Online Astronomy eText: Appendices
Glossary (A - M) / Glossary (N - Z)

aberration — One of a number of optical distortions produced by an optical instrument, caused not by a defect in the instrument, but by the nature of its design, and the laws of optical physics (e.g., see chromatic aberration).
 Also, a change in the apparent direction of a star, caused by that portion of the Earth's orbital velocity toward or away from the star (see stellar aberration).

absolute magnitude — A number specifying the brightness that a star would have if viewed from a distance of 10 parsecs, or 32.6 light-years. Because this is a fixed distance, it is a measure of the star's intrinsic, or actual, brightness (also see apparent magnitude).

absorption lines — Dark lines in the spectrum of an object, caused by the absorption of light by gases lying between us and that object. In the case of a star, the gases may be in the atmosphere, or at the surface of the star, in which case they reveal the physical conditions in those regions (but also see interstellar absorption lines).

accretion — A process of growth in which small things become large things through collisions. In the planetesimal accretion theory, the earliest stages of this process involve random collisions, while the later stages involve random collisions aided by gravitational attraction.

achondrite (literally, not a chondrite) — A stony meteorite which is not a chondrite, and does not contain chondrules. Achondrites are pieces broken off of parent bodies which were large enough to melt and differentiate, then slowly resolidify. This produces crystalline mineral structures which are physically and chemically distinct from meteorites which never underwent such melting and crystallization.

achromat — A lens made of two pieces of glass (or, occasionally, other materials) with differing optical properties, specifically chosen to minimize chromatic aberration.

adiabatic lapse rate — The rate at which temperature decreases as a rising blob of gas expands, to match the decreasing pressure around it. If the atmospheric lapse rate (the decrease in temperature with increasing altitude in the surrounding atmosphere) is greater than the adiabatic lapse rate, the rising blob will accelerate, and convective motion is supported. If the atmospheric lapse rate is less than the adiabatic lapse rate, the rising blob will decelerate, and convective motion is prevented. If the atmospheric lapse rate is the same as, or very close to, the adiabatic lapse rate, the atmosphere is said to be neutrally stratified.

aerolite (also, aerolith) — A stony meteorite.

airglow — A faint glow of the night sky, present even in "perfectly dark" areas, caused by the recombination of particles in the upper atmosphere.

albedo — The reflectivity of a non-luminous body, expressed as a decimal fraction. If an object reflects half the light falling on it, its albedo is 0.5. If it reflects only 10% of the light falling on it, its albedo is 0.1.

Algol variable (also, Algolid) — A binary star which, like the prototype, Algol, varies in brightness because its stars periodically eclipse each other.

altazimuth (also, alt-azimuth) — A type of telescope mount, in which the telescope tube is rotated parallel to the horizon, or perpendicular to it, instead of around the axis of the Earth's rotation.

Altitude-Azimuth coordinate system — The Horizon system of coordinates, based on the direction of gravity, and the division of the sky into visible and non-visible portions by the skyline. So named because its coordinates are called altitude and azimuth.

altitude — The angular height of a celestial object above the astronomical horizon.

analemma — A figure-eight shaped curve, often shown on globes as a graphical representation of the Sun's north-south position on different dates (shown by dates located at various positions along the curve), and the amount by which sundial time differs from mean solar time on those dates (shown by the width of the curve at those dates).

Εngstrφm (Ε) — A unit of distance, equal to a millionth of a millimeter, used to measure very small distances, such as the sizes of atoms, and the wavelengths of light. After Swedish physicist Anders Jonas Εngstrφm (1814 - 1874).

angular diameter — The apparent size of an astronomical object, against the background of the sky, expressed in angular units (degrees, minutes or seconds of arc). For example, the Sun appears to be half a degree across, so it has an angular diameter of half a degree.

anomalistic month — The time required for the Moon to move around its orbit, from perigee to perigee.

anomalistic year — The time required for a planet (usually, the Earth) to move around its orbit, from perihelion to perihelion. Usually very slightly different from the orbital period, because the perihelion point may precess or regress as a result of perturbations.

anomaly — In an orbit, the angle between the position of an object and the position at which it is closest to the object that it is orbiting.

apastron — In an orbit around a star, the furthest point from the star.

aphelion — In an orbit around the Sun, the furthest point from the Sun.

apochromat — A lens made of three pieces of glass (or other materials) with differing dispersions, chosen so as to reduce chromatic aberration to an even greater degree than in an achromat.

apogee — In an orbit around the Earth, the furthest point from the Earth.

aposelene — In an orbit around the Moon, the furthest point from the Moon.

apparent magnitude — A number specifying how bright a star looks without correcting for its distance, or other factors. Because different stars are at different distances, apparent magnitude is not a measure of a star's true brightness. Also see absolute magnitude.

apse — One of the two ends of the major axis of an ellipse. Thus, in the case of an orbit around the Sun, the perihelion or aphelion point.

apsides — The plural form of apse. In the case of an orbit, the line between the perihelion and aphelion points, which are the apsides of the ellipse, is often called the line of apsides.

argument of perihelion — An angle used, in combination with other orbital elements, to specify the orientation of an orbit in space. This particular element specifies the angle, measured in the direction of motion of the body, between the ascending node of its orbit, and the perihelion position.

ascending node — In an orbit, the place where a body crosses the plane of another orbit (usually, the Earth's orbit), heading north.

ashen light — A faint occasional visibility of the night side of Venus, of unknown origin.

asteroid (literally, an object which looks like a star, but is not) — One of tens of thousands of small bodies orbiting the Sun, mostly between the orbits of Mars and Jupiter, in the so-called asteroid belt. Also called minor planets, or planetoids.

astrology — Historically, a part of astronomy involving efforts to determine the character and future of people and nations from the positions of heavenly bodies, based on ancient myths of heavenly interactions with the Earth and its inhabitants. Already discredited in ancient times, and extinct as a science by the early Renaissance. Nowadays, a way for charlatans and frauds to remove money from the pockets of the credulous. Therefore, of interest to historians and social scientists, but not to astronomers. Referring to astronomy as astrology is a good way to convince your instructor that you have less between your ears than you should, and place doubts in his/her mind as to whether you deserve a passing grade. Referring to astrology as having any merit will confirm this poor opinion. The fact that some celebrities subscribe to this rubbish doesn't prove that astrology has any merit — it just proves that you don't need anything between your ears to become a celebrity.

astronomy — The study of the Universe, and its contents.

Astronomical Unit (AU) — Originally, and to about 7 digit accuracy, the semi-major axis of the Earth's orbit, or the Earth's average distance from the Sun. More exactly, the radius of a circular orbit with an orbital period of 2 p / k days, where k = the Gaussian gravitational constant.

astrophysics — The study of the physics and chemistry of the stars.

atom — The smallest unit of matter which has specific chemical properties. Breaking an atom into smaller pieces yields subatomic particles which are in no way like any chemical material. Atoms consist of a nucleus, containing protons and, save for ordinary hydrogen atoms, neutrons, and an electron cloud, surrounding the nucleus, containing one or more electrons. In ordinary atoms, the number of electrons in the electron cloud is identical to the number of protons in the nucleus. The interaction of electron clouds in one atom with electron clouds in other atoms determine the chemical properties of the atoms, so atoms with the same number of protons, and therefore, usually the same number of electrons, are considered to be chemically identical.

Atomic Time — Time determined by passing radio waves through a gas consisting of individual atoms of one material or another, and using the absorption of the radiation to regulate the device which produces the radio waves. This method of 'tuning' the device allows it to be used to drive a clock with exceptional accuracy. An average of a large number of such clocks, kept in various countries, is the basis of Coordinated Universal Time.

Aurora (plural = auroras or aurorae) — Luminous bands, curtains and streamers which appear in the upper atmosphere, caused by bombardment of the atmosphere by high-energy particles from the Sun. Intermittently visible at high and mid-latitudes, but also occurring as a permanent, but fainter display in an oval region centered on the magnetic poles of the Earth.

Aurora australis — The auroral displays visible in the southern hemisphere.

Aurora borealis — The auroral displays visible in the northern hemisphere.

auroral — Pertaining to aurorae.

Autumnal Equinox — The point on the Ecliptic where the Sun crosses the Celestial Equator heading south. Also, the day when it does so, which is the first day of autumn in the northern hemisphere, and the first day of spring in the southern hemisphere.

azimuth — The angle measured along the astronomical horizon from due North to the foot of the vertical circle through a star or other celestial object. Usually, but not always, measured around through east, in which case due or true North is 0 degrees azimuth, due or true East is 90 degrees azimuth, due or true South is 180 degrees azimuth, and due or true West is 270 degrees azimuth.


barycenter — The center of mass of a system of orbiting bodies. If the bodies are not specified, the term usually refers to the center of mass of the Earth-Moon system.

baryon — A heavy sub-atomic particle, such as a proton or neutron.

binary star — A double star system in which the two stars are physically bound by gravity, and actually orbit each other.

black body — An object which absorbs all the radiation which falls on it, and is, therefore perfectly black.

black body radiation (also, blackbody radiation and black-body radiation) — Idealized radiation produced by an object which is a perfect absorber and emitter of radiation. For most stars, approximately equivalent to the radiation of the star, hence its importance in astronomy.

black dwarf — An electron-degenerate star which has cooled until it gives too little radiation to be detected at interstellar distances.

black hole — A region surrounding a singularity where the curvature of space-time (that is, gravity) is so great that nothing, including light, can escape.

blink comparator — An instrument for examining two photographs in rapid succession, to enable the detection of changes in position or brightness of celestial objects. Also, since usually used in conjunction with a microscope, blink microscope. Historically, many Solar System objects (such as Pluto) and variable stars were discovered with the use of blink comparators, but modern computer software has made such mechanical devices obsolete.

blue shift (also, blueshift) — A reduction of light wavelengths, caused by a motion toward the observer. So called because blue light has shorter wavelengths than red light.

bolide — An exceptionally brilliant meteor (synonym = fireball).

bolometer — A device which measures the energy of all of the radiation which falls on it, regardless of the wavelength or visibility of the radiation.

bolometric luminosity — The brightness of a star, taking into account all of its radiation, visible and invisible.

bolometric magnitude — The apparent or absolute magnitude of a star, taking into account all of its radiation, visible and invisible.

brown dwarf — An electron-degenerate star which is so cool that it gives off very little visible light, but is still detectable through its infrared radiation. A star which, as it contracted, became electron-degenerate, and unable to contract or heat up any further, before becoming hot enough to undergo thermonuclear fusion.

butterfly diagram — A diagram showing how the latitudes of sunspots tend to decrease during a solar cycle. So called because the pattern resembles the wings of a butterfly.


Cassini, Giovanni Domenico — 1625-1712. Italian (later, naturalized French) astronomer, discoverer of several moons of Saturn, and the Cassini Division in its ring system. Also, namesake of the Cassini spacecraft. External Link

celestial — Of, or pertaining to, the heavens.

Celestial Equator — A great circle, halfway between the Celestial Poles, dividing the sky into northern and southern halves. Used as the starting point for measurement of declination. Analogous to the Equator in the Terrestrial coordinate system.

celestial globe — A globe showing the relative positions of stars.

Celestial Poles — Two points, the North Celestial Pole, and the South Celestial Pole, which are at the ends of the imaginary axis about which the sky appears to turn.

celestial sphere — An imaginary sphere, centered on the observer, which is used to represent the positions and motions of the heavenly bodies.

centrifugal force — The fictitious force which occurs when moving along a curved path. To an observer moving in such a way, there appears to be a sideways force, which tries to throw them off that path, onto a path which is tangent to their motion. In a car moving along a curved mountain highway, the apparent desire of the car and its contents to go off the side of the mountain, instead of following the road.

centripetal force — The real force, opposite the fictitious centrifugal force, which keeps someone moving on a curved path from moving along a tangent to that path. In a car moving along a curved mountain highway, the frictional force between the tires and the pavement which keeps the car from going off the side of the mountain.

Cepheid variable — A variable star which, like the prototype, Delta Cephei, varies in brightness in a particular way. Cepheid variables which are intrinsically brighter take longer to vary than ones which are intrinsically fainter, making it easy to estimate their true brightness, and from that, their distance.

Čerenkov radiation — (also, Cerenkov or Cherenkov) — Electromagnetic radiation emitted by particles moving faster than light, through a medium in which the speed of light is less than its speed in a vacuum. The blue glow seen around nuclear reactors shielded by a pool of water is Čerenkov radiation. (Named after Russian scientist Pavel Čerenkov.)

chondrite — A stony meteorite which has not been modified by melting and differentiation, and therefore has physical and chemical characteristics which reflect conditions in the Solar Nebula, at the time the Solar System formed. Such meteorites usually have small spherical inclusions, called chondrules, hence their name. Most meteorites are chondrites.

chondrule — A small (typically milllimeter-sized) glassy inclusion found in stony meteorites called chondrites. Such inclusions represent rocky material (usually, the ferromagnesian silicates olivine and pyroxene) which was in a molten state at the time the Solar System formed.

chromatic aberration — A prismatic effect in an image produced by an optical instrument, caused by the tendency of lenses to disperse light, or to refract different colors of light by unequal amounts, thereby turning a point of light into a small spectrum, and causing the image of an extended object to become less sharp, and fringed by the end colors of the spectrum.

chromosphere (literally, the sphere of color) — The region of the Sun's atmosphere lying above the photosphere, and below the corona. So-called because of the pinkish color lent to it by the radiation of hydrogen atoms.

clock drive (also, drive clock) — A device which turns a telescope around an axis parallel to the Earth's axis of rotation, to follow the motion of the stars.

coelostat (literally, stationary sky) — A telescope which uses one or more flat mirrors to reflect the light of a celestial object into an otherwise stationary instrument. This is especially useful when the stationary portion of the telescope is immense, and would be difficult to move, as in the case of the 100-foot long solar telescope at Kitt Peak. When used for observations of the Sun, usually referred to as a heliostat; when used for observations of stars, as a siderostat.

cold trap — The region in the middle atmosphere of most planets where temperatures are lowest. For those planets, essentially the same as the mesosphere.

color index — A number used to specify the color of a star, and due to the relationship between stellar color and temperature, to estimate the temperature of the star. Most often, the value B - V, where B and V are the 'blue' and 'visual' magnitudes of the star.

colure — One of two great circles on the celestial sphere, passing through the Equinoxes or Solstices. The Equinoctial Colure consists of the hour circles through the Vernal and Autumnal Equinoxes, while the Solstitial Colure consists of the hour circles through the Summer and Winter Solstices.

commensurate — Having a common measure, usually as the ratio of two small whole numbers. Commensurate orbital periods have ratios such as 2 to 1, 3 to 2, 4 to 3, 5 to 2, etc.

coma — The fuzzy-looking 'head' of a comet, consisting of gas and solid particles which have escaped from the nucleus of the comet. Also, a kind of telescopic aberration which makes stellar images look like little comet-shaped streaks.

comes — In a double star, the fainter 'companion' of the brighter star.

commensurability — A relationship between orbital periods which involves

conic section — A plane curve obtained by cutting a cone with a plane. Depending upon the angle of the cone, the result is a circle, an ellipse, a parabola or a hyperbola.

conjunction — An apparent close approach of two celestial bodies to each other. If only one body is specified by name, the Sun is presumed to be the other. Hence, if "Mercury and Venus are in conjunction", the two planets appear close to each other in the sky, while if "Mercury is in conjunction", it appears to be close to the Sun. Despite their proximity in the sky, the two bodies may actually be tens or hundreds of millions of miles apart.

constellation — An area of the sky associated with a particular (usually mythological) name or figure, usually originating in ancient times. The sky is divided into eighty-eight officially recognized constellations. An object which is in a direction enclosed by the boundaries of a constellation is said to be "in" that constellation, whether it is close (e.g., the planets in our Solar System) or far away (e.g., some distant galaxy). In general, there is no relationship between objects in a given constellation, save for their being in more or less the same part of the sky.

Coordinated Universal Time (UTC) — Universal Time.

Coriolis, Gaspard Gustave de — 1792-1843. French engineer and mathematician who showed that the ordinary laws of motion, which normally apply only to inertial frames of reference, could be used in rotating, or non-inertial frames of reference, providing that an imaginary force, now called the Coriolis force, was added to those forces which actually exist. External Link

Coriolis effect — An effect which changes the apparent horizontal motions of objects moving on a rotating body. In the case of the Earth, objects in the Northern hemisphere seem to veer off to the right of their 'proper' paths, and objects in the Southern hemisphere seem to veer off to the left of their 'proper' paths, as a result of this effect. The effect is a maximum at the poles, and does not exist at the Equator.

Coriolis force — An imaginary ('fictitious') force, which can be used to explain the Coriolis effect, if one wishes to ignore the rotation which actually causes that effect. The horizontal component of the Coriolis force produces the Coriolis effect, while the vertical component produces the flattening or oblateness predicted for rotating objects, by Newton.

corona — The outermost part of the solar atmosphere; also, by extension, the outermost part of a stellar atmosphere. The solar corona has a very high temperature (several million degrees), but an extremely low density, and its glow, although easily visible during a solar eclipse, is too faint to see when the disc of the Sun is even partially visible.

coronagraph — An instrument for observing the corona, by blocking the light from the brilliant disc of the Sun.

coronal mass ejection (CME) — A violent eruption in the solar corona which ejects hundreds of millions or even billions of tons of ionized gas into interplanetary space.

cosmic rays — Extremely high-energy particles, typically atomic nuclei moving at nearly the speed of light. High-energy cosmic rays originate outside our Solar System, but some low-energy cosmic rays are produced by violent events on the Sun.

cosmology — The study of the structure and evolution of the Universe.

cosmogony — The study of the origin and early evolution of an astronomical system; usually, of the Universe or Solar System, but occasionally of specific systems, such as the Earth-Moon system.

counterglow — An enhancement of the Zodiacal Light in the direction opposite the Sun, caused by a reflection of sunlight from meteoric material in interplanetary space. Particles which are opposite the Sun are 'fully' lit by its light, and reflect far more light than at other positions, in the same way that the full Moon is much brighter than other lunar phases. Also referred to as the Gegenschein.

crescent — A phase of the Moon or a planet (as seen through a telescope) in which less than half the lit side of the object is visible from the Earth. In such circumstances, the object must lie between the Earth and Sun, less than 90 degrees away from the Sun in the sky.

crescent Moon — A phase of the Moon in which its longitude differs from that of the Sun by less than 90 degrees, and it is less than half lit. If it is East of the Sun, and increasing the amount by which it is East of the Sun, the portion which is lit increases from day to day, and we say that the Moon is a waxing crescent. If it is West of the Sun, and decreasing the amount by which it is West of the Sun, the portion which is lit decreases from day to day, and we say that the Moon is a waning crescent.


D'Alembert force — A fictitious force which appears to arises when moving along a straight line, in an accelerated manner. In a car which is rapidly accelerating along a straight-line path, the real force is the one which pushes the car forward. Occupants of the car, however, feel as though they are experiencing a backward force, opposite the motion of the car. This backward force is the fictious D'Alembert force.

Dawes' limit — A practical limit to how close two stars can be, and still be seen as separate objects through a telescope. Expressed in arc-seconds, the limit is approximately 11.5 divided by the telescope's aperture in centimeters, or 4.5 divided by the telescope's aperture in inches. After English astronomer Walter Rutter Dawes (1799 - 1868).

declination — In the Equatorial coordinate system, the north-south angle, measured along an hour circle, between a star and the Celestial Equator.

declination circle — In the Equatorial coordinate system, a circle parallel to the Celestial Equator, at some fixed distance north or south of the Equator. Analogous to a parallel of latitude, in the Terrestrial coordinate system. Also, in a
telescope, a circle used to indicate the declination that the telescope is pointing at.

degeneracy — A situation in which something is so strange, compared to the usual situation, that its characteristics are completely different from the norm (see degenerate ellipse, degenerate energy state, and degenerate gas). For a degenerate gas, a number which compares the degeneracy pressure of the gas to the total gas pressure. If the degeneracy pressure is zero, the degeneracy is zero, and the gas is said to be non-degenerate. If the degeneracy pressure is equal to the total gas pressure (in other words, so high that the normal gas pressure is inconsequential), the degeneracy is 1.00, and the gas is said to be totally degenerate.

degenerate ellipse — The straight line that results when an ellipse has an eccentricity of 1.00. Since an ellipse is a curve, and a straight line is not, calling such a line an ellipse seems to defy rational thought. But if we think of the straight line as the ultimate result of making an ellipse thinner and thinner, until it is as thin as possible, then the 'curve' which results when it becomes a straight line is referred to as a degenerate ellipse.

degenerate energy state — A set of energy states which have exactly the same energy, and are therefore indistinguishable from each other.

degenerate gas — A gas which is so dense that its particles occupy all physically possible positions and energies, and exert forces on each other which cause the gas to behave like a liquid, or a solid, with a temperature of absolute zero. Usually used as a suffix with the name of the particles which are degenerate, as in an electron-degenerate gas, or a neutron-degenerate gas.

descending node — In an orbit, the place where a body crosses the plane of another orbit (usually, the Earth's orbit), heading south.

dichotomy — The phase of a planet or the Moon, when exactly half lit; more often called "quarter" phase.

direct motion — Motion which is in the usual direction. For planets, orbital motion in the same direction as the other planets, or eastward motion among the stars.

direct rotation — Rotation in the same direction as the orbital motion of an object.

direct revolution — Orbital motion in the usual direction of orbital motion. For planets, eastward motion around the Sun. For moons, orbital motion in the same direction as their planet's rotation.

diurnal — Daily.

diurnal motion — The apparent daily rotation of the stars (or other celestial objects) around the globe of the sky.

diurnal path — The path followed by a star across the sky, during one rotation of the Earth. Identical to its declination circle.

Doppler effect (also, Doppler shift) — A change in the wavelength of spectral lines caused by a motion of an object towards or away from the observer, or of the observer towards or away from the object. Motion which reduces the distance between the object and the observer reduces the wavelengths of the spectral lines, whereas motion which increases the distance between the object and the observer increases the wavelengths. For velocities which are small compared to the speed of light, a relative velocity of x percent of the speed of light produces an x percent change in the wavelengths. After Austrian physicist Christian Andreas Doppler (1803 - 1853).

double star — Two stars which are close to each other in the sky. Includes optical doubles and binary stars.

doublet — A lens or eyepiece made of two pieces of glass (or other materials) selected so as to reduce the chromatic aberration of the lens.

draconic month — The time required for the Moon to move around its orbit, from ascending node to ascending node.

drive clock (also, clock drive) — A device which turns a telescope around an axis parallel to the Earth's axis of rotation, to follow the motion of the stars.

dynamo — A device, such as a turbine, used to turn motion into electromagnetic energy. Also see liquid dynamo.


Earthshine — Light reflected onto the Moon by the Earth, causing the dark side of the Moon to be visible from the Earth. Most easily seen at crescent phases, during which the faintly lit dark side, wrapped within the bright crescent Moon, is referred to as "the old Moon in the new Moon's arms".

eccentricity — Mathematically, the ratio of the distance between the foci of an ellipse, compared to the major axis of the ellipse. For planetary orbits, the variation in distance of a planet from the Sun, expressed as a decimal fraction of the average distance from the Sun. Equal to the distance from the center of the orbit to the Sun, expressed as a fraction of the semi-major axis of the orbit. A planet whose eccentricity is .20 would be 20% closer to the Sun at perihelion, than on the average, and 20% further away at aphelion, than on the average.

eclipse — The blockage of sunlight caused by an object passing between the Sun and another object. For solar eclipses, this occurs when the new Moon passes between the Earth and Sun, casting all or a portion of the Moon's shadow on the Earth. For lunar eclipses, this occurs when the full Moon passes through the shadow of the Earth. Similar eclipses can occur for other moons and planets, and even for certain binary stars, or eclipsing binaries.

eclipse year — The time required for the Earth to move around its orbit, relative to the ascending node of the Moon's orbit. Because the Moon's orbit precesses once every 18 years, approximately 1/18 of a year shorter than the sidereal year.

eclipsing binary (also, eclipsing variable) — A binary star system in which, due to the orbits of the two stars being nearly in the line of sight from the Earth, each star passes in front of the other once each orbital period, blocking part or all of the light of the more distant star. Eclipsing binaries are important in astronomy, as their eclipses allow very accurate measurements of their sizes and masses.

Ecliptic — The apparent path followed by the Sun among the stars during the year, as a result of the Earth's motion around the Sun. Also, the apparent path followed by the Earth, as it would be seen by someone on the Sun. Also, the projection of the plane of the Earth's orbit onto the Celestial Sphere. So-called because if a new or full Moon occurs when the Moon is close to the Ecliptic, a solar or lunar eclipse can take place.

Ecliptic coordinate system — A spherical coordinate system in the sky, based on the orbital motion of the Earth, and the apparent motion of the Sun resulting from that orbital motion.

Ecliptic plane — The plane of the Ecliptic, in space. The same as the orbital plane of the Earth.

Ecliptic pole — One of the poles (North and South) of the Ecliptic.

electromagnetic spectrum — The various kinds of electromagnetic radiation, from high-energy gamma and X-rays through moderate-energy ultraviolet radiation, visible light and infrared radiation, to low-energy microwave and radio radiation.

electron — One of the low-mass, negatively charged particles which make up the outer portion of an atom. The optical and chemical properties of an atom are determined by the number, arrangement and energies of its electrons.

electron degenerate gas — A gas in which the degeneracy pressure is provided by electrons.

electron degenerate star — A star consisting of electron degenerate gas. If hot enough to be visible, a white dwarf. Otherwise, a brown dwarf, or a black dwarf.

element — A substance that cannot be split into simpler chemical substances. All the atoms of an element have the same number of positively charged protons in their nuclei, and when in an electrically neutral state, the same number of negatively charged electrons arranged around the nucleus.

ellipse — One of a family of curves produced by rotating a circle around a diameter, and observing the resulting foreshortening. Also, one of the four conic sections.

ellipticity — The elongation, or flattening, of an ellipse, expressed as the decimal fraction by which the minor, or shortest, axis is less than the major, or longest, axis. If the minor axis were 70% less than the major axis, the elllipticity would be 0.7. Often used to express the shape of actual physical structures which are approximately oblate spheroids, such as planets, and elliptical galaxies. Elliptical galaxies are actually classified according to their apparent ellipticity, rounded off to the nearest 10%. Thus, an ellliptical galaxy which appears to have an ellipticity of about 60% would be classified as an E6.

elongation — The angle between two bodies. If the second body is not specified, the angle between a body and the Sun. Thus, the elongation of Mercury is the angle between Mercury and the Sun. Elongations are usually measured from 0 to 180 degrees east or west of the Sun, but for calculations, may be measured eastward only, from 0 to 360 degrees.

emission lines — In a spectrum, wavelengths or colors with an excess of light, produced by emission, or radiation, of energy at those wavelengths by gas particles.

ephemeral — Changing. Inconstant. Not fixed.

ephemeris (plural = ephemerides) — A tabular listing of the positions of heavenly bodies which are moving relative to the stars, and which therefore have inconstant, or ephemeral positions.

Ephemeris Time (ET) — Time used for calculating the positions of the planets. Used for tables of planetary positions, such as in the "Astronomical Ephemeris". Different from ordinary time, because of irregularities in the rotation of the Earth.

epoch — The date used as a reference in quoting the position of stars, planets, etc, in tables. For instance, at the moment, most tables of right ascension and declination are listed for epoch 2000.0.

Equation of Time — The amount by which sundial time differs from mean solar time, as a result of the Earth's motion around the Sun being nonuniform during the year, and at an angle to its equatorial rotation. Often graphically represented on globes by an analemma.

equatorial (also, equatorial mount) — A telescope mount which features an axis parallel to the axis of rotation of the Earth, so that a simple rotation of the telescope around that axis allows it to follow the stars.

Equatorial coordinate system — A spherical coordinate system in the sky, based on the rotation of the Earth, and the apparent rotation of the sky resulting from that rotation.

Equinoctial Colure — a great circle extending from the North Celestial Pole, through the Vernal Equinox, to the South Celestial Pole, then through the Autumnal Equinox, and back to the North Celestial Pole. Consists of the hour circles of 0 and 12 hours right ascension.

equinox (literally, equal night) — One of the two days when the Sun is on the Celestial Equator, and its center is above the horizon half the time, and below the horizon half the time, making day and night of nearly equal length. Also, one of the two positions, the Vernal Equinox, and the Autumnal Equinox, where the Sun is on the Celestial Equator.

escape velocity — The velocity needed by an object to escape the gravitational field of a body, from a given position. Usually, the velocity needed to do so from the surface of the body.

event horizon — The imaginary surface of a black hole. The place at which the curvature of space-time (the gravity) of the singularity which lies at the center of the black hole is so large that nothing, including light, can escape. If an object passes through the event horizon, and into the interior of the black hole, it will never again be visible, and no event involving it can ever be detected — hence the name of the horizon.

exosphere (literally, outer sphere) — The outermost portion of the Earth's atmosphere, in which the gas is so thin that particles moving upwards at more than the escape velocity have more than a 50% chance of not running into other particles, and of escaping into space. Also, the similar region in other planets' atmospheres. On Mercury, the solar wind drives the exosphere all the way to the surface, during the day.

extinction — A dimming of the light of an object, by virtue of material lying between it and the observer, which scatters or absorbs part of that light. Usually refers to such a reduction caused by our own atmosphere, or by clouds of gas and dust in interstellar space.

eyepiece — A small lens or combination of lenses used to magnify the (primary) image formed by a telescope, before its light reaches the eye. The magnification produced by a telescope and eyepiece combination is equal to the focal length of the telescope, divided by the focal length of the eyepiece. A short focal length eyepiece (e.g., 4 to 6 millimeters) will produce a larger magnification, while a long focal length eyepiece (e.g., 35 to 50 millimeters) will produce a smaller magnification.


facula (plural, faculae) — A bright region near a sunspot, as observed (usually, only near the limb of the Sun) in photospheric images. If observed in a chromospheric image, it would be referred to as a plage.

fictitious force — A force which does not exist, from the viewpoint of a person in an inertial reference frame, but which appears to exist, to someone in a non-inertial reference frame, because of the accelerated motion of the non-inertial frame. Examples are the D'Alembert force (in a reference frame which is accelerating along a straight line), the centrifugal force (in a reference frame moving along a curved path) and the Coriolis force (in a rotating reference frame, such as the surface of the Earth).

finder — A small telescope with a wide field of view (similar to a low-power monocular), used to help point a larger telescope at the desired part of the sky. When the larger telescope is small, its finder may simply consist of a lighted sight, similar to that on a rifle.

fireball — An exceptionally brilliant meteor (synonym = bolide).

First Point of Aries — The point on the Ecliptic where the Sun crosses the Celestial Equator heading northward. Identical to the definition of the Vernal Equinox as a point in the sky. So-called because in ancient times, the point was in the constellation of Aries. However, precession has moved the Vernal Equinox to the west at a rate of about one degree per seventy years, so it is now in the constellation of Pisces, and will soon be in Aquarius.

first magnitude — A description of the brightness of a star, as seen in the sky. Traditionally, first magnitude stars were the couple of dozen stars which were noticeably brighter than all the other stars. In recent centuries, first magnitude stars have been subdivided into individual magnitudes, because of the tenfold difference in brightness between the brightest and faintest first magnitude stars; thus, Arcturus and Vega are considered magnitude zero stars, while Altair and Deneb, which are noticeably fainter, are magnitude one stars, and Sirius and Canopus, which are noticeably brighter, are magnitude minus one stars. When we say that a star is a second magnitude star, or a star of magnitude two, the brightness range is the same (only about 2 1/2 times variation in brightness from the faintest magnitude two stars, to the brightest). But for first magnitude stars, saying first magnitude means they are merely among the brightest stars, regardless of their specific brightness; while saying magnitude one, magnitude zero, or magnitude minus one — in other words, putting the number after "magnitude", instead of before it — refers to a more specific brightness range.

flare — A sudden, immense outburst of energy and mass from the Sun. Within a time measured in seconds or minutes, typical flares release energies of several tens of thousands of megatons of TNT. Large flares can release energies hundreds of thousands of times greater than that.

flattening — The ellipticity of a rapidly rotating planet, expressed in terms of the reduction in distance between the poles, compared to the distance through the equator. Thus, if a planet's diameter is 3% less through the poles, than through the equator, it could be said to have a flattening of 3%. Numerically, identical to the oblateness, and only differing from the flattening in terms of the viewpoint. In the case of flattening, the planet is thought of as being squashed through the poles. In the case of oblateness, it is thought of as bulging out at the equator.

focal length — The distance between a lens system and the image it forms of an infinitely distant point. For simple telescopes, this distance is close to the length of the tube enclosing the lens system. For complex telescopes, additional mirrors or lenses may dramatically alter the distance, leading to an effective focal length which is much longer than the length of the tube. For telescopes, a longer focal length produces higher magnifications, all other things being equal. For eyepieces, a longer focal length produces lower magnifications, as the combination of a telescope and eyepiece produces a magnification which is the focal length of the telescope divided by the focal length of the eyepiece.

foci — The plural form of focus. Proper pronunciation is pho-kai, as focus is a word of Latin origin. However, its pronunciation is frequently Anglicized as pho-sigh.

focus (plural, foci) — One of two points used to construct an ellipse. A point on the major axis of the resulting ellipse, offset toward one end of the major axis in such a way that that end of the major axis is just barely the closest place to the focus. In a planetary orbit, the location of the Sun. Also, in a
telescope, the place where light is brought to a focus, producing an image.

foot — The place where a vertical circle crosses the horizon. The point on the horizon directly beneath an object which lies on that vertical circle. So called because a diagram showing the intersection of the vertical circle with the horizon, and the right angles made by their intersection, looks somewhat like the legs of a stick figure.

Fraunhofer lines — The absorption lines in the spectrum of the Sun. So called because they were first observed (and mapped in detail) by the German optician, Joseph von Fraunhofer (1787 - 1826), following his invention of the spectroscope.

free fall — The way in which an object falls under the influence of gravity, in the absence of significant air resistance or other impeding forces. In true free fall (e.g., in a vacuum), all objects fall in exactly the same way, regardless or their size, mass, density or composition.

full Moon — When the longitude of the Sun and Moon differ by 180 degrees, so that they are in opposite directions in the sky. At this time, the lit side of the Moon is the same as the side facing us, so we see the Moon fully lit, hence the name of the phase.


galactic cluster — A few dozen to a few tens of thousands of stars, regularly or irregularly scattered across a few tens of light-years. So-called because they are generally confined to the disc or body of galaxies, as opposed to globular clusters, which are scattered around their outer regions. Also called open clusters, because of their looser structure and appearance than the more thickly clustered globulars.

galaxy (plural, galaxies) — One of countless billions of immense stellar systems, ranging from ten thousand light years to one million light years in size, and containing from a few billion to a few thousand trillion stars. Our Galaxy, the Milky Way Galaxy, is a medium sized galaxy, approximately one hundred thousand light years across, containing several hundred billion stars.

gamma rays (also, gamma radiation, and g-rays) — Electromagnetic radiation of the highest photon energies, and the shortest wavelengths.

gauss — A unit of magnetic field strength, slightly greater than the Earth's magnetic field, which averages about 0.3 gauss near the Earth's magnetic Equator, and 0.6 gauss near the Earth's magnetic poles. Named after Johann Carl Friedrich Gauss (see below), who first measured the strength of the Earth's magnetic field in 1835.

Gauss, Johann Carl Friedrich (1777 - 1855) — German mathematician and astronomer, considered one of the most brilliant mathematicians in history. In astronomy, especially remembered for having devised a method of calculating orbits based on a relatively small number of observations. Prior to this, orbits had to be calculated using a method of triangulation developed by Kepler, hundreds of years earlier, which required observation of at least two orbits around the Sun. With his new method, Gauss was able to calculate the orbit of Ceres using only 9 degrees of motion across the sky, reducing the observational time by nearly a hundred times.
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Gaussian gravitational constant — A number used in precise calculations of orbital motions around the Sun, approximately equal to the average daily angular motion of the Earth around the Sun. Numerically, equal to 0.01720209895 radians per day, or 0.9856076686 degrees per day.

gegenschein (literally, opposite light) — Germanic name for the counterglow.

geomagnetic — Referring to the Earth's magnetic field.

geomagnetic storm — A disruption of the Earth's magnetic field by unusually intense solar winds. Such winds are associated with the ejection of large amounts of high-energy particles from the Sun, typically as a result of a flare, or a coronal mass ejection.

ghost — A false image formed by internal reflections in a telescope or other optical instrument.

gibbous — Referring to the Moon (or a planet, as seen through a telescope), a phase which is more than half lit, but not fully lit, as seen from the Earth.

gibbous Moon — A phase of the Moon in which its longitude differs from that of the Sun by more than 90 degrees, but less than 180 degrees, and it is more than half lit, but not fully lit. If it is East of the Sun, and increasing the amount by which it is East of the Sun, the portion which is lit increases from day to day, and we say that the Moon is waxing gibbous. If it is West of the Sun, and decreasing the amount by which it is West of the Sun, the portion which is lit decreases from day to day, and we say that the Moon is waning gibbous.

globular cluster — A roughly spherical collection of stars, numbering from a few tens of thousands to more than ten million stars, scattered over a region a few tens of light-years in diameter. Tens or hundreds of globular clusters are scattered around the haloes of many galaxies, including our own.

green flash — A momentary change in the color of the Sun, very occasionally observed in the last moments before the topmost reaches of the Sun dip below the western horizon, or in the first few moments after the Sun begins to rise on the eastern horizon. During the green flash, the strong reddish color caused by atmospheric reddening may, under certain circumstances, temporarily change to a vivid green. Usually, the flash, if seen at all, lasts for only a second or two, but when the observer is at a high latitude, so that the Sun is moving nearly parallel to the horizon, the "flash" may last for several minutes.

Gregorian Calendar — A calendar in which normal years have 365 days, leap years have 366 days, and leap years are usually held every four years, but are not held in century years that are not divisible by 400. Instituted by Pope Gregory on October 15, 1582.

gravity — The phenomenon that things have weight, and fall when able to do so. Viewed as a desire to reach a natural place by the ancients, as a force exerted by any mass on all other masses in Newtonian physics, and as a curvature of space-time in Einsteinian physics.

great circle — A circle which cuts a sphere into two equal halves, such as, on the Earth, the Equator, or on the Celestial Sphere, the Celestial Equator, the Ecliptic, or the Horizon. So-called because such a circle has a larger circumference than any other circle which can be inscribed on the sphere.


Helios — The Greek name for the Sun. Also used, in altered forms, as a prefix or suffix to indicate a quantity related to the Sun, such as perihelion, aphelion, heliosphere, spectroheliogram, etc.

heliacal rising — The rising of a star in morning twilight just early enough to see the star before the sky is too bright to see it, or the date on which that occurs. Due to the motion of the Earth around the Sun stars rise about 4 minutes earlier each day than the day before. On days prior to its heliacal rising a star would rise too late to be seen against the bright eastern sky of late twilight. On days after its heliacal rising a star would rise well before the sky was too bright to see it. In ancient Egypt the date of Sirius' heliacal rising occurred about three weeks before the annual flooding of the Nile, and was used as one measure of the length of the year.

heliosphere — The region where the Solar Wind blows away from the Sun, carrying the interplanetary magnetic field (created from the magnetic field of the Sun) throughout the Solar System. A region perhaps 200 Astronomical Units in diameter, within which the interplanetary magnetic field, and the motion of the Solar Wind, is paramount.

heliopause — The outer boundary of the heliosphere. The region where the Solar Wind slows and stops, as it collides and merges with the interstellar medium. Of unknown size, but thought to be about 100 Astronomical Units from the Sun (historically, has been presumed to be about twice as far out as the furthest proven extent of the Solar Wind).

heliostat (literally, stationary Sun) — A coelostat used for observations of the Sun.

heterosphere — (from heterogenous, meaning varied) — The region of the atmosphere where lack of vertical mixing allows lighter gases to diffuse upward, and heavier gases to diffuse downward, creating substantial differences in composition at various altitudes. For the Earth, the thermosphere and exosphere.

homosphere — (from homogeneous, meaning uniform) — The region of the atmosphere where vertical mixing prevents substantial differences in composition at different altitudes. For the Earth, the mesosphere, stratosphere, and troposphere. (The stratosphere, though slightly varied in composition (e.g., the ozone layer), is not considered heterogeneous, as it is more than 99.9% uniformly mixed.)

Horizon — An imaginary circle, halfway between the zenith and the nadir, which divides the sky into equal halves — an upper half, which is mostly above the observer's skyline, and a lower half, which is mostly below the observer's skyline.

Horizon system — A spherical coordinate system in the sky, based on the direction of gravity, and the division of the sky into visible and non-visible portions by the skyline. Also called the Alt-Azimuth coordinate system.

hour — A division of time equal to sixty minutes, 3600 seconds, or 1/24 of a solar day.

hour circle — An arc, in the Equatorial coordinate system, from the North Celestial Pole to the South Celestial Pole, of constant right ascension. Called an hour circle because right ascension is measured in time units. Analogous to a meridian of longitude, in the Terrestrial coordinate system. Also, in a
telescope, a circle used to indicate the right ascension that the telescope is pointing at.

hyperbola — One of two conic sections for which there is only one closed end. At the other, or open, end, the two sides of the hyperbola extend into infinity at an angle to each other, moving steadily further and further apart.


inertia — A property of matter, which causes it to not want to change what it is doing. In ancient physics, a desire not to move. In modern physics, a desire not to change its motion. The amount of inertia which a given body possesses, which determines how hard it is to change its motion, is called its inertia, or its mass.

inertial mass — An object which has inertia.

inertial reference frame — A reference frame in which the Law of Inertia holds (is correct). Any reference frame which has a constant velocity relative to any other inertial reference frame.

inferior (literally, lower) — Closer to the Earth than the Sun (as in an inferior conjunction), or closer to the Sun than the Earth (as for an inferior planet).

inferior conjunction — When an inferior planet (Mercury or Venus) is at conjunction (in the same direction as the Sun), and in between the Earth and the Sun.

inferior planet — A planet closer to the Sun than the Earth (Mercury or Venus).

intercalation — The insertion of an extra day or month into a calendar, to make the average length of the calendar year more nearly equal to the seasonal year. In the Roman, Hebraic and other luni-solar calendars, the insertion of an extra month. In the later Roman and modern (Julian and Gregorian) calendars, the insertion of an extra day.

interstellar — Between the stars.

interstellar absorption lines — Absorption lines in the spectrum of a distant object caused by interstellar gases lying between us and that object. Such lines reveal conditions in the intervening gas, rather than the object itself. Because of the low density of interstellar material, interstellar absorption lines are always very narrow in comparison to stellar absorption lines.

inversion layer — A region in an atmosphere where temperature increases with height, preventing vertical mixing, or convection, in the region. The stratosphere and thermosphere are permanent inversion layers in the Earth's atmosphere, but temporary inversions may occur in the troposphere, near the surface of the Earth, due to local variations in weather.

ion — An atom or molecule which is electrically charged, because of an excess or deficiency of electrons.

ionosphere — The outer part of an atmosphere, which contains a substantial fraction of more-or-less permanently ionized particles, as a result of the absorption of high-energy solar radiation. Also, during the day, the portion of the middle and lower atmosphere in which solar radiation can temporary ionize atoms and molecules. For the Earth, the permanent ionosphere includes the exosphere, thermosphere, and mesosphere, while the temporary ionosphere includes the upper stratosphere. On Mars, the ionosphere extends all the way to the surface, during the day.

iron — A meteorite which is made of crystalline nickel-iron.


Julian Calendar — A calendar in which normal years have 365 days, leap years have 366 days, and leap years are held every fourth year. Created by Sosigenes of Alexandria, but named after Gaius Julius Caesar, who hired Sosigenes for that purpose, and made it the official Roman calendar in 46 BC. Because of errors in intercalation, leap years weren't actually held every four years until 8 AD. (Of course, the terms "BC" and "AD" weren't in use in those days, not being proposed until the 700's.)

Julian Date (JD) — The number of days since noon, Greenwich time, on January 1, 4713 BC. Noon on that day was Julian Day 0.00. At 6 pm on that date, it was Julian Day 0.25. At 6 pm on January 2, 4713 BC, it was Julian Day 1.25. Thus, the Julian Date specifies both the date and the time of day. As this entry was written, at 10:30 am PDT on October 7, 2003, the Julian Date was 2452920.22917. For a calculation of the current (or any other) Julian Date, use this external link.

Julian Day — The day beginning at noon, Greenwich time, and ending at the next Greenwich noon.

Julian Day Number — A running count of Julian Days, beginning with January 1, 4713 BC, which was Julian Day 0. Proposed by Joseph Scaliger in 1583 as a way of reducing the confusion associated with different principalities using different calendars. E.g., for distinguishing between dates in the Gregorian Calendar, adopted in 1582 by Roman Catholic countries, but not adopted by most Protestant countries until the mid 1700's (and by some as late as the 1900's!). Named after the Julian Calendar, and hence indirectly after Gaius Julius Caesar.

(Gaius) Julius Caesar (ee-YOO-lee-oos KAI-zer) — (about 100 - 44 BCE). Roman general and dictator (Caesar). Of astronomical importance because of his decision to reform and regularize the Roman calendar, leading to the adoption of the Julian Calendar.


Kepler, Johannes — 1571-1630. Mathematician who discovered the Laws of Planetary Motion which bear his name. Also, a rayed crater on the Moon. External Link


Laplace, Pierre-Simon — 1749-1827. French originator of the Solar Nebula theory. External Link

lapse rate — The rate at which temperature decreases with height, in the lower atmosphere of a planet. The same as the temperature gradient in that region. For the Earth, the lapse rate is almost 20 Fahrenheit degrees per mile, but for Venus, it is substantially higher, and for most other planets, substantially lower.

last quarter — See quarter Moon.

Law of Inertia, The — Newton's First Law of Motion. Expresses the fact that in an inertial frame of reference, objects which have no force acting on them will maintain constant velocity.

Law of Universal Gravitation, The — The rule that all masses exert a force on each other, called the force of gravity, which varies as the inverse square of the distance between them, and the product of their masses. Discovered by Newton, and in so doing, discovered to apply not only on the Earth, but everywhere else in the Universe, hence its name. Also called Newton's Law of Gravity. Superseded, in modern times, by Einstein's Theory of General Relativity, which describes gravity as a curvature of space-time. However, unless in a very unusual situation, or requiring exceptionally high accuracy, Newton's formulation gives more than adequate results, and being far simpler to comprehend and to calculate mathematically, is always used when possible.

lepton — A light sub-atomic particle, such as an electron.

light curve — A graph showing the changing brightness of an astronomical object (usually, a variable star), plotted against time.

light-time (also, light travel time) — The time required for light to reach the Earth from some distant object. Because the Earth is moving around the Sun, this time changes during the course of the year, and in accurate calculations of periodic changes, a correction is necessary for any change in the light-time. As an example, if observations of the motions of Jupiter's satellites are used to predict their positions at some future date, a change in the position of the Earth, relative to Jupiter, may change the times when particular positions are observed by a quarter of an hour, because it takes light more than a quarter of an hour to cross the orbit of the Earth..

light year (LY) — The distance that light travels in one year. Approximately 60000 AUs, or 6 trillion miles, or 10 trillion kilometers.

line of apsides — In an orbit, the line between the ends of the major axis. In an orbit around the Sun, the line between the perihelion and aphelion points.

line of nodes — In an orbit, the line between the ascending and descending nodes.

liquid — A fluid phase of matter (which also includes gases and plasmas) in which the particles are in direct contact with each other at all times, leaving very little empty space between them, and making the material relatively dense and incompressible.

liquid dynamo theory — A theory of magnetic field creation, in which convective motions driven by the heat of an electrically conducting fluid (such as the molten iron outer core of the Earth) creates a myriad of small magnetic fields, which combine to form the overall magnetic field.

Loof Lirpa — Fictional Scandinavian prankster, primarily famous for his eponymous birthday (April 1).

lunation — The time required for the Moon to pass through a cycle of phases. Identical to a synodic month.

luxon — A particle which travels at the speed of light. Apparently, only light particles (photons) themselves, since neutrinos, the only other massless particles thought to exist, now appear to have mass, and therefore always travel at least a little slower than the speed of light. Used as one of three terms to distinguish between particles which always travel slower than light (tardyons), particles which always travel at the speed of light (luxons), and particles which always travel faster than light (tachyons).


Magellanic Clouds — small companion ('satellite') galaxies of our own Milky Way Galaxy. Only visible from far southern latitudes, and therefore unnoticed by northern (European) astronomers until Magellan's circumnavigation of the globe — hence their name. To the unaided eye, they appear as small patches, similar to the Milky Way, which are somewhat separated from the main body of that fog of faint stars.

magnetic field — A "field" of force surrounding a magnet, or magnetized object, associated with the electromagnetic force. Often represented by lines extending through space, which show the direction of the force on a test object by their direction, and the strength of that force by how far apart they are. Often confused with gravitation by beginning students, who often make statements linking the force of a planet on its neighbors to its magnetic field. In reality, in astronomy, magnetism only affects objects if they are electrically charged or magnetized, and extremely small, or not terribly large, and the magnetic field is exceptionally large. Thus, if an electrically charged atom approaches the Earth's magnetic field, it will be forced to follow the field lines, to a greater or lesser extent, depending upon its speed. Whereas, if a dust grain, even if just big enough to see without a microscope, were to do the same thing, it would go right through the field as if it weren't even there, because only the Earth's gravitational pull, and its own inertia and momentum, would be of any importance.

magnetic pole — One of the places where the magnetic field of a planet comes out of the planet (the South Magnetic Pole), or goes into it (the North Magnetic Pole). Also, on a permanent magnet, or an electromagnet, one of the 'ends' of the magnet.

magnetopause — The boundary between the magnetic field of a planet, and that of the Sun. So called to mimic the practice of naming the top of each layer in our atmosphere as the something-pause, where "something" refers to the name of the layer below that boundary.

magnetosphere — The region in which the magnetic field of a planet is more important than the interplanetary magnetic field. Also, sometimes used to indicate the region where charged particles are trapped in that field (e.g., the Van Allen radiation belts).

magnetotail — A long, tail-like structure in the magnetosphere of a planet, which points away from the Sun. Created by the interaction of the solar wind and the magnetic field of the planet, it can be thought of as caused by a 'dragging' of the magnetic field lines in the direction of the solar wind's motion (which is, of course, away from the Sun).

magnitude — The brightness of a stellar object, as ranked on a scale in which the brightest stars are first magnitude, meaning of the first order of importance, and fainter stars are second, third, or fainter magnitudes. If not prefixed by a descriptive limiter, usually presumed identical to apparent magnitude.

Main Sequence — A region in the Hertzsprung-Russell Diagram, running from the upper left (blue giants) to the lower right (red dwarfs) diagonally across the middle of the Diagram. The vast majority of stars are Main Sequence stars, and therefore, most stars in any given group of stars will have their properties plot somewhere on the Main Sequence.

Main Sequence lifetime — The time that a star can remain a Main Sequence star. In general, less for massive, bright stars, which run through their fuel in just a few million years, longer for middling mass, middling brightness stars, which can last for billions of years, and exceptionally long for low mass, low brightness stars, which can last for trillions of years.

Main Sequence star — A star whose brightness and temperature place it on the Main Sequence, in the Hertzsprung-Russell Diagram. As it happens, a star whose energy source is the fusion of hydrogen to helium, either through the proton-proton cycle (for lower Main Sequence stars), or the carbon cycle (for upper Main Sequence stars).

mantle — For the Earth, and Terrestrial objects, the region lying between the crust, and the core. Also, for those objects, a region made of relatively dense rocks, comprised primarily of ferromagnesian silicates.

mare — Relatively dark-colored, smooth areas on the lunar surface, representing ancient lava flows which filled in and depressed still more ancient craters of relatively large size. The mare lavas are denser than the lighter colored rocks which they overlie, and which surround them, and as a result, they depress the floors of the craters which they 'fill' even further than they would be depressed if they were not covered by the lava.

maria — Plural form of mare.

mass — The measure of a body's inertia, or how difficult it is to change its motion.

Mass-Luminosity Diagram — A diagram showing the relationship between mass and luminosity for Main Sequence stars. The more massive the star, the brighter it is, as a Main Sequence star.

Mass-Luminosity Relationship — An equation approximating the increase of brightness with mass, for Main Sequence stars.

Maxwell, James Clerk — 1831 - 1879, Scottish physicist. Developed the unified theory of electricity and magnetism known as electromagnetism, and in so doing established the nature of light as an electromagnetic wave.
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megaparsec (Mpc) — One million parsecs.

meridian — On the Earth, an arc, extending from the North Pole to the South Pole, of constant longitude east or west of the Prime Meridian.

Meridian, the — In the sky, an arc extending from the North Celestial Pole, through the zenith, to the South Celestial Pole.

meson — A sub-atomic particle of intermediate weight.

mesopause — The boundary between the top of the mesosphere and the bottom of the thermosphere. For the Earth, the mesopause is define by the altitude at which the temperature stops dropping with height. For other planets, it is defined by the altitude at which the temperature starts rising with height (in the lower thermosphere).

mesosphere (literally, middle sphere) — The middle portion of an atmosphere, lying between the lower atmosphere and the outer atmosphere. In the Earth's atmosphere, the mesosphere lies between the stratosphere and thermosphere, and is a large region where temperature decreases with height. In other planets' atmosphere, the mesosphere is limited to a thinner region where temperatures are relatively constant, and very cold (also known as a "cold trap").

meteor — A streak of light in the sky, usually lasting a few seconds at most, caused by the rapid passage of a small piece of interplanetary debris through our upper atmosphere. Sometimes called a 'falling star' or a 'shooting star'. An exceptionally bright meteor is called a bolide, or a fireball.

meteor stream — See meteoroid swarm.

meteoroid — A small piece of interplanetary debris, particularly one on a collision course with the Earth. When such an object passes through the atmosphere and vaporizes, it produces a meteor.

meteoroid swarm (also, meteor stream) — A large number of meteoroids, moving through space in relatively close proximity, and along nearly identical orbital paths. Encountering such a swarm produces a meteor shower. Such swarms are created by the loss of material by a comet, as it passes by the Sun, during one of its orbits. It is not unusual for a number of swarms of different ages to be tracking the orbit of the comet that lost them, at slightly different positions, and with slightly different motions.

meteorite — A piece of interplanetary debris, usually rocky, which fell from space, and survived the journey to the surface of the Earth. If observed to fall, and then recovered, referred to as a 'fall'. If not observed, and subsequently recovered, referred to as a 'find'.

meteor shower — An event in which a relatively large number of meteors appear to be coming from the same area (or, more accurately, going away from the same area) in the sky, during a relatively short time. Most meteor showers only involve a few meteors per hour, but some can involve dozens, hundreds, or even thousands. Exceptional meteor showers are sometimes referred to as meteor storms. All meteor showers, impressive or not, are caused by the Earth passing through meteoric debris lost by a comet at some time in the past.

meteorwrong — A jocular reference to an object thought to be a meteorite, which is not a meteorite.

Metonic cycle — A time equal to 19 years, or 235 synodic months, after which the phases of the Moon repeat themselves on the same calendar dates.

micrometeoroid — A meteoroid of microscopic size. In general, such small meteoroids are stopped in the outer atmosphere of the Earth without producing enough frictional force to result in a visible meteor. However, on an airless body, such as the Moon or Mercury, they can hit the surface with their full interplanetary velocity, and produce a microscopic impact crater.

Milky Way — A faint starry glow which encircles the sky, caused by the combined light of millions of stars too far away and therefore too faint to see individually. Only visible in very dark skies (in fact, considered one way to decide whether the sky is reasonably dark). The glow follows the plane of our galaxy, and helps define that plane.

Milky Way Galaxy — The galaxy in which we reside. So called because the glow of faint stars in that part of the galaxy which is nearest to us creates the Milky Way.

millisecond pulsar — A pulsar whose rotation period is measured in thousandths of a second.

molecule — A group of two or more atoms, chemically bound in a single unit by the electromagnetic interaction of those atoms. E.g., a molecule of oxygen, consisting of two oxygen atoms bound together, or of ozone, consisting of three oxygen atoms bound together.

molecular band — An absorption feature produced by molecules in the atmosphere of a star, or in interstellar space. Because molecules are more complex structures than individual atoms, the absorption of light by molecules often involves a host of absorptions at very similar wavelengths, producing a wide smudge, or 'band', in the spectrum.

molecular cloud — An interstellar cloud in which the temperature is so low that molecules are more predominant than individual atoms or ions. A large collection of such clouds, in a fairly small region, is called a molecular cloud complex.
Glossary (N- Z)