Below are the definitions for some of the most common terms used in astronomy and space exploration. You may click on any blue highlighted word to jump directly to the definition for that word. You can also click on any letter of the alphabet below to jump directly to that section in the listing.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Absolute Magnitude
A scale for measuring the actual brightness of a celestial object without accounting for the distance of the object. Absolute magnitude measures how bright an object would appear if it were exactly 10 parsecs (about 33 light years) away from Earth. On this scale, the Sun has an absolute magnitude of +4.8 while it has an apparent magnitude of -26.7 because it is so close.

Absolute Zero
The temperature at which the motion of all atoms and molecules stops and no heat is given off. Absolute zero is reached at 0 degrees Kelvin or -273.16 degrees Celsius.

Ablation
A process by where the atmosphere melts away and removes the surface material of an incoming meteorite.

Accretion
The theoretical process by where dust and gas accumulated into larger bodies such as stars and planets.

Achondrite
A stone meteorite which contains no chondrules.

Achromatic Doublet (Achromat)
A two-element objective lens used in refractor telescopes. The use of two complimentary glass types (low reflective index crown glass for one element, and high reflective index flint glass for the other) partially cancels out chromatic aberration. While the word "achromatic" means "without color," it actually refers to a refractor that is not totally free of spurious color, but one corrected for chromatic aberration at two wavelengths, or colors, of light.

Airy Disk
Because light behaves in some ways like a wave, it is bent or "diffracted" by a telescope's structure (the edge of the optical tube and diagonal or secondary mirror, for example). Therefore, a star can never be focused to a point. Telescopes always show stars as small disks of light (called Airy disks, after British Astronomer Royal Sir George Airy, 1835 to 1892). The disks are surrounded by very faint diffraction rings. Airy disk size is determined by the aperture of the scope - the larger the aperture, the smaller the Airy disk.

Albedo
The reflective property of a non-luminous object. A perfect mirror would have an albedo of 100% while a black hole would have an albedo of 0%.

Albedo Feature
A dark or light marking on the surface of an object that may or may not be a geological or topographical feature.

Altazimuth Mount
The simplest type of telescope mount, with only up/down (altitude) and left/right (azimuth) motions. Primarily for low to medium power Dobsonian telescopes and terrestrial (spotting scopes) use, as it cannot easily follow at high powers the seemingly curved paths taken by celestial objects and cannot be used for long exposure photography.

Altitude
The angular distance of an object above the horizon.

Annular Eclipse
An annular eclipse differs from a total eclipse in that the Moon appears too small to completely cover the Sun. As a result, the Moon is surrounded by an intensely brilliant ring or annulus formed by the uneclipsed outer perimeter of the Sun's disk. The solar corona is not visible during annular eclipses. Furthermore, a solar filter or projection is needed to observe all phases of an annular eclipse.

Antimatter
Matter consisting of particles with charges opposite that of ordinary matter.  In antimatter, protons have a negative charge while electrons have a positive charge.

Antipodal Point
A point that is on the direct opposite side of a planet.

Apastron
The point of greatest separation of two stars, such as in a binary star system.

Aperture
The size of the opening through which light passes in an optical instrument such as a camera or telescope. With reference to a camera which has a variable aperture, a higher number, e.g. f16, represents a smaller opening while a lower number, e.g. f1.8, represents a larger opening. When using the term to describe telescope size, it means the diameter of the main mirror or objective lens of a telescope. For telescopes, the larger the aperture or opening, the better the resolution and the fainter the objects you can see.

Aphelion
The point in the orbit of a planet or other celestial body where it is farthest from the Sun.

Apochromat
A refractor telescope with a two to four lens optical system that uses one or more lens elements of costly ED (Extra-low Dispersion) or SD (Special Dispersion) glass and/or calcium fluorite crystal to virtually eliminate chromatic aberration. Technically, any lens system that is corrected for spherical aberration at two wavelengths, or colors of light, and for chromatic aberration at three.

Apogee
The point in the orbit of the Moon or other satellite where it is farthest from the Earth.

Apparent Magnitude
The apparent brightness of an object in the sky as it appears to an observer on Earth. Bright objects have a low apparent magnitude while dim objects will have a higher apparent magnitude.

Asteroid
A small planetary body in orbit around the Sun, larger than a meteoroid but smaller than a planet. Most asteroids can be found in a belt between the orbits of Mars and Jupiter. The orbits of some asteroids take them close to the Sun, which also takes them across the paths of the planets.

Astigmatism
An optical defect in which star images are elongated into ovals which change from a radial orientation (pointing towards the center of the field) to a tangential one (at right angles to the radius) as the observer moves the telescope focuser from one side of the best focus to the other. Should not be confused with the star images of an out-of-collimation telescope, which may also be oval but which will not change orientation as the observer passes through the best focus.

Astrochemistry
The branch of science that explores the chemical interactions between dust and gas interspersed between the stars.

Astronomical Unit (AU)
A unit of measure equal to the average distance between the Earth and the Sun, approximately 93 million miles.

Atmosphere
A layer of gases surrounding a planet, moon, or star. The Earth's atmosphere is 120 miles thick and is composed mainly of nitrogen, oxygen, carbon dioxide, and a few other trace gases.

Aurora
A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun. This phenomenon is known as the Aurora Borealis in the Earth's northern hemisphere and the Aurora Australis in the Earth's Southern Hemisphere.

Aurora Australis
Also known as the Southern Lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the southern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Borealis in the northern hemisphere.

Aurora Borealis
Also known as the Northern Lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the northern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Australis in the southern hemisphere.

Averted Vision
At night, the periphery of the eye's retina is more sensitive to light than the center. Looking slightly to one side of a faint object (averting your vision), so that the light falls on the more sensitive outer part of the retina, usually reveals faint details otherwise lost when looking directly at the object.

Axis
Also known as the poles, this is an imaginary line through the centre of rotation of an object, about which the object rotates.

Azimuth
The angular distance of an object around or parallel to the horizon from a predefined zero point.

B

Background Radiation
The background radiation of the universe is the temperature of the extragalactic medium in a non-expanding universe. It has been measured at 2.7 degrees Kelvin with a variation of only 1 in 100,000.

Bar
A unit of measure of atmospheric pressure. One bar is equal to 0.987 atmospheres, 1.02 kg/cm2, 100 kilopascal, and 14.5 lbs/square inch.

Big Bang
The Big Bang Theory is the dominant scientific theory about the origin of the universe. According to the big bang, the universe was created about 13.7 billion years ago from a cosmic explosion that hurled matter and in all directions.

In 1927, the Belgian priest Georges Lemaître was the first to propose that the universe began with the explosion of a primeval atom. His proposal came after observing the red shift in distant nebulas by astronomers to a model of the universe based on relativity. Years later, Edwin Hubble found experimental evidence to help justify Lemaître's theory. He found that distant galaxies in every direction are going away from us with speeds proportional to their distance.

The big bang was initially suggested because it explains why distant galaxies are traveling away from us at great speeds. The theory also predicts the existence of cosmic background radiation (the glow left over from the explosion itself). The Big Bang Theory received its strongest confirmation when this radiation was discovered in 1964 by Arno Penzias and Robert Wilson, who later won the Nobel Prize for this discovery.

Although the Big Bang Theory is widely accepted, it probably will never be proved; consequentially, leaving a number of tough, unanswered questions.

Binary
A system of two stars that revolve around a common centre of gravity.

Black Hole
The collapsed core of a massive star. Stars that are very massive will collapse under their own gravity when their fuel is exhausted. The collapse continues until all matter is crushed out of existence into what is known as a singularity. The gravitational pull is so strong that not even light can escape.

Blueshift
A shift in the lines of an object's spectrum toward the blue end. Blueshift indicates that an object is moving toward the observer. The larger the blueshift, the faster the object is moving. The shift in an object's colour is similar to the effect we hear when an emergency vehicle passes us; the tone of the sound changes from a high pitch to a low pitch.

Bolide
A term used to describe an exceptionally bright meteor. Bolides typically will produce a sonic boom and explode in a bright terminal flash at their end, often with visible fragmentation.

C

Caldera
A type of volcanic crater that is extremely large, usually formed by the collapse of a volcanic cone or by a violent volcanic explosion. Crater Lake is one example of a caldera on Earth.

Cassegrain
Any telescope that folds the light path and directs it through a hole in the center of the primary mirror (called the Cassegrain focus) at the bottom of the telescope.

Catadioptric Telescope
A telescope that uses a combination of mirrors and lenses to increase the effective focal length of the telescope while allowing it to be folded into a more convenient and compact size. The use of a correcting lens in these telescopes virtually eliminates spherical aberration, chromatic aberration, and coma. The word catadioptric is derived by combining the term for an optical system that forms images by using mirrors, (catoptric) with the one of a system that uses lenses (dioptric). The most popular catadioptric designs are the Schmidt-Cassegrain and Maksutov-Cassegrain.

Catena
A series or chain of craters.

Cavus
A hollow, irregular depression.

Celestial Equator
An imaginary line that divides the celestial sphere into a northern and southern hemisphere.

Celestial Poles
The North and South poles of the celestial sphere.

Celestial Sphere
An imaginary sphere around the Earth on which the stars and planets appear to be positioned.

Cepheid Variable
This is a type of variable star whose light pulsates in a regular cycle. The period of fluctuation is linked to the brightness of the star. Brighter cepheids will have a longer period. Cepheids are used to determine the distances to stars and galaxies in the Universe.

Chaos
A distinctive area of broken terrain.

Chasma
Another name used to describe a canyon.

Chondrite
A meteorite which contains chondrules.

Chondrule
Small, glassy spheres commonly found in meteorites.

Chromatic Aberration
An optical defect in refractor telescopes in which all colors of light do not come to focus at the same point, causing stars and planets to have a faint violet halo which masks planetary details and makes it difficult to split faint binary stars. Also known as spurious or secondary colour.

Chromosphere
The part of the Sun's atmosphere just above the surface.

Circumpolar Star
A star that never sets but always stays above the horizon. This depends on the location of the observer.  The further South you go the fewer stars will be circumpolar. Polaris, the North Star, is circumpolar in most of the northern hemisphere.

Collimation
The alignment of the optical elements of a telescope at the correct angles to the light path. If not properly collimated, a telescope will deliver distorted images (lopsided or elongated stars, hazy planetary images, an inability to split close binary stars, etc). Atmospheric turbulence (seeing), thermal currents within the telescope, and dirty or defective eyepieces can often mimic poor collimation.

Coma
An area of dust or gas surrounding the nucleus of a comet. Or in telescopic terms, an optical defect in reflector telescopes in which in-focus star images appear progressively more triangular or comet-like the closer the get to the edge of the field of view. The faster the focal ratio, the more prominent the coma. The visually coma-free field of a telescope in millimeters is roughly equal to the square of the scope's focal ratio- an f/5 focal ratio scope (5squared) has a 25mm coma-free field, an f/6 (6squared) has a 36mm coma-free field, etc. Since a 1.25" eyepiece barrel measures only about 29mm in internal diameter, while 35mm film measures 44mm across its diagonal, it can be seen that even a 25mm coma-free field is more apparent in photos than it is in most visual observing. Coma can superficially appear similar to a star's image in a poorly collimated telescope. With coma, however, the brightest portion of the comatic wedge (actually the Airy disk) always points toward the center of the field. This differs from an out-of-collimation telescope, where the Airy disks are all offset to the same side of the diffraction rings, no matter where in the field the star image is located.

Comet
A gigantic ball of ice and rock that orbit the Sun in a highly eccentric orbit. Some comets have an orbit that brings them close to the Sun where they form a long tail of gas and dust as they are heated by the Sun's rays. Comets have no light generating properties but are illuminated solely by the Sun. The maximum age of any orbiting comet is 1 million years.

Conjunction
An event that occurs when two or more celestial objects appear close together in the sky. See also, Triple Conjunction.

Constellation
A grouping of stars that make an imaginary picture in the sky.

Corona
The outer part of the Sun's atmosphere. The corona is visible from Earth during a total solar eclipse. It is the bright glow seen in most solar eclipse photos.

Cosmic Ray
Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with extremely high amounts of energy.

Cosmic String
A tube like configuration of energy that is believed to have existed in the early universe. A cosmic string would have a thickness smaller than a trillionth of an inch but its length would extend from one end of the visible universe to the other.

Cosmogony
The study of celestial systems, including the solar system, stars, galaxies, and galactic clusters.

Cosmology
A branch of science that deals with studying the origin, structure, and nature of the universe.

Crater
A bowl-shaped depression formed by the impact of an asteroid or meteoroid. Also the depression around the opening of a volcano.

Creation
The general term used to refer to the account in the first chapters of Genesis where God created the heavens, the earth, and mankind. Though completely in harmony with proven science, this part of the Bible has come to be severely criticized, particularly by those who prefer to believe that human life came about as the result of a process of evolution.

Curvature of field
An optical defect in which objects at the edge of the field of view can't be brought into sharp focus at the same time as objects in the center, and vice versa.

D

Dark Matter
A term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies.

Dawes' Limit
If two equally-bright stars are so close together that their Airy disks overlap, they will be seen as one star. Although, usually as an elongated one. If, however, the Airy disk of one star falls in the first dark diffraction ring of the second, both stars can be seen - not as two distinct points, but as a figure 8, in which the intensity of light between the two touching disks drops by a clearly visible 30%. The smallest separation between two stars which shows this 30% drop was empirically determined by English astronomer William R Dawes (1799- 1868) to be 4.56 arc seconds divided by the aperture of the telescope in inches. (Dawes was known as the "eagle eyed" for his acute vision). The larger the telescope aperture, the smaller the separation that can be resolved. Dawes' Limit (determined by testing the resolving ability of many observers on white star pairs of equal magnitude 6 brightness) only applies to point sources of light (stars). Smaller separations can be resolved in extended objects, such as planets. For example, Cassini's Division in the rings of Saturn (0.5 arc seconds across), was discovered using a 2.5" telescope - which has a Dawes' limit of 1.8 arc seconds! The ability of a telescope to resolve to Dawes' limit is usually much more affected by seeing conditions, by the color and brightness differences between the binary star components, and by the observer's visual acuity and observing experience, than it is by the optical quality of the telescope.

Declination
The angular distance of an object north or south of the celestial equator, measured in degrees. One of the two coordinates (right ascension is the other) that let you find celestial objects with the aid of telescope setting circles. Called declination because stellar positions in degrees, "decline" or decrease in numerical value from 90 degrees at the north and south celestial poles (around which everything in the sky appears to rotate) down to 0 degrees at the plane of the celestial equator. Declination is in positive degrees if the object is between the celestial equator and the celestial north pole, and in negative degrees if it is between the celestial equator and the south celestial pole.

Density
The amount of matter contained within a given volume. Density is measured in grams per cubic centimetre (or kilograms per litre). The density of water is 1.0, iron is 7.9, and lead is 11.3.

Diffraction
An optical interference effect due to the bending of light around obstacles in its path (the edges of a telescope tube or its internal light baffles, for example). It's similar to the way waves are bent or deflected around dock pilings or the edge of a jetty. All telescopes show faint light and dark diffraction rings around a star's Airy disk at high power, as the diffracted light waves alternately cancel out and reinforce each other. Diffraction rings are very faint and can be washed out by the glare of a bright star. If you cannot easily see them in your scope, it should not be a cause for concern. For example, in a perfect refractor 84% of the light would be imaged in the Airy disk, with half the remainder falling in first diffraction ring and the balance scattered in diminishing amounts among the second, third, fourth rings, etc. As the first diffraction ring is about six times the area of the Airy disk itself, its fainter light is spread over a larger area. That means the brightness of the first diffraction ring is actually less than 2% that of the Airy disk. The other rings are dimmer still. It is easy to see how the beginning observer can have difficulty separating the very faint diffraction rings from the much brighter Airy disk. Catadioptrics and reflectors start out with diffraction rings about twice as bright as those of a refractor due to the additional light diffracted by their secondary mirror obstructions. However, their brightness is still low in relation to their Airy disk (only 4% as bright in the case of the first ring). The additional light that is scattered by a catadioptric into its diffraction rings shows itself as lower contrast and a loss of sharpness on planets, binary stars, and star clusters when compared with a refractor. The spider vanes holding a reflector's diagonal mirror create additional contrast-lowering spikes radiating from each star's image, an effect particularly visible on long exposure photos. Catadioptric telescopes also have a circular secondary mirror shadow, but do not have diffraction spikes and spider shadows.

Diffraction-Limited
As mentioned above, a star appears in a telescope as a small Airy disk surrounded by faint diffraction rings. A telescope is said to be "diffraction limited" if its optics are made with enough accuracy so that all the light rays from a star fall within that star's Airy disk and diffraction rings, with no access light being scattered out of the disk and rings by defects in the mirrors. Optics that bring all light rays to a focus within 1/4th of a wavelength of light of each other at the final focus are considered to be diffraction limited. Technically, a telescope is diffraction limited if it meets the Rayleigh limit - which specifies the separation in arc seconds of two equally-bright binary stars which appear to be just touching as being equal to 140 divided by the aperture in mm. The Rayleigh limit, which deals with a telescope's ability to separate closely-spaced stars, should not be confused with the Rayleigh criterion, which deals with how accurately an optical system is made. Note that the visual Rayleigh limit for an 8" (203mm) aperture telescope is 0.69 arc seconds (140 divided by 203), a less-stringent specification than the Dawes' limit of 0.57 arc seconds. Telescopes meeting either limit can resolve more detail than the Earth's atmosphere will allow us to see under average seeing conditions, as our atmosphere typically limits the seeing to no better than one arc second resolution (the resolution of a 6" scope) on even a very good night. Five arc second resolution at best is often typical of an average night.

Disk
The surface of the Sun or other celestial body projected against the sky.

Distortion
An optical defect which causes uneven magnification in different directions. It makes straight lines appear curved and is therefore more visible in terrestrial observing, since there are few straight lines in space.

Dobsonian Telescope
A conventional Newtonian reflector optical tube on an inexpensive plywood or fiberboard altazimuth mount. Nylon or Teflon bearings allow smooth telescope motion at a fingers touch, with no vibration or unsteadiness. The scope is moved by hand from object to object (there are no manual slow motion controls or motor drives) using a technique called star-hopping to locate objects. Usually it's a large aperture, fast focal ratio scope deigned for visual deep space observing - although 6" and 8" medium f/ratio (f/6 and f/8) Dobsonians are also suitable for planetary observing. Cannot be used for astrophotography. The Dobsonian is an economical way to get into large aperture astronomy at a fraction of the cost of an equatorially mounted telescope of similar aperture.

Double Asteroid
Two asteroids that revolve around each other and are held together by the gravity between them. Also called a binary asteroid.

Doppler Effect
The apparent change in wavelength of sound or light emitted by an object in relation to an observer's position. An object approaching the observer will have a shorter wavelength (blue) while an object moving away will have a longer (red) wavelength. The Doppler effect can be used to estimate an object's speed and direction.

Double Star
A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system.

E

Eccentricity
The measure of how an object's orbit differs from a perfect circle. Eccentricity defines the shape of an object's orbit.

Eclipse
The total or partial blocking of one celestial body by another.

Eclipsing Binary
A binary system where one object passes in front of the other, cutting off some or all of its light.

Ecliptic
An imaginary line in the sky traced by the Sun as it moves in its yearly path through the sky.

Ejecta
Material from beneath the surface of a body such as a moon or planet that is ejected by an impact such as a meteor and distributed around the surface. Ejecta usually appears as a lighter colour than the surrounding surface.

Electromagnetic Radiation
Another term for light. Light waves created by fluctuations of electric and magnetic fields in space.

Electromagnetic Spectrum
The full range of frequencies, from radio waves to gamma waves, that characterizes light.

Ellipse
An ellipse is an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical in shape rather than circular.

Elliptical Galaxy
A galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.

Elongation
The angular distance of a planetary body from the Sun as seen from Earth. A planet at greatest eastern elongation is seen in the evening sky and a planet at greatest western elongation will be seen in the morning sky.

Ephemeris
A table of data arranged by date. Ephemeris tables are typically to list the positions of the Sun, Moon, planets and other solar system objects.

Equinox
The two points at which the Sun crosses the celestial equator in its yearly path in the sky. The equinoxes occur on or near March 21 and September 22.  The equinoxes signal the start of the Spring and Autumn seasons.

Escape Velocity
The speed required for an object to escape the gravitational pull of a planet or other body.

Event Horizon
The invisible boundary around a black hole past which nothing can escape the gravitational pull - not even light.

Evolved Star
A star that is near the end of its life cycle where most of its fuel has been used up. At this point the star begins to loose mass in the form of stellar wind.

Extinction
The apparent dimming of star or planet when low on the horizon due to absorption by the Earth's atmosphere.

Extragalactic
A term that means outside of or beyond our own galaxy.

Extraterrestrial
A term used to describe anything that does not originate on Earth.

Eyepiece
The lens at the viewing end of a telescope. The eyepiece is responsible for enlarging the image captured by the instrument. Eyepieces are available in different powers, yielding differing amounts of magnification.

Equatorial Mount: A telescope mount designed for astronomical use. It aligns the axis of rotation of a telescope with the axis of the Earth, allowing the scope to follow the seemingly curved paths taken by the stars and planets. When equipped with a motor drive, it automatically tracks celestial objects without the need for constant manual corrections, as is the case with an altazimuth or Dobsonian mount. This is particularly important at high magnification, where objects drift across the field of an unmoving scope in a minute or less. Usually supplied with setting circles that help locate objects by their right ascension and declination coordinates. Convenient for visual observing and essential for astrophotography.

Exit Pupil: The circular image or beam of light formed by the eyepiece of a telescope. To take full advantage of a scope's light-gathering capacity, the diameter of an eyepiece exit pupil should be no larger than the 7mm diameter of your eye's dark-adapted pupil, so that all of the light collected by the telescope enters your eye. (The eyepiece exit pupil diameter is found by dividing the eyepiece focal length by the telescope's focal ratio). Your eye's ability to dilate declines with increasing age (to a dark-adapted pupil of only about 5mm by age 50 or so). For those in this age group, eyepieces with exit pupils larger than their eyes can dilate to, simply waste some of their telescope's light-gathering capacity, as a portion of the telescope's light will fall on their not-fully-dilated iris instead or entering their eye.

Eye Relief: The distance between the lens of an eyepiece and the point behind the eyepiece where the light rays of the exit pupil focus and the image is formed.. This is where your eye should be positioned to see the full field of view of the eyepiece. If you must wear glasses because of astigmatism, you'll need a minimum of 15mm of eye relief if you want to see the full field of view with your glasses on.

Eyepiece: A telescope gathers light and forms a small fixed-size image at a point (called the prime focus) that's determined by the focal length of the optical system. You can see this image by aiming your telescope at something bright, such as the Moon, taking out the eyepiece and star diagonal, and holding a piece of paper behind the focuser. Move the paper back and forth. At some point, you will find a small, but sharp, image of the Moon projected onto the paper. This is the prime focus image formed by the telescope. Unfortunately, human eyes typically cannot focus sharply on an image unless it's more than eight inches from the eye. This makes it difficult to see detail in the small prime focus image formed by the telescope if it's examined solely by the unaided eye. An eyepiece is a small microscope that allows you to get closer than eight inches from that small fixed-focus image - and the closer you can get to an object, the bigger it appears. A 25mm eyepiece, for example, lets you focus on the scope's prime focus image from an effective distance of only 25mm (one inch away from your eye); a 12mm eyepiece puts you half an inch away; etc. The magnification of an eyepiece is found by dividing the telescope focal length by the eyepiece focal length. A 25mm eyepiece used with a 2000mm focal length scope therefore provides 80 power (2000 / 25 = 80x), making objects appear 80 times larger than they do to the bare eye (or 80 times closer, to put it another way).

F

Faculae
Bright patches that are visible on the Sun's surface, or photosphere.

Filament
A strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun.

Finder
A small, wide-field telescope attached to a larger telescope. The finder is used to help point the larger telescope to the desired viewing location.

Fireball
An extremely bright meteor. Also known as bolides, fireballs can be several times brighter than the full Moon. Some can even be accompanied by a sonic boom.

Flare Star
A faint red star that appears to change in brightness due to explosions on its surface.

Focal Length: The length of the effective optical path of a telescope or eyepiece ( the distance from the main mirror or lens where the light is gathered to the point where the prime focus image is formed). Typically expressed in millimeters.

Focal Ratio: The "speed" of a telescope's optics, found by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece or camera. Fast f/4 to f/5 focal ratios are generally best for wide field observing and deep space photography. Slow f/11 to f/15 focal ratios are usually better suited to lunar, planetary, and binary star observing and high power photography. Medium f/6 to f/10 focal ratios work well with either. An f/5 system can photograph a nebula or other faint extended deep space object in one-fourth the time of an f/10 system, but the image will be only one-half as large. Point sources, such as stars, are recorded based on the aperture, however, rather than the focal ratio - so that the larger the aperture, the fainter the star you can see or photograph, no matter what the focal ratio.

Fork Mount: A type of equatorial mount used on short tube catadioptric telescopes in which the telescope tube is mounted between two arms connected to a motor drive. It does not need a counterweight to balance the tube, as with a German equatorial mount. An equatorial wedge and field tripod are used to tilt the scope over at an angle equal to the observer's latitude to align the scope's axis of rotation on the celestial pole for proper tracking. Setting circles are provided to locate celestial objects by their right ascension and declination coordinates. The r.a. setting circle is usually driven by the scope's motor drive to move across the sky at the same speed as the stars, following their apparent motion. This makes fork mount setting circles more convenient to use than the un-powered circles on most German equatorial mounts, as the latter must be readjusted periodically to keep pace with the motion of the stars. Photography near the north celestial pole is difficult with a fork mount.

G

Galactic Halo
The name given to the spherical region surrounding the centre, or nucleus of a galaxy.

Galactic Nucleus
A tight concentration of stars and gas found at the innermost regions of a galaxy. Astronomers now believe that massive black holes may exist in the centre of many galaxies.

Galaxy
A large grouping of stars. Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and contains several billion stars. Some galaxies are so distant the their light takes millions of years to reach the Earth.

Galilean Moons
The name given to Jupiter's four largest moons, Io, Europa, Callisto & Ganymede. They were discovered independently by Galileo Galilei and Simon Marius.

Gamma-ray
The highest energy, shortest wavelength form of electromagnetic radiation.

Geosynchronous Orbit
An orbit in which a satellite's orbital velocity is matched to the rotational velocity of the planet. A spacecraft in geosynchronous orbit appears to hang motionless above one position of a planet's surface.

Giant Molecular Cloud (GMC)
Massive clouds of gas in interstellar space composed primarily of hydrogen molecules. These clouds have enough mass to produce thousands of stars and are believed to be where star formation occurs. Recent observations of M33 (Berkeley-Illinois-Maryland Association (BIMA) Array January 2002) show that these expected star formation sites could not possibly produce stars because the rate of spin of the clouds are between 10 and 100 times slower than that needed for stars to form. Some GMCs are even spinning backwards.

Globular Cluster
A tight, spherical grouping of hundreds of thousands of stars. Globular clusters are composed of older stars, and are usually found around the central regions of a galaxy.

Granulation
A pattern of small cells that can be seen on the surface of the Sun. They are caused by the convective motions of the hot gases inside the Sun.

Gravitational Lens
A concentration of matter such as a galaxy or cluster of galaxies that bends light rays from a background object. Gravitational lensing results in duplicate images of distant objects.

Gravity
A mutual physical force of nature that causes two bodies to attract each other.

Greenhouse Effect
An increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere. Carbon dioxide and water vapour are two of the major gases responsible for this effect.

German Equatorial Mount: A mount used primarily with refractors and reflectors. A counterweight on one side of the polar axis balances the weight of the optical tube on the other. Not as convenient as a fork mount when sweeping from horizon to horizon, as the tube can bump the legs or pedestal mount as the scope passes the zenith, requiring that the tube be "tumbled" or rotated 180° to continue it's tracking of objects down to the western horizon. Its setting circles usually are operated manually. Somewhat more difficult to use and transport than a fork mount telescope, but stable, relatively inexpensive, durable, and capable of astrophotography near the celestial pole.

H

Heliopause
The point at which the solar wind meets the interstellar medium or solar wind from other stars. NASA's Pioneer 10 is expected to reach the heliopause in the coming years.

Heliosphere
The space within the boundary of the heliopause containing the Sun and the solar system.

Hydrogen
An element consisting of one electron and one proton. Hydrogen is the lightest of the elements and is the building block of the universe. Stars form from massive clouds of hydrogen gas.

Hubble's Law
The law of physics that states that the farther a galaxy is from us, the faster it is moving away from us. This is stated due to an apparent relationship between large redshifts and distant galaxies known as the Hubble constant H. Edwin Hubble stated that there is the possibility that the redshift did not mean velocity of recession but might be caused by something else.

Hypergalaxy
A system consisting of a spiral galaxy surrounded by several dwarf white galaxies, often ellipticals. Our galaxy and the Andromeda galaxy are examples of hypergalaxies.

Highest Useful Magnification: The highest visual power at which a telescope can realistically be expected to perform before the image becomes too dim for useful observing (generally about 50x to 60x per inch of telescope aperture). However, turbulence in our atmosphere usually limits the number of nights in which this power is obtainable. Very high powers are best reserved for planetary observations and binary star splitting, as faint nebulas and galaxies appear at their best at relatively low powers (8x to 12x per inch of telescope aperture). On nights of less-than-perfect seeing, medium to low power planetary, binary star, and globular cluster observing (at 25x to 30x per inch of aperture) is often more enjoyable than attempting to push a telescope's magnification to its theoretical limits. Small aperture telescopes can easily use more power per inch of aperture on any given night than larger telescopes, as they look through a smaller column of air and see less of the turbulence in our atmosphere. While some observers use up to 100x per inch of refractor aperture on Mars and Jupiter, the actual number of minutes they spend observing at such powers is small in relation to the number of hours they spend waiting for the atmosphere to stabilize enough for them to use such very high powers.

I

Ice
A term used to describe water or a number of gases such as methane or ammonia when in a solid state.

Inclination
A measure of the tilt of a planet's orbital plane in relation to that of the Earth.

Inferior Conjunction
A conjunction of an inferior planet that occurs when the planet is lined up directly between the Earth and the Sun.

Inferior Planet
A planet that orbits between the Earth and the Sun. Mercury and Venus are the only two inferior planets in our solar system.

Interplanetary Magnetic Field
The magnetic field carried along with the solar wind.

Interstellar Medium
The gas and dust that exists in open space between the stars.

Ionosphere
A region of charged particles in a planet's upper atmosphere. In Earth's atmosphere, the ionosphere begins at an altitude of about 25 miles and extends outward about 250.

Iron Meteorite
A meteorite which is composed mainly of iron mixed with smaller amounts of nickel.

Irregular Galaxy
A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape.

K

Kelvin
A temperature scale used in sciences such as astronomy to measure extremely cold temperatures. The Kelvin temperature scale is just like the Celsius scale except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees Kelvin. Absolute zero, the coldest known temperature, is reached at 0 degrees Kelvin or -273.16 degrees Celsius.

Kepler's First Law
A planet orbits the Sun in an ellipse with the Sun at one focus.

Kepler's Second Law
A ray directed from the Sun to a planet sweeps out equal areas in equal times.

Kepler's Third Law
The square of the period of a planet's orbit is proportional to the cube of that planet's semi-majoraxis; the constant of proportionality is the same for all planets.

Kiloparsec
A distance equal to 1000 parsecs.

Kirkwood Gaps
Regions in the main belt of asteroids where few or no asteroids are found. They were named after the scientist who first noticed them.

Kuiper Belt
The Kuiper belt is a hypothetical massive flattened disc of billions of icy planetesimals beyond the orbit of Neptune. Kuiper Belt objects are believed to be remnants of the original material that formed the solar system. Some astronomers believe Pluto and Charon are Kuiper Belt objects. KBOs are also referred to as Trans-Neptunian Objects or TNOs.

L

Lagrange Point
French mathematician and astronomer Joseph Louis Lagrange showed that three bodies can lie at the apexes of an equilateral triangle which rotates in its plane. If one of the bodies is sufficiently massive compared with the other two, then the triangular configuration is apparently stable. Such bodies are sometimes referred to as Trojans. The leading apex of the triangle is known as the leading Lagrange point or L4; the trailing apex is the trailing Lagrange point or L5.

Lenticular Galaxy
A disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular galaxies tend to look more like elliptical galaxies than spiral galaxies.

Libration
An effect caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon's surface can be seen over a period of time.

Light Year
An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.

Limb
The outer edge or border of a planet or other celestial body.

Local Group
A small group of about two dozen galaxies of which our own Milky Way galaxy is a member.

Luminosity
The amount of light emitted by a star.

Lunar Eclipse
A phenomenon that occurs when the Moon passes into the shadow of the Earth.  A partial lunar eclipse occurs when the Moon passes into the penumbra, or partial shadow.  In a total lunar eclipse, the Moon passes into the Earth's umbra, or total shadow.

Lunar Month
The average time between successive new or full moons. A lunar month is equal to 29 days 12 hours 44 minutes. Also called a synodic month.

Lunation
The interval of a complete lunar cycle, between one new Moon and the next.  A lunation is equal to 29 days, 12 hours, and 44 minutes.

Limiting Magnitude: The magnitude (or brightness) of the faintest star that can be seen with a telescope. The approximate formula for determining the visual limiting magnitude of a telescope is 7.5 + 5 log aperture (in cm). However, this formula does not take into account light loss within the scope due to a secondary mirror obstruction, the reflectivity of the mirrors, seeing conditions, the observers age (visual performance decreases as we get older), etc. The limiting magnitudes specified by manufacturers for their scopes assume very dark skies, trained observers, and excellent atmospheric transparency - and are therefore rarely obtainable under average observing conditions. The photographic limiting magnitude is always greater than the visual (typically by two magnitudes).

M

Magellanic Clouds
Two small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic clouds are visible in the skies of the southern hemisphere.

Magnetic Field
A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles.

Magnetic Pole
Either of two limited regions in a magnet at which the magnet's field is most intense.

Magnetosphere
The area around a planet most affected by its magnetic field. The boundary of this field is set by the solar wind.

Magnification
The ability of a telescope to make a small, distant object large enough to examine in detail. If you look at the Moon (250,000 miles away) with a 125x scope, it's the same as looking at it with your bare eyes from 2000 miles away (250,000 / 125 = 2000). The same telescope used terrestrially will make an object one mile away appear to be only 42 feet away (5280 / 125 = 42).

Magnitude
The degree of brightness of a star or other object in the sky according to a scale on which the brightest star has a magnitude -1.4 and the faintest visible star has magnitude 6. Sometimes referred to as apparent magnitude. In this scale, each number is 2.5 times the brightness of the previous number. Thus a star with a magnitude of 1 is 100 times brighter than on with a visual magnitude of 6. The larger the positive number, the fainter the star or object, with a one digit magnitude change indicating a 256% difference in brightness. 4th magnitude stars are often the faintest visible to the naked eye from a light-polluted suburb. For comparison, 14th magnitude stars (those visible in an 8" scope) are a mere 1/10,000th as bright. 6th magnitude stars are typically the faintest naked eye stars visible from a reasonably dark sky observing site. On extended objects (galaxies and nebulas), the magnitude is the one the object would have if all its light was gathered into a single point, like a star. A good example is M33, the face-on spiral galaxy in Triangulum. It's a 6th magnitude object, but is often difficult to see in even an 8" telescope (whose visual limiting magnitude is 14), because its mag 6 brightness is spread over nearly one square degree of sky. Such an object is said to have low surface brightness and is quite often masked by light pollution when observing from city or suburban sites.

Main Belt
The area between Mars and Jupiter where most of the asteroids in our solar system are found.

Major Planet
A name used to describe any planet that is considerably larger and more massive than the Earth, and contains large quantities of hydrogen and helium. Jupiter and Neptune are examples of major planets.

Mare
A term used to describe a large, circular plain. The word mare means "sea". On the moon, the mare are the smooth, dark-coloured areas.

Mass
A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.

Matter
A word used to describe anything that contains mass.

Meridian
An imaginary circle drawn through the North and South poles of the celestial equator.

Metal
A term used by astronomers to describe all elements except hydrogen and helium.

Meteor
A small particle of rock or dust that burns away in the Earth's atmosphere.  Meteors are also referred to as shooting stars.

Meteor Shower
An event where a large number of meteors enter the Earth's atmosphere from the same direction in space at nearly the same time.  Most meteor showers take place when the Earth passes through the debris left behind by a comet.

Meteorite
An object, usually a chunk or metal or rock, that survives entry through the atmosphere to reach the Earth's surface. Meteors become meteorites if they reach the ground.

Meteoroid
A small, rocky object in orbit around the Sun, smaller than an asteroid.

Millibar
A measure of atmospheric pressure equal to 1/1000 of a bar. Standard sea-level pressure on Earth is about 1013 millibars.

Minor Planet
Another name used to describe a large asteroid.

Molecular Cloud
An interstellar cloud of molecular hydrogen containing trace amounts of other molecules such as carbon monoxide and ammonia.

Maksutov-Cassegrain: A catadioptric telescope that uses a thick and deeply-dished spherical corrector lens to correct for the spherical aberration of its spherical primary mirror - an all- spherical design that keeps its collimation virtually indefinitely. Its typically long focal ratio and small secondary obstruction yield higher contrast and resolution than any other catadioptric or reflector design.

N

Nadir
A term used to describe a point directly underneath an object or body.

Nebula
A cloud of dust and gas in space, usually illuminated by one or more stars. Nebulae represent the raw material the stars are made of.

Neutrino
A fundamental particle produced by the nuclear reactions in stars. Neutrinos are very hard to detect because the vast majority of them pass completely through the Earth without interacting.

Neutron Star
A compressed core of an exploded star made up almost entirely of neutrons. Neutron stars have a strong gravitational field and some emit pulses of energy along their axis. These are known as pulsars.

Newtonian Reflector
This classic 300 year old Sir Isaac Newton design uses a large primary mirror at the bottom of the telescope tube, with a flat diagonal mirror at the top that brings the light out to the Newtonian focus at the side of the tube. Totally color-free, for excellent planetary observing. Offers more light-gathering aperture per pound than any other telescope design, as well, for very good deep space performance.

Newton's First Law of Motion
A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

Newton's Second Law of Motion
For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

Newton's Third Law of Motion
In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.

Nova
A nova is a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star. This occurs in a binary star system when a red giant fills its Roche Lobe and begins accreting gas from the star's outer atmosphere to the white dwarf.

Nuclear Fusion
The nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. Nuclear fusion is the reaction that fuels the Sun, where hydrogen nuclei are fused to form helium.

O

Objective
The main light-gathering lens or mirror of a telescope.

Obliquity
The angle between a body's equatorial plane and orbital plane.

Oblateness
A measure of flattening at the poles of a planet or other celestial body.

Occultation
An event that occurs when one celestial body conceals or obscures another. For example, a solar eclipse is an occultation of the Sun by the Moon.

Oort Cloud
A theoretical shell of comets that is believed to exist at the outermost regions of our solar system though has never been directly detected. The Oort cloud is said to exist at somewhere between 40,000 and 50,000 AU but at this distance, perturbations (either from the outer solar system planets or from nearby stars) to cause "new" comets to begin an orbit around the Sun, are basically impossible. This theory of the origins of comets is the most widely accepted today. The oort cloud was named after the Dutch astronomer who first proposed it.

Open Cluster
A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open clusters are still embedded in the gas and dust from which they formed.

Opposition
The position of a planet when it is exactly opposite the Sun as seen from Earth. A planet at opposition is at its closest approach to the Earth and is best suitable for observing.

Orbit
The path of a celestial body as it moves through space.

P

Parallax
The apparent change in position of two objects viewed from different locations.

Parsec
A large distance often used in astronomy.  A parsec is equal to 3.26 light years.

Patera
A shallow crater with a complex, scalloped edge.

Penumbra
The area of partial illumination surrounding the darkest part of a shadow caused by an eclipse.

Perigee
The point in the orbit of the Moon or other satellite at which it is closest to the Earth.

Perihelion
The point in the orbit of a planet or other body where it is closest to the Sun.

Perturb
To cause a planet or satellite to deviate from a theoretically regular orbital motion.

Phase
The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits.

Photon
A particle of light composed of a minute quantity of electromagnetic energy.

Photosphere
The bright visible surface of the Sun.

Planet
A very large body in orbit around a star. Planets can be composed mainly of rock or of dense gases.

Planetary Nebula
A shell of gas surrounding a small, white star. When a star begins to run out of fuel, its core shrinks and heats up, boiling off the star's outer layers. Leftover material shoots outward, expanding in shells around the star. This ejected material is then bombarded with ultraviolet light from the central star's fiery surface, producing huge, glowing gas clouds in a variety of colours and shapes - planetary nebulas.

Planitia
A low plain.

Planum
A high plain or plateau.

Plasma
A form of ionized gas in which the temperature is too high for atoms to exist in their natural state. Plasma is composed of free electrons and free atomic nuclei.

Precession
The apparent shift of the celestial poles caused by a gradual wobble of the Earth's axis.

Prominence
An explosion of hot gas that erupts from the Sun's surface. Solar prominences are usually associated with sunspot activity and can cause interference with communications on Earth due to their electromagnetic effects on the atmosphere.

Proper Motion
The apparent angular motion across the sky of an object relative to the solar system.

Protostar
Dense regions of molecular clouds where stars are forming.

Pulsar
A spinning neutron star that emits energy along its gravitational axis. This energy is received as pulses as the star rotates.

Q

Quadrature
A point in the orbit of a superior planet where it appears at right angles to the Sun as seem from Earth.

Quasar
Quasars are mysterious point sources of light with very large redshifts. Their redshifts, however, do not conform to the Hubble Law and are not related to the speed of movement away from us, but possibly the age of the quasar itself. Quasars give off both radio and X-ray emissions and have a variable luminosity, the period of which is related to the size of the object. They have been found lying close to active galaxies and may be associated with the formation of new galaxies.

Quasi-Stellar Object
Sometimes also called quasi-stellar source, this is a star-like object with a large redshift that gives off a strong source of radio waves. They are highly luminous and presumed to be extragalactic.

R

Radial Velocity
The movement of an object either towards or away from a stationary observer.

Radiant
A point in the sky from which meteors in a meteor shower seem to originate.

Radiation
Energy radiated from an object in the form of waves or particles.

Radiation Belt
Regions of charged particles in a magnetosphere.

Radio Galaxy
A galaxy that gives off large amounts of energy in the form of radio waves.

Rayleigh Criterion
Lord J. W. R. Rayleigh (1842-1919), the Nobel Prize-winning English physicist, empirically determined that telescope optics that yield 1/4th wave accuracy at the final focus (so that all light gathered by the system comes to a focus within 1/4th the wavelength of green light to which the eye is most sensitive) will produce results on stars that are visually indistinguishable from an optically perfect system. This is known as the Rayleigh Criterion and is a handy yardstick by which telescope quality can be measured. To achieve a 1/4th wave accuracy overall, each mirror in a reflector must be finished to 1/8th wave smoothness. When observing extended deep space objects (such as nebulas and galaxies), most amateur astronomers find it difficult to see any visible difference between optics made at 1/4th wave accuracy and those made to 1/10 wave accuracy - although experienced observers usually find the higher accuracy to be beneficial on planets. Large optics polished to higher levels of accuracy than 1/10th wave usually gain the observer little additional benefit visually, however, as the performance of the telescope will be limited more by atmospheric conditions than it will be by mirror accuracy. Inexpensive scopes can have mirrors polished to 1/4th wave accuracy and still have a rough surface marred by micro-ripple whose errors might be 1/50th wave or less. Such rough mirrors will have visibly lower contrast and less-sharp images than a well-finished mirror.

Recollimation
The need to realign the optical elements of a telescope after the telescope has been disassembled, frequently moved, or given rough treatment. Usually required frequently with reflector telescopes (particularly with large fast focal ratio systems), occasionally with Schmidt-Cassegrains, and almost never with a refractor. Recollimation is very easy with a Schmidt; still relatively easy with a reflector; and difficult and best left to the manufacturer with a refractor or Maksutov.

Red Giant
A stage in the evolution of a star when the fuel begins to exhaust and the star expands to about fifty times its normal size. The temperature cools, which gives the star a reddish appearance.

Redshift
A shift in the lines of an object's spectrum toward the red end. Redshift indicates that an object is moving away from the observer. The larger the redshift, the faster the object is moving.

Refractor
A telescope that uses a lens to bring light to a focus at the end of a long tube.

Resolution
The ability of a telescope to separate closely-spaced binary stars into two objects, measured in seconds of arc. One arc second equals 1/3600th of a degree and is about the width of a 25 cent coin at a distance of three miles. In essence, a measure of how much detail a telescope can reveal. In theory, resolution equals 5.45 arc seconds divided by the aperture of the scope (in inches), so that an 8" scope has a resolution of 0.69 arc seconds, and can show as two joined points binary stars separated by that small an angular distance - refer back to Dawes' Limit above.

Resonance
A state in which an orbiting object is subject to periodic gravitational perturbations by another.

Retrograde Motion
The phenomenon where a celestial body appears to slow down, stop, them move in the opposite direction. This motion is caused when the Earth overtakes the body in its orbit.

Rich Field Telescope (RFT)
A fast focal ratio reflector that gives wide-angle views of star clouds, nebulas, large galaxies, etc. Most large Dobsonians are rich field telescopes.

Right Ascension
The amount of time that passes between the rising of Aries and another celestial object. Right ascension is one unit of measure for locating an object in the sky. Typically, the angular distance of a celestial object east of the vernal equinox, measured in hours and minutes. Simply stated, one of the two coordinates (declination is the other) that let you find celestial objects by using a telescopes setting circles and a star chart or star atlas. If you face the north celestial pole, the stars will rise (ascend) on your right - hence the term "right ascension". The same point on the 360 degree celestial sphere passes overhead approximately every 24 hours, making each hour of right ascension equal to 1/24th of a circle, or 15 degrees. Each degree of sky therefore moves past a stationary telescope in four minutes - a rapid rate when observing at high power.

Ring Galaxy
A galaxy that has a ring-like appearance. The ring usually contains luminous blue stars. Ring galaxies are believed to have been formed by collisions with other galaxies.

Roche Limit
The smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary. At a lesser distance the tidal forces of the primary would break up the secondary.

Roche Lobe
In a binary star system, the Roche Lobe is the boundary of each star's gravitational field. At the point where these lobes touch their gravitational pulls cancel each other out.

Rotation
The spin of a body about its axis.

S

Satellite
A natural or artificial body in orbit around a planet.

Scopulus
A lobate or irregular scarp.

Scarp
A line of cliffs produced by erosion or by the action of faults.

Schmidt-Cassegrain Telescope (SCT)
A catadioptric telescope that uses a thin aspheric corrector lens to compensate for the spherical aberration of its primary mirror.

Seeing
The steadiness of telescopic images due to conditions in the Earth's atmosphere. Seeing is bad when the high altitude winds and temperature differentials cause the image to twinkle or undulate, or appear blurred or distorted - typically when the barometer is low or falling. The seeing is good when the air is still and the image appears sharp and steady - as is the case when there's a high pressure ridge over the observing site. Poor seeing affects the resolution of a telescope, putting an upper limit on the maximum usable magnification on any given night. On most nights, seeing conditions limit the resolution of even large telescopes to no better than five arc seconds or so and bloat small Airy disks into "seeing disks" three or four arc seconds in diameter.

Setting Circles
Circular scales on an equatorial mount telescope that are used to point it at the position (in right ascension and declination) of a celestial object. Setting circles and a star chart make it possible to find objects even when they are too faint to see through the finder scope.

Seyfert Galaxy
A main-sequence star which rotates rapidly, causing a loss of matter to an ever-expanding shell.

Shell Star
A satellite which constrains the extent of a planetary ring through gravitational forces. Also known as a shepherd moon.

Shepherd Satellite
A satellite which constrains the extent of a planetary ring through gravitational forces. Also known as a shepherd moon.

Sidereal
Of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite.

Sidereal Month
The average period of revolution of the Moon around the Earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time.

Sidereal Period
The period of revolution of a planet around the Sun or a satellite around its primary.

Singularity
The centre of a black hole, where the curvature of spacetime is maximal. At the singularity, the gravitational tides diverge. Theoretically, no solid object can survive hitting the singularity.

Solar Cycle
The approximately 11-year quasi-periodic variation in frequency or number of solar active events.

Solar Eclipse
A phenomenon that occurs when the Earth passes into the shadow of the Moon. A total solar eclipse occurs when the Moon is close enough to completely block the Sun's light. An annular solar eclipse occurs when the Moon is farther away and is not able to completely block the light. This results in a ring of light around the Moon.

Solar Flare
A bright eruption of hot gas in the Sun's photosphere.  Solar prominences are usually only detectable by specialized instruments but can be visible during a total solar eclipse.

Solar Nebula
The cloud of dust and gas out of which the solar system was believed to have formed about 13 billion years ago.

Solar Wind
A flow of charged particles that travels from the Sun out into the solar system.

Solstice
The time of the year when the Sun appears furthest north or south of the celestial equator. The solstices mark the beginning of the Summer and Winter seasons.

Spectrometer
The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.

Spectroscopy
The technique of observing the spectra of visible light from an object to determine its composition, temperature, density, and speed.

Spectrum
The range of colours produced when visible light passes through a prism.

Spicules
Grass-like patterns of gas seen in the atmosphere of the Sun.

Spherical Aberration
An optical defect that causes light rays from an object, passing through an o9ptical system at different distances from the optical center, to come to focus at different points along the axis. This can cause a star, for example, to be seen as a discrete disk, but should not be confused with the star's normally smaller Airy disk. Spherical aberration is most often seen in inexpensive small aperture imported reflectors, which use spherical mirrors rather than the costly and more parabolic mirrors found in a quality reflector.

Spiral Galaxy
A galaxy that contains a prominent central bulge and luminous arms of gas , dust, and young stars that wind out from the central nucleus in a spiral formation. Our galaxy, the Milky Way, is a spiral galaxy.

Spotting Scope
A small refractor or catadioptric telescope on an altazimuth mount or photo tripod for terrestrial observing. Usually has an image-erecting prism for correctly-oriented terrestrial views. (Astronomical reflectors have inverted and reversed images, while astronomical catadioptrics have upright mirror image views).

Star
A giant ball of hot gas that creates and emits its own radiation through nuclear fusion.

Star Cluster
A large grouping of stars, from a few dozen to a few hundred thousand, that are bound together by their mutual gravitational attraction.

Star-Hopping
A way to locate celestial objects by moving to them in a series of small 4 or 5 degree steps or "hops" from a known star or object, using the 4 or 5 degree field of view of a conventional finder scope or non-magnifying illuminated finder to follow a path previously marked out on a star chart.