are EROS and OGLE. A MACHO is an object with a mass from about 10 billion tons to solar masses. If it is made out neutrons and protons then it is baryonic dark matter, but primordial black holes are a non-baryonic dark matter version of MACHOs.”
  • Magellanic Clouds - “Two small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic Clouds are visible in the skies of the southern hemisphere.”
  • Magnetic Field - “A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles.”
  • Magnetic Pole - “Either of two limited regions in a magnet at which the magnet’s field is most intense.”
  • Magnetosphere - “The area around a planet most affected by its magnetic field. The boundary of this field is set by the solar wind.”
  • m1 - “Total, integrated magnitude of a comet’s head (meaning coma + nuclear condensation). This can be estimated visually, as the comet’s “total visual magnitude”. The variable m1 is usually found in ephemerides predicting a comet’s future motion, position on the sky, and brightness. See also definition for “Magnitude”, below. [Note that m1 is also used by stellar spectrophotometrists to define a “metal index” on the Stroemgren ubvy photometric system.]”
  • m2 - “The magnitude value measured (or predicted) for a comet’s nuclear condensation. Note that the true comet nucleus is rarely, if ever, directly observed from the earth because of the large amount of gas and dust that is ever-present in the inner coma close to the nucleus, serving to hide the true nucleus’ surface. So-called “nuclear magnitudes” are therefore fraught with problems as to true meaning, especially because such nuclear magnitudes are extremely dependent upon instrumentation (aperture, focal-ratio, magnification) and wavelength. Nuclear magnitudes are chiefly used for astrometric purposes, in which predictions are made for the brightness of the comet’s nuclear condensation so that astrometrists can gauge how faint the condensation is likely to be and thus how long an exposure is needed to get a good, measurable image. (Astrometrists are only concerned about measuring the nuclear condensation, which is considered to be the site of the main mass of any comet.) See also definition for “Magnitude”, below.”
  • Magnitude - “The units used to describe brightness of astronomical objects. The smaller the numerical value, the brighter the object. The human eye can detect stars to 6th or 7th magnitude on a dark, clear night far from city lights; in suburbs or cities, stars may only be visible to mag 2 or 3 or 4, due to light pollution. The brightest star, Sirius, shines at visual magnitude -1.5. Jupiter can get about as bright as visual magnitude -3 and Venus as bright as -4. The full moon is near magnitude -13, and the sun near mag -26. Comet C/1996 B2 (Hyakutake) reached magnitude about 0 in late March 1996. The magnitude scale is logarithmic, with a difference of one magnitude corresponding to a change of about 2.5 times in brightness; a change of 5 magnitudes is defined as a change of exactly 100 times in brightness.”
  • Main Belt - “The area between Mars and Jupiter where most of the asteroids in our solar system are found.”
  • Major Planet - “A name used to describe any planet that is considerably larger and more massive than the Earth, and contains large quantities of hydrogen and helium. Jupiter and Neptune are examples of major planets.”
  • Mare - “A term used to describe a large, circular plain. The word mare means “sea”. On the Moon, the maria are the smooth, dark-colored areas.”
  • Mass - “A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.”
  • Matter - “A word used to describe anything that contains mass.”
  • Meridian - “An imaginary circle drawn through the North and South poles of the celestial equator.”
  • Metal - “A term used by astronomers to describe all elements except hydrogen and helium, as in “the universe is composed of hydrogen, helium and traces of metals”. This astronomical definition is quite different from the traditional chemistry definition of a metal.”
  • Meteor - “A small particle of rock or dust that burns away in the Earth’s atmosphere.  Meteors are also referred to as shooting stars.”
  • Meteor Shower - “An event where a large number of meteors enter the Earth’s atmosphere from the same direction in space at nearly the same time.  Most meteor showers take place when the Earth passes through the debris left behind by a comet.”
  • Meteorite - “An object, usually a chunk or metal or rock, that survives entry through the atmosphere to reach the Earth’s surface. Meteors become meteorites if they reach the ground.”
  • Meteoroid - “A small, rocky object in orbit around the Sun, smaller than an asteroid.”
  • Micron - “One micrometer, 0.000001 meters. Visible light has wavelengths between 0.4 and 0.7 microns. 1 micron is 10,000 Angstroms, or 1000 nanometers (nm). 1 nanometer is 0.000000001 meters.”
  • Millibar - “A measure of atmospheric pressure equal to 1/1000 of a bar. Standard sea-level pressure on Earth is about 1013 millibars.”
  • Minor Planet - “A term used since the 19th century to describe objects, such as asteroids, that are in orbit around the Sun but are not planets or comets. In 2006, the International Astronomical Union reclassified minor planets as either dwarf planets or small solar system bodies.”
  • Molecular Cloud - “An interstellar cloud of molecular hydrogen containing trace amounts of other molecules such as carbon monoxide and ammonia.”

 

N

  • Nadir - “A term used to describe a point directly underneath an object or body.”
  • Nanowatt or nW - “One billion’th of a watt.”
  • Nebula - “A cloud of dust and gas in space, usually illuminated by one or more stars. Nebulae represent the raw material the stars are made of.”
  • Neutralino - “A particle predicted by supersymmetry models of the forces of nature. These models predict that each type of known particle will have a supersymmetric partner. The neutralino is the lightest electrically neutral supersymmetric partner, and it is a candidate for cold dark matter. As of 1999, no supersymmetric partner particles of any kind have been observed experimentally. A neutralino is one type of WIMP.”
  • Neutrino - “A fundamental particle produced by the nuclear reactions in stars. Neutrinos are very hard to detect because the vast majority of them pass completely through the Earth without interacting.”
  • Neutron Star - “A compressed core of an exploded star made up almost entirely of neutrons. Neutron stars have a strong gravitational field and some emit pulses of energy along their axis. These are known as pulsars.”
  • Newton’s First Law of Motion - “A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.”
  • Newton’s Second Law of Motion - “For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.”
  • Newton’s Third Law of Motion - “In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.”
  • Non-barcyonic - “Not made up of neutrons, protons and electrons, and thus not like any of the known chemical elements.”
  • Nova - “A star that flares up to several times its original brightness for some time before returning to its original state.”
  • Nuclear Fusion - “The nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. Nuclear fusion is the reaction that fuels the Sun, where hydrogen nuclei are fused to form helium.”
  • Nucleon - “A neutron or a proton - one of the particles inside an atomic nucleus.”

 

O

  • Obliquity - “The angle between a body’s equatorial plane and orbital plane.”
  • Oblateness - “A measure of flattening at the poles of a planet or other celestial body.”
  • Occultation - “An event that occurs when one celestial body conceals or obscures another. For example, a solar eclipse is an occultation of the Sun by the Moon.”
  • Omega or Ω - “The ratio of the density of the Universe to the critical density.”
  • Oort Cloud - “A theoretical shell of comets that is believed to exist at the outermost regions of our solar system. The Oort cloud was named after the Dutch astronomer who first proposed it.”
  • Open Cluster - “A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open clusters are still embedded in the gas and dust from which they formed.”
  • Opposition - “The position of a planet when it is exactly opposite the Sun in the sky as seen from Earth. A planet at opposition is at its closest approach to the Earth and is best suitable for observing.”
  • Orbit - “The path of a celestial body as it moves through space.”
  • Orbital elements - “Parameters (numbers) that determine an object’s location and motion in its orbit about another object. In the case of solar-system objects such as comets and planets, one must ultimately account for perturbing gravitational effects of numerous other planets in the solar system (not merely the sun), and when such account is made, one has what are called “osculating elements” (which are always changing with time and which therefore must have a stated epoch of validity). Six elements are usually used to determine uniquely the orbit of an object in orbit about the sun, with a seventh element (the epoch, or time, for which the elements are valid) added when planetary perturbations are allowed for; initial (“preliminary”) orbit determinations shortly after the discovery of a new comet or minor planet (when very few observations are available) are usually “two-body determinations”, meaning that only the object and the sun are taken into account —- with, of course, the earth in terms of observing perspective. The six orbital elements used for comets are usually the following: time of perihelion passage (T) [sometimes taken instead as an angular measure called “mean anomaly”, M]; perihelion distance (q), usually given in AU; eccentricity (e) of the orbit; and three angles (for which the mean equinox must be specified) —- the argument of perihelion (lower-case Greek letter omega), the longitude of the ascending node (upper-case Greek letter Omega), and the inclination (i) of the orbit with respect to the ecliptic.”

 

P

  • Parallax - “The apparent change in position of two objects viewed from different locations.”
  • Parsec - “A large distance often used in astronomy.  A parsec is equal to 3.26 light-years.”
  • Patera - “A shallow crater with a complex, scalloped edge.”
  • Penumbra - “The area of partial illumination surrounding the darkest part of a shadow caused by an eclipse.”
  • Perigee - “The point in the orbit of the Moon or other satellite at which it is closest to the Earth.”
  • Perihelion - “The point in the orbit of a planet or other body where it is closest to the Sun.”
  • Perturb - “To cause a planet or satellite to deviate from a theoretically regular orbital motion.”
  • Perturbations - “Gravitational influences (“tugging” and “pulling”) of one astronomical body on another. Comets are strongly perturbed by the gravitational forces of the major planets, particularly by the largest planet, Jupiter. These perturbations must be allowed for in orbit computations, and they lead to what are known as “osculating elements” (which means that the orbital element numbers change from day to day and month to month due to continued perturbations by the major planets, so that an epoch is necessarily stated to denote the particular date that the elements are valid.”
  • Phase - “The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits.”
  • Phase angle - “For a solar system object besides the earth and sun, the angle between the earth and the sun (or the earth’s elongation from the sun) as seen from that third object. The phase angle is given in ephemerides on IAU Circulars and Minor Planet Circulars is denoted by either of the lower-case Greek letters beta or phi.”
  • Photometry - “In astronomy, the measurement of the light emitting from astronomical objects, generally in the visible or infrared bands, in which a specific or general wavelength band is normally specified. An excellent reference on this topic is Astronomical Photometry: A Guide, by C. Sterken and J. Manfroid (1992, Dordrecht: Kluwer Academic Publishers).”
  • Photon - “A particle of light composed of a minute quantity of electromagnetic energy.”
  • Photosphere - “The bright visible surface of the Sun.”
  • Planemo - “A large planet or planetary body that does not orbit a star. Planemos instead wander cold and alone through the cosmos. It is believed that most planemos once orbited their mother star but were ejected from the star system by gravitational interaction with another massive object.”
  • Planet - “A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals.”
  • Planetary Nebula - “A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing a variety of colors and shapes.”
  • Planetesimal - “A solid object that is believed to exist in protoplanetary disks and in debris disks. Planetesimals are formed from small dust grains that collide and stick together and are the building blocks that eventually form planets in new planetary systems.”
  • Planitia - “A low plain.”
  • Planum - “A high plain or plateau.”
  • Plasma - “A form of ionized gas in which the temperature is too high for atoms to exist in their natural state. Plasma is composed of free electrons and free atomic nuclei.”
  • Precession - “The apparent shift of the celestial poles caused by a gradual wobble of the Earth’s axis.”
  • Prominence - “An explosion of hot gas that erupts from the Sun’s surface. Solar prominences are usually associated with sunspot activity and can cause interference with communications on Earth due to their electromagnetic effects on the atmosphere.”
  • Prograde Orbit - “In reference to a satellite, a prograde orbit means that the satellite orbits the planet in the same direction as the planet’s rotation. A planet is said to have a prograde orbit if the direction of its orbit is the same as that of the majority of other planets in the system.”
  • Proper Motion - “The apparent angular motion across the sky of an object relative to the Solar System.”
  • Protoplanetary Disk - “A rotating circumstellar disk of dense gas surrounding a young newly formed star. It is thought that planets are eventually formed from the gas and dust within the protoplanetary disk.”
  • Protostar - “Dense regions of molecular clouds where stars are forming.”
  • Pulsar - “A spinning neutron star that emits energy along its gravitational axis. This energy is received as pulses as the star rotates.”

 

Q

  • QSO  - “Quasi-stellar object. The first discovered QSO’s were radio sources, leading to the name quasi-stellar radio sources, or QSRS, or quasars. These objects look like stars on an image of the sky, but theirspectrashow strongemission linesat high redshift. Theredshiftmeans that quasars are very far away, and are thus the most luminous objects in the Universe.”
  • Quadrature - “A point in the orbit of a superior planet where it appears at right angles to the Sun as seem from Earth.”
  • Quadrupole - “A type of pattern on the sky which generally has two high spots and two low spots.”
  • Quantum Fluctuations - “The uncertainty principle in quantum mechanics leads to all allowed interactions having some probability of occurrence.”
  • Quark - “An elementary, strongly interacting constituent of matter. Quarks come in six flavors: up, down, charm, strange, top and bottom. The up, charm and top quarks have electric charges of +(2/3)e, while the down, strange and bottom quarks have charges of -(1/3)e. The proton which has a charge of +e is constructed of two up quarks and one down quark: (uud), while the neutron is (udd).”
  • Quasar - “An unusually bright object found in the remote areas of the universe. Quasars release incredible amounts of energy and are among the oldest and farthest objects in the known universe. They may be the nuclei of ancient, active galaxies.”
  • Quasi-Stellar Object - “Sometimes also called quasi-stellar source, this is a star-like object with a large redshift that gives off a strong source of radio waves. They are highly luminous and presumed to be extragalactic.”

 

R

  • r - “The alphabetic letter (“variable”) used to denote the distance between the sun and the object being discussed, also called the object’s heliocentric distance; in most ephemerides of objects such as comets and minor planets, r is given in AU. Similarly, the upper-case Greek letter Delta gives the distance between the object and the earth (its geocentric distance).”
  • Radial Velocity - “The movement of an object either towards or away from a stationary observer.”
  • Radiant - “A point in the sky from which meteors in a meteor shower seem to originate.”
  • Radiation - “Energy radiated from an object in the form of waves or particles.”
  • Radiation Belt - “Regions of charged particles in a magnetosphere.”
  • Radio Galaxy - “A galaxy that gives off large amounts of energy in the form of radio waves.”
  • Red Giant - “A stage in the evolution of a star when the fuel begins to exhaust and the star expands to about fifty times its normal size. The temperature cools, which gives the star a reddish appearance.”
  • Redshift - “A shift in the lines of an object’s spectrum toward the red end. Redshift indicates that an object is moving away from the observer. The larger the redshift, the faster the object is moving.”
  • Reflector - “A telescope that uses as its primary optical element a mirror. Nearly all large telescopes in use today by amateur and professional astronomers are reflecting telescopes.”
  • Refractor - “A telescope that uses as its primary optical element a lens. Binoculars are a type of refractor. In general, refractors are much more expensive to build and buy than are reflectors.”
  • Regular Satellite - “A satellite that orbits close to a planet in a nearly circular, equatorial orbit. Regular satellites are believed to have been formed at the same time as the planet, unlike irregular satellites which are believed to have been captured by the planet’s gravity.”
  • Resonance - “A state in which an orbiting object is subject to periodic gravitational perturbations by another.”
  • Retrograde Motion - “The phenomenon where a celestial body appears to slow down, stop, them move in the opposite direction. This motion is caused when the Earth overtakes the body in its orbit.”
  • Retrograde Orbit - “The orbit of a satellite where the satellite travels in a direction opposite to that direction of the planet’s rotation.”
  • Right Ascension - “The amount of time that passes between the rising of Aries and another celestial object. Right ascension is one unit of measure for locating an object in the sky.”
  • Ring Galaxy - “A galaxy that has a ring-like appearance. The ring usually contains luminous blue stars. Ring galaxies are believed to have been formed by collisions with other galaxies.”
  • Roche Limit - “The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary. At a lesser distance the tidal forces of the primary would break up the secondary.”
  • Rotation - “The spin of a body about its axis.”

 

S

  • Satellite - “A natural or artificial body in orbit around a planet.”
  • Scarp - “A line of cliffs produced erosion or by the action of faults.”
  • Secular motion - “Secular variations in the motions of the planets are those that have very slow changes that proceed through ages (secula) in a way such that they are nearly proportional with time for a relatively large number of years. Precession is considered a secular variation, arising from the motions of the mean equator and the mean ecliptic. Compare this with periodic variations, which are rather rapid changes; nutation is a periodic variation.”
  • Seyfert Galaxy - “A main-sequence star that rotates rapidly, causing a loss of matter to an ever-expanding shell.”
  • Shell Star - “A type of star which is believed to be surrounded by a thin envelope of gas, which is often indicated by bright emission lines in its spectrum.”
  • Shepherd Satellite - “A satellite that constrains the extent of a planetary ring through gravitational forces. Also known as a shepherd moon.”
  • Sidereal - “Of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite.”
  • Sidereal Month - “The average period of revolution of the Moon around the Earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time.”
  • Sidereal Period - “The period of revolution of a planet around the Sun or a satellite around its primary.”
  • Singularity - “The center of a black hole, where the curvature of space time is maximal. At the singularity, the gravitational tides diverge. Theoretically, no solid object can survive hitting the singularity.”
  • Small Solar System Body - “A term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neitherplanets or dwarf planets. These include most of the asteroids, comets, and other small bodies in the Solar System.”
  • Solar Cycle - “The approximately 11-year quasi-periodic variation in frequency or number of solar active events.”
  • Solar Eclipse - “A phenomenon that occurs when the Earth passes into the shadow of the Moon. A total solar eclipse occurs when the Moon is close enough to completely block the Sun’s light. An annular solar eclipse occurs when the Moon is farther away and is not able to completely block the light. This results in a ring of light around the Moon.”
  • Solar Flare - “A bright eruption of hot gas in the Sun’s photosphere.  Solar prominences are usually only detectable by specialized instruments but can be visible during a total solar eclipse.”
  • Solar Nebula - “The cloud of dust and gas out of which the Solar System was believed to have formed about 5 billion years ago.”
  • Solar Wind - “A flow of charged particles that travels from the Sun out into the Solar System.”
  • Solstice - “The time of the year when the Sun appears furthest north or south of the celestial equator. The solstices mark the beginning of the Summer and Winter seasons.”
  • Spectrum - “Result of spreading out light by wavelengths. A rainbow is a naturalspectrum. The eye is sensitive to waves from violet at 380 nm wavelength to red at 700 nm wavelength, but astronomers now study electromagnetic radiation from gamma rays through X-rays, ultraviolet, violet, blue, green, yellow, orange, red, infrared and radio waves.”
  • Spectrometer - “The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.”
  • Spectroscopy - “The technique of observing the spectra of visible light from an object to determine its composition, temperature, density, and speed.”
  • Spectrum - “The range of colors that make up visible white light. A spectrum is produced when visible light passes through a prism.”
  • Spicules - “Grass-like patterns of gas seen in the atmosphere of the Sun.”
  • Spiral Galaxy - “A galaxy that contains a prominent central bulge and luminous arms of gas, dust, and young stars that wind out from the central nucleus in a spiral formation. Our galaxy, the Milky Way, is a spiral galaxy.”
  • Star - “A giant ball of hot gas that creates and emits its own radiation through nuclear fusion.”
  • Star Cluster - “A large grouping of stars, from a few dozen to a few hundred thousand, that are bound together by their mutual gravitational attraction.”
  • Steady State Theory - “The theory that suggests the universe is expanding but exists in a constant, unchanging state in the large scale. The theory states that new matter is being continually being created to fill the gaps left by expansion. This theory has been abandoned by most astronomers in favor of the big bang theory.”
  • Steradian - “The unit of solid angle. There are 4*pi steradians in the entire celestial sphere. One square degree is (1/57.3)2steradians because one degree is (1/57.3) radians.”
  • Stellar Wind - “The ejection of gas from the surface of a star. Many different types of stars, including our Sun, have stellar winds. The stellar wind of our Sun is also known as the Solar wind. A star’s stellar wind is strongest near the end of its life when it has consumed most of its fuel.”
  • Strong Nuclear Force - “One of the four forces of nature. The strong nuclear force holds the particles in the nucleus of atoms together.”
  • Stone Meteorite - “A meteorite that resembles a terrestrial rock and is composed of similar materials.”
  • Stony Iron - “A meteorite that contains regions resembling both a stone meteorite and an iron meteorite.”
  • Sublimation - “The change of a solid (such as ice) directly into a gaseous state (bypassing the liquid state). This happens in the vacuum of space with comets, as the heating effects of solar radiation cause ices in comets to “steam off” as gasses into space. The ice molecules present in the nucleus actually break up (or dissociate) into smaller atoms and molecules after leaving the nucleus in gas form.”
  • Sunspot - “Areas of the Sun’s surface that are cooler than surrounding areas. The usually appear black on visible light photographs of the Sun. Sunspots are usually associated disturbances in the Sun’s electromagnetic field.”
  • Supergiant - “The stage in a star’s evolution where the core contracts and the star swells to about five hundreds times its original size. The star’s temperature drops, giving it a red color.”
  • Superior Conjunction - “A conjunction that occurs when a superior planet passes behind the Sun and is on the opposite side of the Sun from the Earth.”
  • Superior Planet - “A planet that exists outside the orbit of the Earth. All of the planets in our solar system are superior except for Mercury and Venus. These two planets are inferior planets.”
  • Supernova - “A supernova is a cataclysmic explosion caused when a star exhausts its fuel and ends its life.  Supernovae are the most powerful forces in the universe. All of the heavy elements were created in supernova explosions.”
  • Supernova Remnant - “An expanding shell of gas ejected at high speeds by a supernova explosion. Supernova remnants are often visible as diffuse gaseous nebulae usually with a shell-like structure. Many resemble “bubbles” in space.”
  • Synchronous Rotation - “A period of rotation of a satellite about its axis that is the same as the period of its orbit around its primary. This causes the satellite to always keep the same face to the primary. Our Moon is in synchronous rotation around the Earth.”
  • Synodic Period - “The interval between points of opposition of a superior planet.”

 

T

  • Tektite - “A small, glassy material formed by the impact of a large body, usually a meteor or asteroid. Tektites are commonly found at the sites of meteor craters.”
  • Telescope - “An instrument that uses lenses and sometimes mirrors to collect large amounts of light from distant objects and enable direct observation and photography. A Telescope can also include any instrument designed to observe distant objects by their emissions of invisible radiation such as x-rays or radio waves.”
  • Terminator - “The boundary between the light side and the dark side of a planet or other body.”
  • Terrestrial - “A term used to describe anything originating on the planet Earth.”
  • Terrestrial Dynamical Time (TDT or TT) - “Time scale used in orbital computations; TDT is tied to atomic clocks (International Atomic Time, TAI), whereas Universal Time is tied to observations. Prior to 1992, Ephemeris Time (ET) was used in publications of the ICQ/CBAT/MPC; since then, TT has been used. The difference between TDT and UTC in 1994 was 60 seconds (i.e., UT + 60 seconds = TDT).”
  • Terrestrial Planet - “A name given to a planet composed mainly of rock and iron, similar to that of Earth.”
  • Tidal Force - “The differential gravitational pull exerted on any extended body within the gravitational field of another body.”
  • Tidal Heating - “Frictional heating of a satellite’s interior due to flexure caused by the gravitational pull of its parent planet and/or other neighboring satellites.”
  • Time Dialation - “In special relativity, moving clocks appear to run slowly when compared to stationary clocks. This clock slowing is called time dilation.”
  • Total (visual) magnitude - “Total, integrated magnitude of a comet’s head (meaning coma + nuclear condensation). This can be estimated visually, as the comet’s “total visual magnitude”. The variable m1, usually found in comet ephemerides, is used to denote the total (often predicted) magnitude. See also definition for “Magnitude”, above.”
  • Transit - “The passage of a celestial body across an observer’s meridian; also the passage of a celestial body across the disk of a larger one.”
  • Trans-Neptunian Object (TNO) - “Any one of a number of celestial objects that orbit the Sun at a distance beyond the orbit of the planet Neptune.”
  • Trojan - “An object orbiting in the Lagrange points of another (larger) object. This name derives from a generalization of the names of some of the largest asteroids in Jupiter’s Lagrange points. Saturn’s moons Helene, Calypso and Telesto are also sometimes called Trojans.”

 

U

  • Ultraviolet - “Electromagnetic radiation at wavelengths shorter than the violet end of visible light. The atmosphere of the Earth effectively blocks the transmission of most ultraviolet light, which can be deadly to many forms of life.”
  • Umbra - “The area of total darkness in the shadow caused by an eclipse.”
  • Universal Time (UT) - “Also known as Greenwich Mean Time, this is local time on the Greenwich meridian. Universal time is used by astronomers as a standard measure of time.”

 

V

  • Vacuum Energy Density  - “Quantum theory requires empty space to be filled with particles and anti-particles being continually created and annihilated. This could lead to a net density of the vacuum, which if present, would behave like a cosmological constant.”
  • Van Allen Belts - “Radiation zones of charged particles that surround the Earth. The shape of the Van Allen belts is determined by the Earth’s magnetic field.”
  • Variable Star - “A star that fluctuates in brightness. These include eclipsing binaries.”
  • Vernal equinox - “The point on the celestial sphere where the sun crosses the celestial equator moving northward, which corresponds to the beginning of spring in the northern hemisphere and the beginning of autumn in the southern hemisphere (in the third week of March). This point corresponds to zero (0) hours of right ascension.”
  • Visible Light - “Wavelengths of electromagnetic radiation that are visible to the human eye.”
  • Virgo Cluster - “A gigantic cluster of over 2000 galaxies that is located mainly within the constellation of Virgo. This cluster is located about 60 million light-years from Earth.”
  • Visual Magnitude - “A scale used by astronomers to measure the brightness of a star or other celestial object. Visual magnitude measures only the visible light from the object. On this scale, bright objects have a lower number than dim objects.”

 

W

  • Weak Nuclear Force  - “One of the four forces of nature. The weak nuclear force is responsible for radioactive decay as well as the fusion reactions in the Sun that provide heat and light for the Earth.”
  • WIMP “A Weakly Interacting Massive Particle, a possible form for cold dark matter.”
  • Wavelength - “The distance between consecutive crests of a wave. This serves as a unit of measure of electromagnetic radiation.”
  • White Dwarf - “A very small, white star formed when an average sized star uses up its fuel supply and collapses. This process often produces a planetary nebula, with the white dwarf star at its center.”

 

X

 

Y

  • Yellow Dwarf - “An ordinary star such as the Sun at a stable point in its evolution.”

 

Z

  • Z - “z is the symbol used for the redshift. The formula is 1+z = λobsem= (Observed wavelength)/(Emitted wavelength) where a line is emitted at one wavelength but observed at a different wavelength.”
  • Zero Point Energy - “The uncertainty principle does not allow a quantum mechanical system to have a definite position and definite velocity at the same time. Thus a harmonic oscillator like a pendulum or a mass on a spring has a minimum energy that is larger than zero, since zero energy would require a definite position to zero the potential energy and a definite (zero) velocity to zero the kinetic energy. This minimum energy is 0.5*h*f, where h is Planck’s constant and f is the frequency of the oscillator.
  • Zenith - “A point directly overhead from an observer.”
  • Zodiac - “An imaginary belt across the sky in which the Sun, moon, and all of the planets can always be found.”
  • Zodiacal Light - “A faint cone of light that can sometimes be seen above the horizon after sunset or before sunrise.  Zodiacal light is caused by sunlight reflecting off small particles of material in the plane of the Solar System.”

 

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