Astronomy Glossary Of Terms (Vocabulary About Space A-Z)

Astronomy Glossary Of Terms

There are many good reasons to learn space words. For one thing, it can help you learn more about the science behind space travel and exploration. Also, if you know a lot of space-related words, you can talk to other people who are interested in space-related topics more easily. Lastly, learning this vocabulary can be fun and interesting, and it can help you understand the world around us better. No matter why you want to learn space words, you can have a good time doing it.

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  • accretion – Accumulation of dust and gas into larger bodies.
  • albedo – Reflectivity of an object; the ratio of reflected light to incident light.
  • albedo feature – A dark or light marking on the surface of an object that might not be a geological or topographical feature.
  • allocthonous -(1) Material that is formed or introduced from somewhere other than the place it is presently found. (2) Fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets its outer flanks after the impact event.
  • Alpha Centauri – The closest bright star to our solar system.
  • angstrom – A unit of length = 1.0E-08cm.
  • antipodal point – The point that is directly on the opposite side of the planet; e.g., the Earth’s north pole is antipodal to its south pole.
  • aphelion – The point in its orbit where a planet is farthest from the Sun.
  • apoapsis – The point in orbit farthest from the planet.
  • apogee – The point in orbit farthest from the Earth.
  • ash – The fine-grained material produced by a pyroclastic eruption. An ash particle is defined to have a diameter of fewer than 2 millimeters.
  • asteroid number – Asteroids are assigned a serial number when they are discovered; it has no particular meaning except that asteroid N+1 was discovered after asteroid N.
  • astronomical unit (AU) – The average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometers (92,960,116 miles).
  • atmosphere – One atmosphere is 14.7 pounds per square inch (105 Newtons per square meter); the average atmospheric pressure at sea level on Earth.
  • aurora – A glow in a planet’s ionosphere caused by the interaction between the planet’s magnetic field and charged particles from the Sun.
  • aurora borealis – The Northern Lights caused by the interaction between the solar wind, the Earth’s magnetic field, and the upper atmosphere; a similar effect happens in the southern hemisphere where it is known as the aurora australis.


  • bar – A unit of pressure, equal to the sea-level pressure of Earth’s atmosphere; 1 bar = 0.987 atmosphere = 101,300 pascals = 14.5 lbs/square inch = 100,000 Newtons per square meter.
  • basalt – A general term for dark-colored, igneous rocks composed of minerals that are relatively rich in iron and magnesium.
  • blackbody temperature – The temperature of an object if it is reradiating all the thermal energy that has been added to it; if an object is not a blackbody radiator, it will not reradiate all the excess heat and the leftover will go toward increasing its temperature.
  • black hole – An object whose gravity is so strong that the escape velocity exceeds the speed of light.
  • bolide – An exploding meteorite.
  • bow shock – The outermost part of a planetary magnetosphere; the place where the supersonic flow of the solar wind is slowed to subsonic speed by the planetary magnetic field.
  • breccia – A course-grained rock, composed of angular, broken rock fragments held together by a mineral cement or a fine-grained matrix.
  • butte – A conspicuous, isolated, flattop hill with steep slopes.


  • calcium K – A narrow wavelength of blue light which is emitted and absorbed by ions of the element calcium.
  • caldera – A large, basin-shaped volcanic depression that is more or less circular in form. Most volcanic calderas are produced by the collapse of the roof of a magma chamber due to the removal of magma by voluminous eruptions or subterranean withdrawal of the magma, although some calderas may be formed by explosive removal of the upper part of a volcano.
  • carbonate – A compound containing carbon and oxygen; an example is calcium carbonate (limestone).
  • cataclastic – A texture found in metamorphic rocks in which brittle minerals have been broken, crushed, and flattened during shearing.
  • catena – A chain of craters.
  • cavus – Hollows, irregular depressions.
  • central peak – The exposed core of uplifted rocks in complex meteorite impact craters; the central peak material typically shows evidence of intense fracturing, faulting, and shock metamorphism.
  • chaos – A distinctive area of broken terrain.
  • chasma – A canyon.
  • chromosphere – The lower level of the solar atmosphere between the photosphere and the corona.
  • cinders – Loose, vesicular volcanic ejecta 4 to 32 millimeters (.16 to 1.28 inches) in diameter.
  • cinder cone – A conical hill formed by the accumulation of pyroclastic fragments that fall to the ground in an essentially solid condition.
  • clast – A fragment of rock that has been transported, either by volcanic or sedimentary processes.
  • colles – A small hill or knob.
  • coma – The dust and gas surrounding an active comet’s nucleus.
  • composite volcano – A volcano composed of interbedded lava and pyroclastic material commonly with steep slopes.
  • convection – Fluid circulation is driven by temperature gradients; the transfer of heat by this automatic circulation (see also Educator’s Guide to Convection).
  • corona – 1) The upper level of the solar atmosphere, characterized by low densities and high temperatures (> 1.0E+06 K); it is not visible from the Earth except during a total eclipse of the sun or by use of special telescopes called coronagraphs. 2) An ovoid-shaped feature.
  • coronagraph – A special telescope that blocks light from the disk of the Sun in order to study the faint solar atmosphere.
  • cosmic rays – Electromagnetic rays of extremely high frequency and energy; cosmic rays usually interact with the atoms of the atmosphere before reaching the surface of the Earth. Some cosmic rays come from outside the solar system while others are emitted from the Sun and pass through holes in the corona.
  • crater – 1) A depression formed by the impact of a meteorite. 2) A depression around the orifice of a volcano.
  • cratons – The relatively stable portions of continents composed of shield areas and platform sediments; typically, cratons are bounded by tectonically active regions characterized by uplift, faulting, and volcanic activity.
  • Cretaceous period – A geological term denoting the interval of Earth’s history beginning around 144 million years ago and ending 66 million years ago.
  • Cretaceous-Tertiary boundary – A major stratigraphic boundary on Earth marking the end of the Mesozoic Era, best known as the age of the dinosaurs. The boundary is defined by a global extinction event that caused the abrupt demise of the majority of all life on Earth.
  • crystalline – Rock types made up of crystals or crystal fragments, such as metamorphic rocks that recrystallized in high temperature or pressure environments, or igneous rocks that formed from the cooling of a melt.


  • density – 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.
  • diaplectic glass – A natural glass formed by shock pressure from any of several minerals without melting; it is found only in association with meteorite impact craters.
  • dielectric constant – The ratio of electric flux density to the electric field.
  • disk – The visible surface of the Sun (or any heavenly body) projected against the sky.
  • Doppler effect – The apparent change in wavelength of sound or light caused by the motion of the source, observer, or both.
  • dorsum – A ridge.


  • e=mc2 – Einstein’s famous theory of relativity formula is known as the energy-mass relation. The energy e is equal to the mass m multiplied by the speed of light squared c2. A small mass produces an enormous amount of energy.
  • eccentric – Noncircular; elliptical (applied to an orbit).
  • eccentricity – A value that defines the shape of an ellipse or planetary orbit; the ratio of the distance between the foci and the major axis.
  • eclipse – The cutting off of light from one celestial body by another.
  • ecliptic – The plane of Earth’s orbit about the Sun
  • effusive eruption – A relatively quiet volcanic eruption that puts out basaltic lava that moves at about the speed one walks. The lava is fluid in nature. The eruptions at the Kilauea volcano on the island of Hawaii are effusive
  • ejecta – Material such as glass and fragmented rock thrown out of an impact crater during its formation.
  • ellipse – A closed curve that is formed from two foci or points in which the sum of the distances from any point on the curve to the two foci is a constant. Johannes Kepler first discovered that the orbits of the planets are ellipses, not circles; he based his discovery on the careful observations of Tycho Brahe.
  • en echelon fissures – Fissures that are parallel in trend to each other, but offset to either the left or right.
  • eolian – Related to wind deposits and associated effects.
  • eruption – The ejection of volcanic materials (lavas, pyroclasts, and volcanic gases) onto the surface, either from a central vent, a fissure, or a group of fissures.
  • explosive eruption – A dramatic volcanic eruption that throws debris high into the air for hundreds of miles. The lava is low in silicate and can be very dangerous for people nearby. An example is Mount St. Helens in 1980.


  • faculae – A bright region of the photosphere seen in white light, seldom visible except near the solar limb.
  • fault – A crack or break in the crust of a planet along which slippage or movement can take place.
  • 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.
  • fissure – A narrow opening or crack of considerable length and depth.
  • flare – A sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted.
  • flexus – A cuspate linear feature
  • fluctus – A flow terrain
  • fossa – A long, narrow, shallow depression.


  • Gaia Hypothesis – Named for the Greek Earth goddess Gaea, this hypothesis holds that the Earth should be regarded as a living organism. British biologist James Lovelock first advanced this idea in 1969.
  • Galilean moons – Jupiter’s four largest moons: Io, Europa, Ganymede, and Callisto; discovered independently by Galileo and Marius.
  • graben – An elongated, relatively depressed crustal unit or block that is bounded by faults on its sides.
  • 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.
  • granulation – A pattern of small cells seen on the surface of the Sun caused by the convective motions of the hot solar gas.
  • greenhouse effect – An increase in temperature caused when the atmosphere absorbs incoming solar radiation but blocks outgoing thermal radiation; carbon dioxide is the major factor.
  • gravity – A mutual physical force attracting two bodies.


  • H-alpha – A narrow wavelength of red light that is emitted and absorbed by the element hydrogen; this wavelength is often used to study the Sun.
  • heliocentric – Sun-centered; see Copernicus, Kepler, Galileo.
  • 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.
  • hemisphere – A half of the celestial sphere that is divided into two halves by either the horizon, the celestial equator, or the ecliptic.
  • high-pressure mineral phase – In this phase, mineral forms are stable only at the extremely high pressures typical of Earth’s deep interior but not it’s surface. Such pressures are generated instantaneously during meteorite impact. Stishovite is the high-pressure polymorph of quartz, a common crustal mineral.
  • hot spot – Center of persistent volcanism, thought to be the surface expression of a rising hot plume in Earth’s mantle.
  • hummocky – Uneven, lumpy terrain.


  • ice – Planetary scientists use this word to refer to water, methane, and ammonia, which usually occur as solids in the outer solar system.
  • igneous – Rock or mineral that solidified from molten or partly molten material.
  • impact melt – Rocks melted during impact, including small particles dispersed in various impact deposits and ejecta, and larger pools and sheets of melt that coalesce in low areas within the crater. Impact melts are extremely uniform in their composition but highly variable in texture. They are composed predominantly of the target rocks but can contain a small but measurable amount of the impactor.
  • inclination – The inclination of a planet’s orbit is the angle between the plane of its orbit and the ecliptic. The inclination of a moon’s orbit is the angle between the plane of its orbit and the plane of its primary equator.
  • inferior planets – The planets Mercury and Venus are inferior planets because their orbits are closer to the Sun than Earth’s orbit.
  • interplanetary magnetic field (IMF) – The magnetic field carried with the solar wind.
  • ion – An atom or molecular fragment that has a positive electrical charge due to the loss of one or more electrons; the simplest ion is the hydrogen nucleus, a single proton.
  • ionosphere – A region of charged particles in a planet’s upper atmosphere; the part of the earth’s atmosphere beginning at an altitude of about 400 kilometers (25 miles) and extending outward 400 kilometers (250 miles) or more.


  • Jovian planet – Any of the four outer, gaseous planets: Jupiter, Saturn, Uranus, and Neptune.


  • Kelvin (K) – Zero K is absolute zero; ice melts at 273 K (0° C, 32° F); water boils at 373 K ( 100° C, 212° F).
  • kilogram (kg) – One kilogram is equivalent to 1,000 grams or 2.2 pounds; the mass of a liter of water.
  • kilometer (km) – One kilometer is equivalent to 1,000 meters or 0.62 miles.


  • labes – A landslide.
  • labyrinthus – An intersecting valley complex.
  • lacus – A lake.
  • Lagrangian point – One of the solutions to the three-body problem discovered by the eighteenth-century French mathematician Lagrange; the two stable Lagrangian points, L-4 and L-5, lie in the orbit of the primary body, leading and trailing it by a 60-degree arc.
  • lava – A general term for molten rock that is extruded onto the surface.
  • lava tube – A tunnel formed underneath the surface of a solidifying lava flow.
  • leading hemisphere – The hemisphere that faces forward, into the direction of motion of a satellite that keeps the same face toward the planet.
  • lee – The side of an object that is sheltered from the wind.
  • levee – An embankment, continuous dike, or ridge.
  • light – Electromagnetic radiation that is visible to the eye.
  • light year – The distance light travels in a year, at the rate of 300,000 kilometers per second (671 million miles per hour); 1 light-year is equivalent to 9.46053e12 km, 5,880,000,000,000 miles or 63,240 AU.
  • limb – The outer edge of the apparent disk of a celestial body.
  • linea – An elongated marking.
  • lineament – A linear topographic feature that may depict crustal structure.
  • lobate – Having lobes or resembling a lobe.


  • macula – A dark spot.
  • magma – Molten rock within the crust of a planet that is capable of intrusion into adjacent crustal rocks or extrusion onto the surface. Igneous rocks are derived from magma through solidification and related processes or through the eruption of the magma at the surface.
  • magnetic field – A region of space near a magnetized body where magnetic forces can be detected.
  • magnetograph – A special telescope that analyzes the color and polarization of sunlight in order to measure the magnetic field of the Sun.
  • magnetopause – The boundary of the magnetosphere, lying inside the bow shock.
  • magnetosphere – The region of space in which a planet’s magnetic field dominates that of the solar wind.
  • magnetotail – The portion of a planetary magnetosphere that is pushed in the direction of the solar wind.
  • magnitude – The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512; also called apparent magnitude.
  • mare – Latin word for “sea.” Galileo thought the dark featureless areas on the Moon were bodies of water, even though the Moon is essentially devoid of liquid water. The term is still applied to the basalt-filled impact basins common on the face of the Moon visible from Earth.
  • mensa – A mesa, flat-topped elevation.
  • mesa – A broad, flattop, erosional hill or mountain, commonly bounded by steep slopes.
  • meteor – The luminous phenomenon is seen when a meteoroid enters the atmosphere, commonly known as a shooting star.
  • meteorite – A part of a meteoroid that survives through the Earth’s atmosphere.
  • meteoroid – A small rock in space.
  • millibar – This is 1/1000 of a bar; the standard sea-level pressure is about 1,013 millibars.
  • minor planets – Another term used for asteroids.
  • mons – A mountain.


  • nebula – A diffuse mass of interstellar dust and gas.
  • neutrino – A fundamental particle supposedly produced in massive numbers by the nuclear reactions in stars; they are very hard to detect because the vast majority of them pass completely through the Earth without interacting.
  • nuclear fusion – A 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. The difference in mass is converted to energy by Einstein’s famous equivalence E=mc2. This is the source of the Sun’s energy and, ultimately, of (almost) all energy on Earth.


  • Oceanus – An ocean.
  • obliquity – The angle between a body’s equatorial plane and orbital plane.
  • occultation – The blockage of light by the intervention of another object; a planet can occult (block) the light from a distant star.
  • old – A planetary surface that has been modified little since its formation typically featuring large numbers of impact craters; (compare to young).
  • optical depth – Optical depth is a measure of the transparency of a ring system. When a ring is “optically thick” (i.e., the optical depth is large), the ring is nearly opaque, and very little light passes through. When a ring is “optically thin” (i.e., the optical depth is small), very little material is present and most of the light passes through.
  • orbit – The path of an object that is moving around a second object or point.
  • ovoid – Shaped like an egg.


  • Paleozoic – A geological term denoting the time in Earth’s history between 570 and 245 million years ago.
  • pahoehoe – A type of basalt lava flow characterized by smooth glassy skin, and constructed of innumerable “flow units” called “toes”; pahoehoe flows advance at rates of 1 to 10 meters (3 to 33 feet) hour and are associated with low-effusion-rate eruptions with little to no fountaining.
  • palimpsest – A circular feature on the surface of dark icy moons such as Ganymede and Callisto lacking the relief associated with craters; Pamlimpsests are thought to be impact craters where the topographic relief of the crater has been eliminated by slow adjustment of the icy surface.
  • palus – A swamp.
  • patera – Shallow crater; scalloped, complex edge.
  • peak ring – A central uplift characterized by a ring of peaks rather than a single peak; peak rings are typical of larger terrestrial craters above about 50 kilometers (30 miles) in diameter.
  • penumbra – The outer filamentary region of a sunspot.
  • periapsis – The point in the orbit closest to the planet.
  • perigee – The point in the orbit closest to the Earth.
  • perihelion – The point in its orbit where a planet is closest to the Sun.
  • perturb – To cause a planet or satellite to deviate from a theoretically regular orbital motion.
  • photosphere – The visible surface of the Sun; the upper surface of a convecting layer of gases in the outer portion of the sun whose temperature causes it to radiate light at visible wavelengths; sunspots and faculae are observed in the photosphere.
  • phreatic eruption – A volcanic eruption or explosion of steam, mud or other material that is not incandescent; this form of eruption is caused by the heating and consequent expansion of groundwater due to an adjacent igneous heat source.
  • plage – Bright regions seen in the solar chromosphere.
  • planar features – Microscopic features in grains of quartz or feldspar consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain’s crystal structure.
  • planitia – Broad plains that occupy lowlands on planetary surfaces.
  • planum – A plateau or high plain.
  • plasma – A low-density gas in which the individual atoms are charged, even though the total number of positive and negative charges is equal, maintaining overall electrical neutrality.
  • polarization – A special property of light; light has three properties, brightness, color, and polarization.
  • Precambrian – A geological term denoting the time in Earth’s history prior to 570 million years ago.
  • pressure ridge – A ridge formed by the uplift of a lava flow crust due to the pressure of the flowing lava.
  • prominence – An eruption of hot gases above the photosphere of the Sun. Prominences are most easily visible close to the limb of the Sun, but some are also visible as bright streamers on the photosphere.
  • promontorium – A cape.
  • pseudocrater – A generally circular crater produced by a phreatic eruption resulting from the emplacement of a lava flow over wet ground.
  • pyroclastic – Pertaining to clastic (broken and fragmented) rock material formed by volcanic explosion or aerial expulsion from a volcanic vent.
  • pumice – A light vesicular form of volcanic glass with a high silica content; it is usually light in color and will float on water.



  • radiation – Energy radiated in the form of waves or particles; photons.
  • radiation belt – Regions of charged particles in a magnetosphere.
  • red giant – A star that has a low surface temperature and a diameter that is large relative to the Sun.
  • regio – Region.
  • regolith – The layer of rocky debris and dust made by the meteoritic impact that forms the uppermost surface of planets, satellites, and asteroids.
  • relativity, the Theory of – More accurately describes the motions of bodies in strong gravitational fields or at near the speed of light than Newtonian mechanics. All experiments done to date agree with relativity’s predictions to a high degree of accuracy. (Curiously, Einstein received the Nobel prize in 1921 not for Relativity but rather for his 1905 work on the photoelectric effect.)
  • resolution – The amount of small detail visible in an image; low resolution shows only large features, high resolution shows many small details.
  • resonance – A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
  • retrograde – The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic; moving in the opposite sense from the great majority of solar system bodies.
  • rhyolite – Fine-grained extrusive igneous rock, commonly with phenocrysts of quartz and feldspar in a glassy groundmass.
  • rift – A fracture or crack in a planet’s surface caused by an extension. On some volcanoes, subsurface intrusions are concentrated in certain directions; this causes tension at the surface and also means that there will be more eruptions in these “rift zones.”
  • 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 the approximately parallel strike.
  • rima – A fissure.
  • Roche limit – The Roche Limit was first described by Edouard Roche in 1848. It is the closest distance a body can come to a planet without being pulled apart by the planet’s tidal (gravity) force. As a result, large moons cannot survive inside the Roche Limit. On July 7, 1992, Comet Shoemaker-Levy 9 broke apart into 21 pieces due to tidal forces when it passed within Jupiter’s Roche Limit; on the subsequent pass, each of the comet’s pieces collided with Jupiter.
  • If a planet and a moon have identical densities, then the Roche Limit is 2.446 times the radius of the planet. The Roche Limits for the ringed planets are:
    • Jupiter – 175,000 km (108,000 miles)
    • Saturn – 147,000 km ( 92,000 miles)
    • Uranus – 62,000 km ( 39,000 miles)
    • Neptune – 59,000 km ( 37,000 miles)

This limit represents the rough boundary between each planet’s ring system and its innermost moons.

  • rupes – The term applied to scarps on planetary surfaces; many scarps are thought to be the surface expression of faults within the crust of the planetary object.


  • sapping – A process of erosion where water leaks to the surface through the pores of rocks; as the water flows away, it slowly removes material to form valleys and channel networks.
  • satellite – A body that revolves around a larger body.
  • scarp – A line of cliffs produced by faulting or erosion; a relatively straight, clifflike face or slope of considerable linear extent, breaking the general continuity of the land by separating surfaces lying at different levels.
  • scopulus – A lobate or irregular scarp.
  • semimajor axis – One-half of the longest dimension of an ellipse.
  • shatter cone – Striated conical fracture surfaces produced by meteorite impact into fine-grained, brittle rocks such as limestone.
  • shepherd satellite – A satellite that constrains the extent of a planetary ring through gravitational forces.
  • shield – Any of several extensive regions where ancient Precambrian crystalline rocks are exposed at the Earth’s surface.
  • shield volcano – A volcano in the shape of a flattened dome, broad and low, built by flows of very fluid lava.
  • shock metamorphism – The production of irreversible chemical or physical changes in rocks by a shock wave generated by the impact, or detonation of high-explosive or nuclear devices.
  • siderial – Of, relating to, or expressed in relation to stars or constellations.
  • siderial rotation – Rotation time measured with respect to the fixed stars rather than the Sun or body orbited.
  • siderophile elements – This phrase literally means iron-loving elements. It includes Iridium, Osmium, Platinum, and Palladium, which are found in the metal-rich interiors of chemically segregated asteroids and planets; consequently, these elements are extremely rare on Earth’s surface.
  • silicate – A rock or mineral whose structure is dominated by bonds of silicon and oxygen atoms (ie. olivine).
  • sinus – A bay.
  • Small Bodies – Aside from the planets and their satellites, the solar system features a plethora of smaller yet intriguing objects. There are hundreds of known asteroids and comets, and there are certainly many more that are undiscovered. The majority of asteroids orbit between Mars and Jupiter. A few are farther away. Some asteroids have orbits that bring them closer to the Sun than the Earth (Aten, Icarus, Hephaistos). Most comets have very eccentric orbits, spending most of their time in the solar system’s outer regions, with just short passes near the Sun. The difference between comets and asteroids is debatable. The primary difference seems to be that comets contain more volatiles and elliptical orbits. However, there are several intriguing unclear examples, such as 2060 Chiron (aka 95 P/Chiron), 3200 Phaethon, and Kuiper Belt objects, which seem to have some characteristics of both classifications. Asteroids are also known as small planets or planetoids at times (not to be confused with “lesser planets” which refers to Mercury and Pluto). To differentiate them from bigger asteroids, very tiny pebbles circling the Sun are often referred to as meteoroids. When such a body penetrates the Earth’s atmosphere, it becomes incandescent, and the visible streak in the sky is referred to be a meteor. A meteorite is defined as a chunk of it that survives and reaches the Earth’s surface. Every day, millions of visible meteors impact the Earth (amounting to hundreds of tons of material). Except for a minuscule proportion, all of them burn up in the atmosphere before reaching the earth. The handful that does is our primary source of physical data about the remainder of the solar system. Finally, the space between the planets is far from empty. It is densely packed with tiny particles and gas, as well as radiation and magnetic fields.
  • solar cycle – The approximately 11-year, quasi-periodic variation in the frequency or number of solar active events.
  • solar nebula – The large cloud of gas and dust from which the Sun and planets condensed 4.6 billion years ago.
  • solar wind – A tenuous flow of gas and energetic charged particles, mostly protons and electrons — plasma — which stream from the Sun; typical solar wind velocities are almost 350 kilometers (217 miles) per second.
  • spatter cone – A low, steep-sided cone built up from fluid pyroclasts coating the surface around a vent.
  • spectroradiometer – A device that measures the amount of reflected or radiated energy from a surface in two or more wavelengths.
  • spectrum – The distribution of wavelengths and frequencies.
  • speed of light – Light speed equals 299,792,458 meters/second (186,000 miles/second). Einstein’s Theory of Relativity implies that nothing can go faster than the speed of light.
  • spicules – The grass-like patterns of gas seen in the solar atmosphere.
  • stratosphere – The cold region of a planetary atmosphere above the convecting regions (the troposphere), usually without vertical motions but sometimes exhibiting strong horizontal jet streams.
  • stishovite – A dense, high-pressure phase of quartz that has so far been identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters.
  • subduction – The process of one lithospheric plate descending beneath another.
  • sublime – Sublimation occurs when a substance changes directly from a solid to a gas without becoming liquid.
  • sulcus – Subparallel furrows and ridges.
  • sulfuric acid – A heavy, corrosive, oily, dibasic strong acid H2SO4 that is colorless when pure; it is a vigorous oxidizing and dehydrating agent.
  • sunspot – An area seen as a dark spot on the photosphere of the Sun. Sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere.
  • superior planets – The planets Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are superior planets because their orbits are farther from the Sun than Earth’s orbit.
  • 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. These orbits are commonly used for communications satellites.
  • synchronous rotation – A satellite’s rotational period is equal to its orbital period; this causes the same side of a satellite to always face the planet. The synchronous rotation occurs when a planet’s gravity produces a tidal bulge in its satellite. The gravitational attraction and bulge act like a torque, which slows down the satellite until it reaches a synchronous rotation.
  • synthetic-aperture radar – SAR is a side-looking imaging system that uses the Doppler effect to sharpen the effective resolution in the cross-track direction.


  • target rocks – The surface rocks that an asteroid or comet impactor smashes into in a meteorite impact event.
  • tectonic – The deformation forces acting on a planet’s crust.
  • tektites – Natural, silica-rich, homogeneous glasses produced by complete melting, and dispersed as droplets during terrestrial impact events. Tektites range in color from black or dark brown to gray or green and most are spherical in shape. They have been found in four regional deposits or strewn fields on the Earth’s surface: North America, Czechoslovakia, Ivory Coast and Australasia.
  • terminator – The dividing line between the illuminated and the unilluminated part of the moon’s or a planet’s disk.
  • terra – An extensive land mass.
  • tessera – A tile; polygonal ground.
  • tholus – A small domical mountain or hill.
  • tidal forces – The gravitational pull on planetary objects from nearby planets and moons. When the tidal forces of a planet and several moons are focused on certain moons, particularly if the orbits of the various objects bring them into alignment on a repeated basis, the tidal forces can generate a tremendous amount of energy within the moon. The intense volcanic activity of Io is the result of the interaction of such tidal forces.
  • tidal heating – The frictional heating of a satellite’s interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites.
  • trailing hemisphere – The hemisphere that faces backward, away from the direction of motion of a satellite that keeps the same face toward the planet.
  • Trojan satellites – Satellites that orbit at the Lagrangian points, 60° ahead of and 60° behind another satellite. For example, Telesto and Calypso are trojans of Saturn’s satellite Tethys.
  • troposphere – The lower regions of a planetary atmosphere where convection keeps the gas mixed and maintains a steady increase of temperature with depth. Most clouds are in the troposphere.
  • tuff – The general term for consolidated pyroclastic debris.


  • 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.
  • umbra – The dark central region of a sunspot.
  • undae – Dunes.


  • vallis – A sinuous valley.
  • vastitas – Widespread lowlands.
  • vent – The opening in the crust through which volcanic material erupts.
  • volatile – Compounds with low melting temperatures, such as hydrogen, helium, water, ammonia, carbon dioxide, and methane.
  • volcano – (1) A vent in the planetary surface through which magma and associated gases and ash erupt. (2) The form or structure produced by the erupted materials.


  • weight – The gravitational force exerted on a body.
  • white dwarf – A whitish star of high surface temperature and low intrinsic brightness with a mass approximately equal to that of a Sun but with a density many times larger.


  • x-ray – Electromagnetic radiation of a very short wavelength and very high energy; x-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays.


  • young – When used to describe a planetary surface, “young” means that the visible features are of relatively recent origin, i.e. that older features have been destroyed by erosion or lava flows. Young surfaces exhibit few impact craters and are typically varied and complex; in contrast, an “old” surface is one that has changed relatively little over geologic time. The surfaces of Earth and Io are young; the surfaces of Mercury and Callisto are old.


  • zodiac – Twelve constellations dividing the ecliptic into approximately equal parts. Each month the Sun is in a different constellation of the zodiac.

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