A. B. C. D. E. F. G. H. I. J. K. L. M. N. O. P. Q. R. S.T. U. V. W. X. Y. Z.
Absolute magnitude- the brightness a star would be as seen from a distance of 10 parsecs
Absolute Zero- the lowest possible temperature, at which substances contain no heat energy, and atomic movement has stopped
Accretion- accumulation of dust and gas into larger bodies such as stars, planets, and moons
Accretion disk- a disk of hot, glowing matter spiraling into a black hole
Active galaxy- a galaxy under-going a violent outburst in its central regions
Adaptive optics- the technology that allows, based on a laser beam aimed through the atmosphere, a computer to make very slight modifications to a telescope’s mirror, which will correct for atmospheric distortions
Albedo- the reflecting power of a non-luminous body; a perfect reflector would have an albedo of 100 per cent
Albedo feature- a dark or light marking on the surface of an object that may not be a geological or topographical feature
Altazimuth mounting- a telescope mounting that swings from side to side parallel to the horizon, and up and down
Angstrom unit- the hundred-millionth part of a centimeter (10-10 m)
Annihilation- total destruction of matter in a burst of energy
Anthropic principle- we see the universe the way it is because if it were different we would not be here to observe it
Antigravity- a gravitational field that repels, rather than attracts, matter and light rays
Antimatter- the exact opposite of matter; when matter meets the tiny amount of antimatter in the universe, the two annihilate each other
Antiparticle- an atomic particle that has exactly the opposite properties of its counter-part (e.g. a positron and an electron)
Antipodal point- the point that is directly on the opposite side of the planet
Aperture- the diameter of an opening through which light passes in an optical instrument
Aphelion- the position of a planet (or other body) when it is at its furthest from the sun
Apparent magnitude- the brightness of an object as seen from Earth
Arcuate- having the form of a bow; curved; arc-shaped
Asteroid- a small rocky object orbiting the sun, less than 1,000 kilometers in diameter
Astronomical Unit (A.U.)- the average distance between the Earth and the sun, roughly 150 million km (93 million miles)
Atmosphere- the layer of gases enveloping a celestial object
Atom- the smallest part of an element that can take part in a chemical reaction; most of the mass of the an atom is concentrated in its nucleus, which is about .000000000001 meters (.01 angstroms) across
Aurora- curtains and arks of light in the sky visible over middle and high latitudes; they are caused by particles from the sun hitting the Earth’s atmosphere and causing some of its gases to glow
Axis- the imaginary line through the center of a planet, star, or galaxy around which it rotates; also, a similar line through a telescope mounting
Bar- equals 0.987 atmosphere = 1.02 kg/cm2= 100 kilopascal = 14.5 lbs/inch2
Baryon- baryons are composed of three quakrs; they include protons and neutrons
Big Bang- the violent event that gave birth to our universe
Big Crunch- the ultimate collapse of the universe that may take place in the future if the universe starts to contract
Binary system- system of two stars that orbit around each other
Black dwarf- a dead star with a maximum possible mass of 1.4 solar masses that has cooled to a point where it no longer glows with residual heat
Black hole- a collapsed object with such strong gravity that nothing can escape it; as a result, the object is black, and it is a hole because nothing can escape from it
Blazar- a type of active galaxy that is angled in such a way to us that we look almost directly at its accretion disk and jet
Blue shift- a shift in the wave-length of radiation emitted by an object when it is approaching us; the Doppler shift makes the wave fronts bunch closer together, causing the light to appear of a shorter wavelength, and hence bluer
Bolide- a fireball that produces a sonic boom
Brown dwarf- a “failed star” in the sense that when it was finished forming, it did not have enough mass to begin fusion; it does not shine as a star does, but can generate heat through very slow gravitational contraction — this works because when a gas is compressed, it gains temperature
Caldera- a volcanic crater
Carbonate- a compound containing carbon and oxygen (e.g. calcium carbonate AKA limestone)
Casimir effect- the attractive pressure between two flat, parallel metal plates placed very near to each other in a vacuum; the pressure is due to a reduction in the usual number of virtual particles in the space between the plates
Catadioptric telescope- a telescope that uses both mirrors and lenses to form and image
Catena- a chain of craters
Cavus- a hollow, irregular depression
Charge-coupled device (CCD)- a computer-controlled electronic detector that can record an image
Celestial equator- the imaginary line encircling the sky midway between the two celestial poles
Celestial poles- the imaginary points on the sky where Earth’s rotation axis, extended infinitely, would touch the imaginary celestial sphere
Celestial sphere- the imaginary sphere enveloping the Earth upon which the stars, galaxies, and other celestial objects all appear to lie
Cepheid variable- a variable star of short period; the fluctuations are regular and are linked with its real luminosity; the longer the period, the more luminous the star
Chandrasekhar limit- the maximum possible mass of a stable cold star, above which it must collapse into a black hole
Chaos- a distinctive area of broken terrain
Chasma- a canyon
Chromosphere- that part of the sun’s atmosphere that lies just above its visible surface, or photosphere
Circumpolar stars- stars that never set when seen from a given location
Colles- small hills or knobs
Collimation- the procedure of aligning a telescope’s optics
Coma- the dust and gas surrounding an active comet’s nucleus
Comet- a small body composed of ices and dust which orbits the sun on an elongated path
Conjunction- the moment when two celestial objects lie closest together in the sky
Conservation of angular momentum- the law of science that states that momentum must be conserved within a system
Conservation of energy- the law of science that states that energy (or its equivalent in mass) can neither be created nor destroyed
Constellation- one of the eighty-eight official patterns of stars into which the night sky is divided
Convection- fluid circulation driven by large temperature gradients; the transfer of heat by this automatic circulation
Corona- the high-temperature outermost atmosphere of the sun, visible from Earth only during a total solar eclipse
Coronagraph- a special telescope which blocks light from the disk of the Sun in order to study the faint solar atmosphere
Cosmic censor- mythical being who dictates that singularities must be surrounded by an event horizon
Cosmic ray- an extremely energetic (relativistic) charged particle
Cosmological Constant- a mathematical device used by Einstein to give space-time an inbuilt tendency to expand
Cosmology- the study of the universe as a whole
Core- in a star, the central region that is undergoing nuclear fusion; in a galaxy, the innermost few light-years
Crater- bowl-shaped depression formed by the impact of a meteoroid; depression around the orifice of a volcano
Culmination- the maximum altitude of a celestial body from the celestial equator
Dark adaptation- the process by which the human eye increases sensitivity under conditions of low, or none, illumination
Dark matter- invisible matter that is believed to make up 99% of the mass of the universe
Declination- the angular distance of a celestial object north or south of the celestial equator; it corresponds to latitude on the Earth
Density- degree of “solidity” of a body: its mass divided by its volume
Direct motion- the movement of a celestial body from west to east – that is, in the same direction as that of the Earth around the sun
Disk- the visible surface of the Sun (or any heavenly body) projected against the sky
Doppler effect- change in the observed frequency of sound or radiation that takes place when the observer and the source are moving relative to each other
Dorsum- a ridge
Double-star system- a system of two stars in orbit around each other
Duality- a correspondence between apparently different theories that lead to the same physical results
Dust- microscopic grains in space that absorb starlight; the grains are “soot” left by dying stars, and they will sometimes clump together in huge dark clouds
Dwarf star- a star, such as the sun, that lies on the Main Sequence
Eccentricity- a measure of how closely a planet’s orbit approximates to a perfect circle
Eclipse- when one celestial body passes in front of another, dimming or obscuring its light (e.g. solar, lunar, and eclipsing binaries)
Ecliptic- the apparent yearly path of the sun against the stars
Einstein-Rosen bridge- the “throat” of a black hole in one universe connecting up with one in a different universe; in theory, it is a bridge from one universe to another
Electromagnetic radiation- radiation made up of magnetic and electrical fields that move at the speed of light
Electron- tiny particle with a negative charge, often in orbit around the nucleus of an atom
Electroweak unification energy- the energy (around 100 GeV) above which the distinction between the electromagnetic force and the weak force disappears
Elementary particle- a particle that, it is believed, cannot be subdivided
Ellipse- the oval, closed path followed by a celestial object moving under gravity (e.g. a planet around the sun)
Elongation- the angular distance of a planet from the sun or a satellite from its primary planet
Equatorial mounting- a telescope mounting which has one axis parallel to the Earth’s rotational axis, so the motion of the heavens can be followed with a single movement
Equinox- the two points at which the sun crosses the celestial equator; the spring equinox is about March 21, and the autumnal equinox is about September 22
Ergosphere- region surrounding a spinning black hole, between the static limit and the outer event horizon, in which it is impossible to be at rest
Escape velocity- speed a body needs to travel in order to escape the surface gravity of a star or planet
Event- a point in space-time, specified by its time and place
Event horizon- the “edge” of a black hole: and imaginary surface where the escape velocity reaches the speed of light
Exclusion Principle- the idea that two identical spin-1/2 particles cannot have (within the limits set by the uncertainty principle) both the same position and the same velocity
Extinction- the apparent reduction in the brightness of a star or planet when low over the horizon because more of its light is absorbed by the Earth’s atmosphere
Eyepiece- a set of lenses used to magnify the image produced by a telescope’s objective
Faculae- the bright patches on the sun’s photosphere
Farrum- a pancake-like structure (like those on Venus)
Field- something that exists throughout space and time, as opposed to a particle that exists at only one point at a time
Filament- a strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun; a filament on the limb of the Sun seen in emission against the dark sky is called a prominence
Finder- a small, low-power telescope attached to and aligned with a larger one; its wider field of view makes it useful for locating celestial objects
Fireball- any meteor brighter than Venus, about magnitude -4
Flare, solar- brilliant outbreaks in the solar atmosphere, normally detectable only by spectroscopic methods
Flare star- a faint red star that has short-lived explosions on its surface; these explosions cause the star to appear temporarily brighter
Fluctus- flow terrain
Fossa- long, narrow, shallow depression
Fraunhofer Lines- the dark lines in the spectrum of the sun
Frequency- for a wave, the number of complete cycles per second
Gaia Hypothesis- named for the Greek Earth goddess Gaea, holds that the Earth as a whole should be regarded as a living organism and that biological processes stabilize the environment. First advanced by British biologist James Lovelock in 1969
Galactic cluster- (see open cluster)
Galaxy- a huge gathering of stars, gas, and dust, bound by gravity and having a mass ranging from 100,000 to 10 trillion times that of the sun; there are spiral, elliptical, and irregular types of galaxies
Gamma rays- the highest energy, shortest wavelength electromagnetic radiation of all
Gegenschein- a round or elongated spot of light in the sky at a point 180 degrees from the sun; also called counter glow
General relativity- the theory of relativity that describes how matter behaves in the presence of strong gravitational fields
Geodesic- the shortest (or longest) path between two points
Geosynchronous orbit- a direct, circular, low inclination orbit in which the satellite’s orbital velocity is matched to the rotational velocity of the planet; a spacecraft appears to hang motionless above one position of the planet’s surface
Globular cluster- a spherical cluster that may contain up to a million stars
Grand unification energy- the energy above which, it is believed, the electromagnetic force, weak force, and strong force become indistinguishable from each other
Grand Unified Theory (GUT)- a theory that unifies the electromagnetic, strong, and weak energy forces
Granulation- a pattern of small cells seen on the surface of the Sun caused by the convective motions of the hot solar gas
Gravitational lens- distortion of an image – or the production of many images – by a powerful gravitational field
Gravitational waves- ripples in space that travel at the speed of light, produced by the movement of very massive bodies
Gravity- force of attraction that is felt between two masses, such as the pull between the Earth and the Moon
Hadron- particles made of quarks that are influenced by the Strong Nuclear Force; includes mesons and bosons, but excludes leptons
Heliocentric- sun-centered solar system theories
Heliopause- the point 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 solar system
Hertzsprung-Russel (HR) Diagram- a graph whose horizontal axis plots star color (or temperature) against a vertical axis plotting stellar luminosity (or absolute magnitude)
Ice- used by planetary scientists to refer to water, methane, and ammonia which usually occur as solids in the outer solar system
Imaginary time- time measured using imaginary numbers (i.e. the square root of negative one)
Inclination- measure of the tilt of a planet’s orbital plane, in relation to that of the Earth
Inferior planet- either of the two planets, Mercury and Venus, that orbit between the sun and the Earth
Inferometry- if one combines two or more telescope’s light, the resulting image would have the resolution of a telescope the size of the distance separating them
Infrared- heat radiation, inter../mediate in wavelength between light and radio waves
Interplanetary Magnetic Field (IMF)- the magnetic field carried with the solar wind
Kelvin (K)- 0 Kelvin is absolute zero; water melts at 273 K; water boils at 373 K; developed by William Thomson
Lacus- lake
Lagrange points- Lagrange showed that three bodies can 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
Last stable orbit- the closest an object can circle a black hole without being pulled in
Lepton- one of the fundamental types of particles, with quarks being the other; common examples are electrons and neutrinos
Libration- an effect caused by the apparent slight “wobbling” of the Moon from side to side, as seen from Earth; as a result, a total of fifty-nine per cent of the Moon can be observed from Earth, though no more than fifty per cent at one time
Lidar- an instrument similar to radar that operates at visible wavelengths
Light-year- distance covered by a ray of light traveling at 300,000 kps (186,000 mps) in a year; it is about 9,460,528,404,880 km (5,878,499,814,140 miles)
Light cone- a surface in space-time that marks out the possible directions for light rays passing through a given event
Limb- an edge or border, as of the sun, Moon, or any planet
Local Group- a gathering of roughly thirty galaxies to which the Milky Way belongs
Luminosity- the total intrinsic brightness of a star or galaxy
Lunar month- The average time between successive new or full moons, equal to 29 days 12 hours 44 minutes
Lunation- the interval between one new moon and the next: that is, 29 days, 12 hours, and 44 minutes
MACHO- an acronym for “Massive Compact Halo Object” – supposed dark, massive objects surrounding our galaxy
Magnetosphere- the are around a planet in which its magnetic field is dominant
Magnetotail- the portion of a planetary magnetosphere which is pushed away from the sun by the solar wind
Magnetic field- the field responsible for magnetic forces, now incorporated along with the electric field, into the electromagnetic field
Magnitude- a logarithmic unit used to measure the optical brightness of celestial objects; numerically lower magnitudes are brighter than numerically larger ones; a five-magnitude difference represents a 100-fold change in brightness
Main Sequence- the band on the HR Diagram where stars lie for much of their life
Mare- literally “sea” (a very bad misnomer, still in use for historical reasons); really a large circular plain
Mass- amount of matter making up a body
Matter- what everything that we know of is made of; the opposite of antimatter
Meridian- an imaginary line on the sky that runs due north and south and passes through your zenith
Meson- matter that is composed of two quarks
Metal- used by astrophysicists to refer to all elements except hydrogen and helium, as in: “The universe is composed of hydrogen, helium and traces of metals;” (note: this is quite different from the usual chemistry definition)
Meteor- the bright, transient streak of light produced by a piece of space debris burning up as it enters the atmosphere at high speed
Meteorite- any piece of space debris that reaches the Earth’s surface intact
Milky Way- a soft, glowing band of light encircling the sky, it is the disk of the spiral galaxy in which the sun lies, seen from the inside
Microwave background radiation- the radiation from the glowing of the hot early universe, now so greatly red-shifted that it appears not as light but as microwaves (radio waves with a wavelength of a few centimeters)
Mini black hole- one of many tiny black holes with the mass of a mountain but the size of an atom that are believed to have been created in the Big Bang
Minor planets- the official term used for asteroids
Missing mass- several seperate experiments show that there is more matter in the universe than we can see; more than that, this so-called “dark matter” accounts for about 25% of the universe’s total mass-energy, but there is a “dark energy” which appears to be moving everything apart from everything else (acting only at very large scales) that seems to account for 70% of the universe’s mass-energy content
Mons- mountain (plural is montes)
Naked singularity- a singularity that is not surrounded by an event horizon
Nebula- a cloud of dust and gas in space, from which new stars are created
Neutrino- miniscule particle with little or no mass and no charge that travels at the speed of light
Neutron- electrically neutral particle that makes up part of the nucleus of an atom
Neutron star- collapsed star composed mainly of neutrons; pulsars are young, fast-spinning neutron stars
No boundary condition- the idea that the universe is finite but has no boundary (in imaginary time)
Nova- a white dwarf star in a binary system that brightens suddenly by several magnitudes as gas pulled away from its companion star explodes in a thermonuclear reaction
Nuclear fusion- nuclear reaction in which one kind of atom, under extreme heat and pressure, is combined with another and forms a different one
- the central part of an atom, consisting only of protons and neutrons, held together by the strong force
- the central part of a comet, whose existence is not a function of its distance from the sun — the only “permenant” feature of a comet
Objective- the main light gathering optical element in a telescope; it may be a lens or a mirror
Oblateness- the degree of flattening at the poles of a celestial body
Occulation- the covering up of one celestial object by another, such as the Moon passing in front of a star or planet as seen from Earth
Open cluster- a group of some few hundred stars bound by gravity and moving through space together
Opposition- the position of a superior planet when exactly opposite the sun in the sky, as seen from Earth; the planet is then best placed for observation
Orbit- the path followed by any celestial object moving under the control of another’s gravity
Orbital family- a group of asteroids that follows the same relative orbital path, velocity, and is usually seen close together; they are thought to have once been one asteroid that was broken apart due to a collision in the past
Parallax- the apparent change in position of a nearby star due to Earth’s orbital motion around the sun
Parsec- a unit of distance equal to 3.26 light-years; it is the distance at which a star would have a parallax of one second of arc
Particle accelerator- a machine that, using electromagnets, can accelerate moving charged particles, giving them more energy
Patera- a shallow crater with a scalloped, complex edge
Penumbra- the outer part of an eclipse shadow; also, the lighter area surrounding the center of a sunspot
Perihelion- a planet or comet’s closest approach to the sun
Perturb- to cause a planet or satellite to deviate from a theoretically regular orbital motion
Phase- the varying illuminated part of the Moon and planets caused by the relative locations of the object, the Earth, and the sun
Photosphere- the visible surface of the sun or a star
Plage- bright regions seen in the solar chromosphere
Planetary nebula- a shell of gas puffed off by a star late in its life; their often round appearance led to the name
Planitia- plateau or high plain
Plank’s Quantum Principle- the idea that light (or any other classical waves) can be emitted or absorbed only in discrete quanta, whose energy is proportional to their wavelength
Positron- the positively charged antiparticle of the electron
Precession- a slow periodic wobble in the Earth’s axis caused by the gravitational pull of the sun and Moon
Prominence, solar- a loop of cooler gas seen above the sun’s surface, which sometimes erupts outwards into space
Proton- a positively charged particle that forms part of the nucleus of an atom
Protoplanet- a stage in the formation of a planet which implies the body is nearly full-size
Protostar- a stage in the formation of a star which implies the body is nearly full-size; the star is still within its parent nebula, and does not yet produce energy through nuclear fusion
Pulsar- collapsed star composed mainly of neutrons; pulsars are young, fast-spinning neutron stars
Quantum- the indivisible unit in which waves may be emitted or absorbed
Quantum chronodynamics (QCD)- the theory that describes the interactions of quarks and gluons
Quark- a (charged) elementary particle that feels the strong force; protons and neutrons are composed of three quarks each
Radar- a system using pulsed radio waves to detect the position of objects by measuring the time it takes a single pulse to reach the object and be reflected back
Radial velocity- the movement of a celestial body toward or away from an observer
Radiant- the point on the sky from where a shower of meteors appears to come
Radiation- made up of magnetic and electrical fields that move at the speed of light
Radio galaxy- active galaxy that gives out as much energy in radio waves as it does in light
Radio telescope- a telescope that picks up radio waves from objects in space
Radioactivity- the spontaneous breakdown of one type of atomic nucleus into another
Red giant- an old star whose outer layers have billowed out and cooled down
Red shift- shift in the light of a retreating object toward red wavelengths, caused by the Doppler effect
Reflector- a telescope that forms an image with mirrors
Refractor- a telescope that forms an image with a lens
Relativity- (see general relativity and special relativity)
Resolving power- the ability of a telescope to image two closely spaced objects as separate
Resonance- a state in which one orbiting object is subject to periodic gravitational perturbations by another
- an apparent westward motion of a planet, asteroid, or comet relative to the stars
- the motion of a moon that orbits contrarty to the spin of its planet — a strong indication that the moon was captured and did not form in the vicinity of the planet
Rift valley- an elongated valley formed by the depression of a block of the planet’s crust between two faults or groups of faults of approximately parallel strike
Right ascension- the celestial coordinate analogous to longitude on Earth
Roche Limit- the closest one celestial object can get to another before the weaker object is broken apart by tidal effects
Satellite- any small object orbiting a larger one, although the term is most often used for rocky or man-made objects orbiting a planet
Scarp- line of cliffs produced by faulting or erosion
Schwarzchild Radius- radius of the event horizon around a black hole
Seeing- a measure of the steadiness of the atmosphere; good seeing is essential to using high magnification
Semimajor axis- the semimajor axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing thru the foci of the ellipse with endpoints on the ellipse itself; the semimajor axis of a planetary orbit is also the average distance from the planet to its primary
Seyfert galaxy- a galaxy that has a small, bright nucleus and faint spiral arms; it is often a strong radio source
Shepherd moon- a satellite which constrains the extent of a planetary ring through gravitational forces
Sidereal period- the time, relative to the stars, needed for a planet or moon to make one rotation or revolution around its primary body
Singularity- the center of a black hole; a point (or ring) of infinite density that occupies zero space
Singularity theorem- a theorem that shows that a singularity must exist under certain circumstances – in particular, that the universe must have started with a singularity
Solar cycle- the approximately 11-year quasi-periodic variation in frequency or number of solar active events
Solar filter- a filter that reduces the sun’s light to a level where you can view it with a telescope; only those filters which fit over a telescope’s objective are safe to use
Solar mass- the mass of the sun; it is used as a standard weight against which other celestial objects can be compared
Solar nebula- the cloud of gas and dust that began to collapse about 5 billion years ago to form the Solar System
Solar System- our sun with everything that orbits it: nine planets (plus their satellites), thousands of asteroids, and countless comets, meteors, and other debris
Solar wind- charged particles from the sun that travel into the Solar System at about 1.5 million kph (932,000 mph)
Solstice- the time when the sun reaches its greatest northern or southern declination
Space-Time- the four-dimensional description of the universe in which length, breadth, and height make up the first three spatial dimensions, while time makes up the fourth dimension
Spaghettification- gravitational stretching of a body falling into a black hole
Spatial dimension- any of the three dimensions that are space-like – that is, any except the time dimension (i.e. length, width, depth)
Special relativity- a branch of relativity dealing with the behavior of objects traveling close to the speed of light
Spectrograph- an instrument that breaks the light from a celestial object into its component colors for analysis
Spectroscopic binary- a very close double that is recognizable only by the periodic splitting of lines in the combined spectrum of the two stars, owing to the opposite Doppler effects resulting from their motions
Spectrum- the range of color produced when light is split up by a prism of diffraction grating
Spicules- grass-like patterns of gas seen in the solar atmosphere
Static limit- a limit close to a black hole inside of which is impossible to remain at rest
Stationary state- one that is not changing with time; a sphere spinning at a constant rate is stationary because it looks identical at any given instant
Stellar mass black hole- a black hole produced by the explosion of a massive star as a supernova; most weigh about ten solar masses
String Theory- a theory of physics in which particles are described as waves on strings; strings have length but no other dimension
Strong force- the strongest of the four fundamental forces, with the shortest range of all; it holds the quarks together within protons and neutrons, and holds the protons and neutrons together to form atoms
Sunspot- a highly magnetized dark spot on the sun’s surface, cooler than surrounding area
Supergiant- the stage in the evolution of a massive star when its core contracts, its surface expands to about 500 solar radii, and its temperature drops, giving the star its red color
Superior planet- any planet beyond the orbit of the Earth in the Solar System
Supermassive black hole- a black hole located at the center of a galaxy; these holes, formed by material falling onto the galaxy’s core, may weight billions of solar masses
Supernova- an explosion of a massive star at the end of its life; the star may briefly equal an entire galaxy in brightness
Supernova remnant- the gaseous debris, rich in heavy elements, thrown off by a supernova
Synchronous orbit radius- the orbital radius at which the satellite’s orbital period is equal to the rotational period of the planet; a synchronous satellite with an orbital inclination of zero (same plane as the planet’s equator) stays fixed in the sky from the perspective of an observer on the planet’s surface (such orbits are commonly used for communications satellites; it is also seen in the cases of Pluto and Charon)
Synchronous rotation- said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary; this implies that the satellite always keeps the same hemisphere facing its primary (e.g. the moon); it also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite’s motion while the other (trailing) one always faces backward; most of the satellites in the solar system rotate synchronously
Synodic period- the interval between the successive oppositions, conjunctions, etc., of a celestial body
Telrad- a sighting device for telescopes which projects a bull’s-eye on the sky
Terminator- the boundary between the illuminated and dark portions of a planet or satellite
Terra- extensive land mass
Terrestrial- refers to either Mercury, Venus, Earth, or Mars, due to them all having a solid surface, and they are within the inner solar system
Tholus- small dome-shaped mountain or hill
Tidal heating- frictional heating of a satellite’s interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites (e.g. Jupiter’s moon Io)
Tidally locked- two bodies are tidally locked when their rotations are such that they always present the same side to the other — their day is as long as their year; examples include Earth’s moon, and the Pluto-Charon system
- the instant when a celestial object crosses the meridian
- when an object passes in front of another object
Ultraviolet (UV)- the portion of the spectrum with wavelengths just shorter than the bluest light visible
Umbra- the dark inner part of an eclipse shadow; also, the dark central part of a sunspot
Uncertainty Principle- the principle, formulated by Heisenberg, that one can never be exactly sure of both the position and the velocity of a particle; the more accurately one knows the one, the less accurately one can know the other
Undae- dunes (literally ‘waves’)
Vallis- sinuous valley (plural: valles)
Van Allen Belts- radiation zones of charged particles surrounding the Earth
Variable star- any star, the brightness of which appears to change, with periods ranging from minutes to years
Virtual particle- in quantum mechanics, a particle that can never be directly detected, but whose existence does have measurable effects
Visible spectrum- the wavelengths of light to which the human eye is sensitive
Vastitas- widespread lowlands
Volatile- As a noun, this refers to substances that are gases at ordinary temperatures. In astronomy it includes Hydrogen (H), Helium (He), water (H20), ammonia (NH3), carbon dioxide (CO2), and methane (CH4)
Wave/particle duality- the concept in quantum mechanics that there is no distinction between waves and particles; particles may sometimes behave like waves, and waves like particles
Wavelength- the distance between eave crests on any train of electromagnetic radiation; short ones are more energetic than long ones
Weak force- the second weakest of the four fundamental forces, with a very short range; it affects all matter particles, but not force-carrying particles
Weight- the force exerted on a body by a gravitational field; it is proportional to, but not the same as, its mass
White dwarf- a collapsed core of a normal star such as the sun after it has lost its outer layers
White hole- the exact opposite of a black hole; an object that spews out matter and energy
Wormhole- an object with two mouths in different parts of our universe connected by a tunnel that allows two-way traffic; they may be safe shortcuts through space
X-ray source- a region of extremely hot gas; matter torn away from a normal star by a black hole or a neutron star becomes violently heated and emits x rays
YYellow dwarf- an ordinary star such as the sun at a comparatively stable and long-lived stage of evolution
Zenith- the point on the celestial sphere directly overhead
Zodiac- the twelve constellations straddling the ecliptic through which the sun, Moon, and planets appear to move during the year
Zodiacal light- a faint cone of light rising from the horizon after sunset or before sunrise; it is caused by sunlight reflected from thinly spread interplanetary material lying in the main plane of the Solar System
