A

Active Sun

The state of the sun when it has many center of activity-sunspots, prominences, flares, and other phenomena. 

 

Accretion

The process in which star, planet, or other object gain mass by sweeping up surrounding gas or matter.

Acretion disk

A disk of matter surrounding a star or black hole, formed of matter pulled in toward the object by its gravity. 

Albedo

The reflecting power of a body. The fraction of the incident light that a body reflect. 

Altitude

The angular distance above the horizon of a celestial body measured along its vertical circle. 

Angular diameter

The angle subtended by the diameter of a body. 

Annular Eclipse

A solar eclipse in which the sun's apparent disk is larger than the moon's apparent disk so that a ring (the sun's disk) appears to surround the moon. 

Aperture

The diameter of the objective lens or mirror of a telescope.

 

Aphelion

The point in the elliptical orbit of a planet or comet at which the body is at its greatest distance from the sun.

 

Apogee

The point in the elliptical orbit of the moon or artificial satellite at which the body is at greatest distance from the earth.

 

Apparent magnitude

A measure of the visual brigthness of a celestial body as observed from the earth.

 

Apparent noon

The time when the sun's center is on the observer meridian.

 

Apparent solar day

The period between two successive transits of the sun's center across the observer's meridian.

 

Apparent sun

The true sun.

 

Appulse

A penumbral lunar eclipse.

 

Ascending node

The point in the orbit of a celestial body at which it crosses the references plane-either the ecliptic or the celestial equator-from south to north. 

Association

A loose cluster of stars with physical characteristics that indicate a common origin.

 

Asteroid

A small body whose orbital path around the sun is usually between the orbit of Mars and Jupiter. A synonim for a minor planet or planetoid. 

Astrology

The pseudoscience that claims there is an influence of a celestial bodies on human affairs and that allegedly foretells terestial events by the positions of the stars and planets. 

Astronomy

The science that discribes the celestial bodies according to their location, sizes, motions, constitutions, and evolution. 

Astronomical unit

A unit of length defined by international agreement as the mean distance at he earth from the sun. Its value is about 93,000,000 miles, and its symbol is a.u.

 

Astrophysics

A branch of astronomy in which the method, tools, laws, and theories of physics are applied to the study of celestial bodies. 

Aurora

The beautiful streamers of ligth visible in the arctic region, produced when solar particles interact with atmospherics gases. Aurora borealis are the nothern lights, and Aurora australis are the southern ligths. 

Autumnal equinox

The intersection of the celestial equator and the ecliptic at the point where the sun crosses from north to south. Autumn begins when the sun reaches the autumnal equinox.

 

Azimuth

The angle measured from the north point, eastward along the horizon to the vertical circle that passes through the celestial body.

B

Barred spiral

A spiral galaxy with a bright bar through its nucleus. The arm extend from the two ends of the bar.

 

Barycenter

The center of mass (gravity) around which two bodies orbit.

 

Big bang theory

A cosmological theory which states that the universe evolved from a primeval axplosion.

 

Binary star

A double star system held together by mutual gravitation with its components revolving around their barycenter.

 

BL Lacertae

A peculiar variable galaxy showing no strong spectral features but emitting radiowaves. It was originally mistaken for a star.

 

Black body

A theoritically perfect radiator-abody that absorps and reemits all the radiation that fall on it.

 

Black dwarf

A star whose mass is too small for nuclear reaction to occur. It is therefore nonluminous.

 

Black hole

An object that has suffered gravitational collapse. The gravitational attraction at its "surface" or horizon is so large that not even light is able to escape. Matter falling into a black hole is similarly unable to escape, and loses its identity. Black holes may be remnants of supernovae.

 

Bode-Titius relationship

A sequence of number (empirical relationship) which gives the approximate mean distances of the planets in astronomical units.

 

Bolide

A very bright meteor that often breaks up with a loud sound; a fireball.

 

Bolometric magnitude

Stellar magnitude based on the radiation emitted at all wavelengths from the star. The measurement is based on the radiation of the entire electromagnetic spectrum corrected for the effect of the earth's atmosphere.

 

Brigthness

A measure of luminosity of a body.

C

Cardinal points

The four principal points of the compass: north, south, east, and west.

 

Cassegrain focus

A telescope arrangement in which a convex hyperboloidal secondary mirror reflects the beam back through an opening in the primary mirror, where the image is formed and viewed. Since the image is formed at the back of the telescope, it is easier to mount and use equipment.

 

Cassini division

The 4800 km (3000 mi) gap between the outer and the middle ring of saturn.

 

Celestial equator

The projection of the earth's equator on tha celestial sphere. It is a great circle whose points are 90^o from the north and south celestial poles.

 

Celestial horizon

The great circle on the celestial sphere whose points are 90o from the zenith and nadir.

 

Celestial mechanics

A branch of astronomy that deals with the gravitation and motion of bodies in space.

 

Celestial meridian

The great circle on the celestial sphere that passes through the zenith and nadir and crosses the observer's horizon at 90^o.

 

Celestial poles

The points on the celestial sphere established by the extension of the earth's axis of rotation.

 

Celestial sphere

An apparent sphere of infinite radius at whose center the observer is located and on whose surface the celestial bodies appear to be located.

 

Center of mass (center of gravity)

One point in a single body or within a system of several bodies that remains fixed or moves uniformly through space as through the entire mass of the body or system were concentrated at that point.

 

Centrifugal force

A term used to describe the resistance of a particle to being accelerated along a curved path.

 

Centripetal force

A center-directed force which diverts a body from a straight-line path into a curved path.

 

Cepheid variable

A yellow giant or supergiant pulsating star whose brigthness varies because it expands and contracts periodically.

 

Ceres

The first and largest asteroid (minor planet) discovered.

 

Chromatic aberration

The failure of a simple lens to bring visible light to a single focus because it refracts the wavelengths of visible light differently, producing a dispersion.

 

Chromosphere

The nearly transparent, hot layer of the sun's atmosphere that lies directly above the solar visible surface, or photosphere.

 

Circumpolar star

Stars whose angular distances from the north or south celestial poles are less than the latitude of the observer, so that they always remain above the horizon.

 

Cluster of stars

A group of stars with a common region, bound together by mutual gravitation, and moving in space along nearly parallel paths at about the same speed.

 

Cluster variables

A class of high-velocity pulsating stars (RR Lyrae stars) with period of less than one day. They are very numerous in the Milky Way and in globular clusters.

 

Coelostat

An instrument that permit the sun's image to be viewed in a fixed positon throughout the day. This is accomplished by use of a telescope with an equatorially mounted, clock-driven plane mirrror that follows the sun and a fixed mirror that directs the sunlight to the objective of a fixed tower telescope.

 

Collimator

A lens system that renders a diverging beam of light into parallel beam.

 

Color index

The different between the photographic and photovisual magnitudes of a star. C.I.= m_pg - m_pv.

 

Comparison spectrum

The spectrum of a vaporized element (often iron) photographed next to a stellar spectrum for comparison and identification of their spectral line.

 

Coma (comet)

The diffuse gaseous shell around the nucleus. The nucleus and coma comprise the head of the comet.

 

Coma (optical)

An optical defect in which light rays that strike a lens or mirror at an angle to its axis are not focused at the same place.

 

Configuration

A special orientation of a planet or the moon with respect to the sun.

 

Conjunction

A configuration of two or more celestial bodies that are in line longitudinally and have the same right ascension.

 

Constellation

A configuration of stars with which the ancients associated the name and shape of a person, animal, or object in their mythology. Today, it designates the definite are in the sky in which the configuration is located.

 

Continuous spectrum

The spectrum of light that appears as a continuous blend of colors emitted by an incandescent solid, liquid, or gas under pressure.

 

Coordinates

Angular distances that are used to locate points on the terrestrial sphere (longitude and latitude) and positions of celestial bodies on the celestial sphere (right ascension and declination).

 

Core

The central part of any celestial body.

 

Coriolis effect

The deflection with respect to the earth's surface of a moving body, air, or water, due to the earth's rotation. The deflection is to the right in the northern hemisphere and to the left in the southern hemisphere.

 

Corona

The tenuous outer layer of the sun's atmosphere that is visible during a total solar eclipse or with a coronagraph.

 

Coronagraph

An instrument for photographing the sun's corona whenever the sun is visible.

 

Corpuscolar radiation

Charged particles, such as atomic nuclei and electrons. The solar wind and cosmis rays are sometimes so termed.

 

Cosmic rays

High energy particles (mostly protons) from space that strike the earth's atmosphere and produce secondary particles. They are believed to originate in supernova explosions, although some are produced in solar flares.

 

Cosmogony

The study of the creation and evolution of the universe.

 

Cosmological principle

The assumption that the universe at any time is the same from any position and in any direction.

 

Cosmology

The study of design, character, and extent of the universe.

 

Coude focus

A secondary optical system in a reflecting telescope that directs the reflected light down the telescope's polar axis to a focus that remains fixed regardless of any movement of the telescope. 

D

Dark nebula

A nonluminous cloud of interstellar dust and gas that obscures the light of the stars behind it.

 

Declination

The angular distance north or south of the celestial equator measured along the hour circle that passes through the body.

 

Deferent

The circumference of the large circle in the ptolemaic system on which the center of the epicycle of a planet moves.

 

Descending node

The point in the orbit of celestial body at which it crosses the reference plane-either the ecliptic or the celestial equator-from north to south.

 

Diffuse nebula

A cloud of interstellar dust and gas dense enough to be seen as a dark nebula or close enough to nearby stars to either reflect light (reflecting nebula) or emit light (emission nebula).

 

Distance modulus

The difference between the apparent and absolute magnitudes of a stars. A quantity used to express the star's distance (d) in parsecs, mathematically equal to 5 log d - 5.

 

Diurnal circle

The daily apparent path of a celestial body yhat is parallel to the celestial equator.

 

Diurnal motion

The daily apparent motion of a celestial body caused by the daily rotation of the earth.

 

Doppler effect

The apparent change in the frequency of radiation caused by the relative motion between the radiation source and the observer.

 

Dwarf star

A main sequence stars comparable to the sun in size, mass, and luminosity.

E

Earthshine

The sunlight reflected from the earth that dimly illuminates the dark position of the moon.

 

Eccentricity

The measure of the degree of flattening of an ellipse. It is the ratio of the distance between the foci to length of the major axis. A circle has zero eccentricity.

 

Eclipse

The cutting off the light, partially or totally, from one body when another body passes in front of it.

 

Eclipse limit

The maximum angular distance from either side of the node at which an eclipse can occur.

 

Eclipsing binary

A binary star whose orbital plane is seen nearly edgewise so that alternately one star appears to eclipse the other.

 

Ecliptic

The annual apparent path of the sun among the stars. It is the projection of the earth's orbit on the celestial sphere.

 

Electromagnetic radiation

Radiation consisting of wave phenomena associated with electrical and magnetic fields whose wavelengths include gamma rays, x-rays, ultraviolet, visible light, infrared, and radio.

 

Electromagnetic spectrum

The entire family of electromagnetic radiation.

 

Ellipse

A conic section. A closed curve formed by the intersection of a nonhorizontal plane and a circular cone.

 

Elliptical galaxy

An ellipsoidal galaxy that lacks spiral arms and any appreciable ainterstellar material.

 

Elongation

The apparent angle between the sun and a planet as seen from the earth. The difference between the longitude of the sun and a planet.

 

Emission line

A discrete bright line in a spectrum.

 

Emission nebula

A bright gaseuos nebula whose light is derived from the ultrviolet light of a star within or near the nebula.

 

Emission spectrum

A series of bright lines (emission lines) produce by a low-pressure incandescent gas.

 

Ephemeris

A table that gives the positions of celestial bodies at regular interval of time.

 

Epicycle

The small circle in the Ptolemaic system whose center moves along the circumference of the larger circle (defedent) while the planet moves along the circumference of the small circle.

 

Epoch

An arbitrary date selected as a point of reference to which astronomical observations are referred.

 

Equation of time

The difference between the apparent and mean solar time.

 

Equator

A great circle on the terrestrial sphere  whose points are 90^o from the north and south poles.

 

Equatorial mount

A telescope mount with one axis parallel to the earth's axis which rotates at the same rate as the earth by means of a clock drive.

 

Equinox

One of two intersection of the ecliptic and the celestial equator.

 

Erg

A unit of energy in the metric system. The amount of work accomplished by a force of one dyne moving a body a distance of one centimeter.

 

Escape velocity

The initial speed that a body must attain to overcome the gravitational pull of another body and escape into space.

 

Exosphere

The top layer of the earth's atmosphere where the molecules readily escape from the earth's gravitational pull.

 

Extragalactic

Outside or beyond the Milky Way. 

F

Faculae

Extended bright regions seen near the sun's limb.

 

Filar micrometer

An instrument attached to the eyepiece of a telescope to measure the angle (separation) between two stars and their relative position.

 

Fireball

An unusually bright meteor.

 

Flare

A very bright flash of light over a small area of the sun's surface, especially near an active sunspot, which is due to an intense outburst of energy.

 

Flash spectrum

The spectrum of the sun's limb that is the bright line spectrum of the chromosphere visible for an instant just before totality in a solar eclipse.

 

Flocculi (plages)

Bright regions in the chromosphere in the magnetic fields around sunspots visible in spectroheliograms in monochromatis light.

 

Flourescence

The absorption of radiation of one wavelength (especially ultrviolet) and its re-emission in an another wavelength (visible light).

 

Focal length

The distance from the center of a lens or mirror to the focal point.

 

Focal ratio

The ratio of the focal length of a lens or mirror to its diameter.

 

Forbidden lines

Spectral lines not usually obtainable under laboratory conditions because their emissions result from atomic transitions that are most improbable.

 

Fraunhofer line

An absorption (dark) line in the spectrum of the sun or a star.

 

Fullmoon

The phase of the moon when it is in opposition (opposite side of the earth from the sun) and its entire visible hemisphere is illuminated.

G

Galactic cluster

An open cluster of stars found in the spiral arms of the Milky Way.

 

Galactic equator

The plane of the great circle on the celestial sphere that locates the center line of the Milky Way.

 

Galactic poles

The north and south galactic poles that are 90^o from the galactic equator.

  

Galaxy

A large assemblage of stars and interstellar material held together by gravitation. A galaxy usually contains from millions to hundred of billions of stars.

 

Gamma ray

The most energetic from of electromagnetic radiation yet discovered, and the one possesing the shortest known wavelength.

 

Giant star

A star of large radius and luminosity.

 

Gibbous

A phase of the moon or a planet during which more than half but less than the whole disk appears illuminated.

 

Globular cluster

A large spherical cluster of stars located  in the halo which surrrounds the Milky Way and other galaxies.

 

Globule

a small, dark, relatively dense nebula that may be a protostar.

 

Granules

Small, bright spots in the sun's photosphere that give it a mottled appearance similar to rice grains. They are produced by hot gases rising from below the photosphere.

 

Gravitation

The property of matter  by which one  mass exerts a force of attraction on another.

 

Great circle

The curve formed on the surface of a sphere  where a plane that passes through the sphere intersects its center. It divides the sphere into two equal parts.

 

Greenwich meridian

The meridian that passes through a point at the Royal Greenwhich Observatory in England, the reference or prime meridian for determining longitude.

 

Gregorian calendar

The calendar in common use today, which was introduced by Pope Gregory XIII in 1582.