Glossary

Abrasion (Glacial)	A glacial erosion process of scraping produced by the impact of rock debris.
Abyssal plain	Large zone of relatively low-relief sea-floor.
Acid Precipitation	Acidic rain, snow or fog resulting from high level of oxides of sulfur and nitrogen in the air.
Advection	The horizontal movement of material in the atmosphere.
Aerosols	Tiny solid or liquid particles suspended in the atmosphere.
Aggradation	The combination of processes that builds up the surface through the deposition of material that was removed from elsewhere through degradation.
Alluvial fan	A fan-shaped deposit consisting of alluvial material located where a mountain stream emerges onto a plain; primarily a desert land-form.
Alluvium	Sediment laid down by a stream on its valley floor; deposition occurs when the steam velocity decreases and the valley fills with a veneer of unconsolidated material. Since, soil particles washed from slopes in the drainage basin form a large part of these deposits, alluvial soils are usually fertile and productive.
Antarctic Circle	The latitude 66.5 degree south marking the northern boundary of southern hemisphere portion of earth’s surface that receives a 24 hour period of sunlight at least once each year.
Antarctic Ice Sheet	The continental glacier that covers almost all of Antarctica, enabling the study of what conditions were like elsewhere in the world during much of the late Cenozoic Ice Age.
Anticline	An arc-like upfold with the limbs dipping away from its axis.
Archipelago	A group of islands, often elongated tnto a chain.
Arctic Circle	The latitude (66.5 degree north) marking the southern boundary of the Northern hemisphere portion of earth’s surface that receives a 24 hour sunlight at least once a year.
AGN	Active Galactic Nucleus. The massive black hole in the core of an active galaxy.
Active Galaxy	A galaxy with a central black hole which gives radiation.
Antimatter	Matter composed of the antiparticles ot those that form normal matter.
Aphelion	The farthest distance from the Sun in the elliptical orbit of a planet, comet or an asteroid.
Apogee	The point in the orbit of a satellite at which it is farthest from Earth.
Arc-minute	A unit of angular measure equal to 1/6 of a degree. The sun and the moon are 30 arc-minutes across.
Arc-second	A unit of angular measure equal to 1/60 of an arc-minute. Jupiter measures 44 arc-seconds in respect of its width.
Asteroid	Also called minor planet. It is a body which circles around the sun between the orbits of Mars and Jupiter. There are about 10 thousands of Asteroids.
AU	Astronomical Unit. It is the mean difference between the Earth and the Sun. An AU is equal to 150 million Kilometer.
Atmosphere	A layer of gases attached to a planet due to its gravity.
Aurora	Beautiful lights seen in the northern and the southern skies.
Axis	The imaginary line through the center of a planet, Sun or Galaxy around which it rotates.
Ash (volcanic)	Solid cinder like lava fragments, smaller than volcanic bombs, that are exploded into the air during an eruption; most fall to the ground around the erupting volcano.

Bay	A broad indentation into a coastline.
Biome	The broadest justifiable subdivision of the plant and animal world, as assemblage and association of plants and animals that forms a regional ecological unit of Sub-continent dimensions.
Breccia	In elastic sedimentary rocks when pebbles-sized fragments in a conglomerate are not rounded but angular and jagged.
Biosphere	The zone of terrestrial life, habitat of all living things, and the part of soil layer below that hosts living organisms.
Big-Bang	The event marked by a large explosion 13.7 billion years ago, which resulted in the birth of the Universe.
Binary Star	Two Stars linked by mutual gravity and revolving around common center of gravity.
Black Hole	A collapsed celestial body which is so dense that no light or radiation can escape it.
Blue Giant	A very big hot and luminous blue star.

Cadera	A steep walled, circular volcanic basin usually formed by the collapse of a volcano whose magma chamber emptied out; Can aslo result from a particularly powerful eruption that blows off the peak and crater of a volcano.
Cartography	The science, art and technology of map making and map use
Cenozoic	The era of the recent life on the geological time scale extending from 66 million years ago to the present. this period is subdivided into Tertiary and Quaternary period.
Chinook	Name given to Foehn Winds that affect the leeward areas of mountains zones in the western plateaus of North America.
Cinder Cone	Volcanic land-form consisting mainly of pyroclastics; often formed during brief periods of explosives activity, they normally remain quite small.
Circum-Pacific Earthquake Belt	An aspect of the specific ring of fire, the lengthy belt of subduction zones that girdles the pacific basin; here occur the heaviest concentration of earthquake epicenters as well as active volcanoes on the world map.
Cirque	An amphitheater-like basin, high up on a mountain, that is source area of a mountain glacier.
Celestial Equator	An imaginary line encircling the sky midway between the celestial poles.
Comet	A small celestial body compressed of ice and dust that orbits the Sun.
Constellation	The recognized pattern of stars which divide the sky into sections e.g., Virgo, Sagittarius, etc.

Disc Galaxy	A galaxy with a flattened circular disc of stars.
Dwarf Star	A star with less than average size.

Eclipse	The phenomenon when one celestial body passes in front of another.
Ecliptic	The path of the Sun around a celestial sphere. It is equivalent to Earth’s orbital plane.
Equator	Imaginary line which lies half way between the two poles of a celestial body.
Escape Velocity	The minimum velocity which an object requires to leave the surface of planet for entering the space.
Equinox	The moment when the Sun appears to stand directly above a planet’s equator.