Astronomy Glossary

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
The actual brightness of a star. It is defined as the apparent magnitude a star would have if it were located at a distance of 10 parsecs, and is often determined by measuring a star's temperature and comparing it to the Hertzsprung-Russell diagram.

absorption line
A narrow dark line within a spectrum. It is caused by specific gases absorbing a portion of radiation, and its width represents the relative abundance of the gases.

accretion
The process of growth of an object due to the gradual accumulation of material.

achromatic refractor
A long, slender telescope which uses a main lens to focus the incoming light into an eyepiece. Achromatic refractors do not have perfect optics and produce a certain degree of colour surrounding bright objects.

active galaxies
Energetic galaxies that release huge amounts of energy, mostly in the form of long-wavelength radiation. The two most common types are Seyfert galaxies and Radio galaxies.

adaptive optics
Recent technology utilized by the most advanced telescopes in the world. Computers measure the atmospheric turbulence and thousands of actuators slightly deform the mirror to compensate for it.

airglow
The background light in the night sky caused by the atmospheric scattering of man-made light.

albedo
The fraction of incoming light that is reflected by a celestial body. An albedo of one reflects all incoming light while an albedo of zero absorbs all light.

altazimuth
A telescope mount which has its two axis' of movement aligned with the horizon and the zenith.

altitude
The height in degrees than an object is above the horizon an a given location. Zero degrees is at the horizon while 90 degrees is overhead.

Andromeda Galaxy
The nearest major galaxy to the Milky Way. The Andromeda Galaxy is approximately 2.2 million light years away and is very similar in appearance to our galaxy.

angular size
A measure of the apparent size of an object in the sky, measured in angles.

annular eclipse
A solar eclipse in which the Moon is near apogee and is, therefore, too small to cover the whole disk of the Sun, leaving a visible edge or ring of sunlight.

aperture
The size of the main mirror or lens of a telescope. The aperture of a telescope is a measure of its light gathering power.

aphelion
The point in an orbit where a planet is at its furthest distance from the Sun. Opposite of perihelion.

apogee
The point in its orbit where the Moon is at its furthest distance from the Earth. Opposite of perigee.

Apollo
The American space program in the late 1960's and early 1970's which sent six successful manned missions to the surface of the Moon.

arc minute
A unit of measurement that describes the apparent angular size of an object. It is equivalent to one sixtieth (1/60) of a degree. The angular size of the Moon is about 30 arc minutes.

asterism
A pattern formed by a collection of stars within a constellation.

asteroid
A large piece of rock, generally between 100 metres and several hundred kilometres across. Also known as a minor planet.

asteroid belt
The belt of asteroids between the orbits of Mars and Jupiter where most asteroids are located.

astrology
The non-scientific study of the relationship between the planets and the stars, and their supernatural effects on life on earth.

Astronomical Unit (AU)
A measure of distance equal to the average distance between the Earth and the Sun. It is defined as 149,597,870.691 kilometres.

astronomy
The scientific study of the universe and all the matter beyond the Earth.

asynchronous orbit
An orbit which is not fixed over a particular location on the surface of the Earth. Satellites in an asynchronous orbit pass overhead at different times each day.

atmosphere
The layer of gas which surrounds a celestial body. Stars, many planets, and a few satellites have atmospheres, and vary in their composition.

atoms
The building blocks of all matter, composed of positively charged protons and neutral neutrons in the nucleus, surrounded by negatively charged electrons.

aurora
The colourful displays in the upper atmosphere which occur when molecules are energized by charged particles in the solar wind. The light is emitted as the molecules release energy when they return to their original energy state.

aurora australis
The aurora in the Southern Hemisphere, also known as the Southern Lights.

aurora borealis
The aurora in the Northern Hemisphere, also known as the Northern Lights.

autumnal equinox
The point at which the Sun crosses the celestial equator moving southward. It occurs around September 22 and marks the first day of autumn in the Northern Hemisphere.

azimuth
The coordinate which describes an object's location in relation to its compass direction, expressed in degrees between zero and 360.

B

barred spiral galaxy
A galaxy with a bar of interstellar material passing through the central bulge. Spiral arms of stars and interstellar material originate from the ends of the bars.

Big Bang
The theory of cosmology which describes the explosive creation of the universe from a singularity, expanding into its current form.

binary star
A system of two stars in orbit around each other.

blackbody
An object which does not reflect any radiation, it is all absorbed.

black dwarf
A ball of gas which is not capable of nuclear fusion within its core, which is required to produce energy. A black dwarf results after a white dwarf has exhausted its fuel source, or after a gas cloud condenses but does not have the mass required to ignite nuclear fusion within its core.

black hole
An incredibly dense object whose gravity is so strong that not even light can escape from it. Black holes warp space and time around them.

Bondar, Roberta
The first Canadian woman in space, aboard the Space Shuttle Discovery in 1992.

Brahe, Tycho (1546 - 1601)
Danish astronomer whose extremely detailed observations of the planets helped Johannes Kepler formulate the laws of planetary motion. Brahe also observed a supernova explosion which contradicted the Church's theory of a static universe.

C

Canada-France-Hawaii Telescope (CFHT)
A 3.6 metre telescope in Hawaii used by many Canadian astronomers for research.

Canadarm
The Canadian robotic arm which was deployed on the Space Shuttle in 1981.

Canadarm2
The second generation Canadarm, a larger and more advanced version which is involved in the construction of the International Space Station. It is capable of supporting heavier loads and is not fixed at either end, allowing it to travel around the ISS.

Canadian Galactic Plane Survey (CGPS)
A project of the DRAO which involves the study of the galactic plane in radio waves in order to better understand the interstellar medium.

carbon asteroid
Dim asteroids which are located near the outer regions of the asteroid belt. The majority of asteroids are of this type.

Cassini
A spacecraft en route to Saturn, scheduled to arrive in 2004. Named after the astronomer who discovered the dark gap between the two main rings of Saturn.

celestial body
Any object beyond the Earth and visible in the sky.

celestial equator
The imaginary line dividing the northern and southern hemispheres of the celestial sphere. Equivalent to the equator on the Earth.

celestial pole
The two poles (north and south) of the celestial sphere. The stars rotate around the stationary poles.

celestial sphere
The imaginary sphere on which the stars and deep sky objects lie. Although an inaccurate representation of the three dimensional universe, the celestial sphere is a useful way to represent the night sky.

Cepheid variable
A variable star whose luminosity and period are related, and are used as an indicator of the star's distance.

Chandra X-ray Telescope
The most sophisticated X-ray observatory ever built, it was launched and deployed into Earth orbit by the Space Shuttle in 1999.

charge-coupled device (CCD)
A computer chip with thousands of light-gathering pixels used in place of a photographic plate. CCD's have greatly enhanced the capabilities of both amateur and professional telescopes.

chromosphere
The lower layer of the atmosphere of the Sun.

circumpolar
Areas of the night sky which do not travel below the horizon over the course of a year, from a particular location on the Earth. Further from the equator, more of the sky is circumpolar.

collision ejection theory
A theory of the formation of the Moon. It states that the Moon was formed after a large asteroid collided with the Earth and the ejected material cooled and formed our Moon.

Colour Magnitude Diagram
A plot of magnitude versus temperature for a specific group of stars within the same star cluster. A colour magnitude diagram gives important information regarding a star cluster, such as age and distance.

coma
The bright envelope of vaporized gases surrounding the nucleus of a comet.

comet
A chunk of dirty ice and snow in orbit around the Sun. Comets have highly eccentric orbits and are composed of the nucleus, the coma, and an ion tail and dust tail.

condensation theory
A theory which describes the slow collapse of a nebular cloud and interstellar dust into a protostar along with planet formation.

constellation
One of 88 divisions in the night sky, each containing an asterism of stars. Constellations were officially set in 1930 and are used to organize the sky into regions.

convection zone
The region of the interior of the Sun which lies just below the surface. Hot material is brought up to the surface and cooler material flows down towards the centre in a constant cycle.

Copernicus, Nicolaus (1473 - 1543)
Polish astronomer who proposed the heliocentric model of the solar system after analyzing recorded observations of Tycho Brahe.

corona
The thin and hot upper atmosphere of the Sun, visible only with special filters or during a total solar eclipse.

Cosmic Background Radiation
The nearly uniform radiation received from all regions of the sky. It is a radio signal with a temperature of 2.7K, and is thought to be the cooled afterglow of the Big Bang.

Cosmic Distance Ladder
A collection of progressive distance measuring techniques that build upon each other to determine distances within the universe.

cosmology
The scientific study of the formation, evolution and structure of the universe.

crater
A circular divot caused by the impact of a rocky object on a celestial body. The size of the crater depends on the mass and velocity of the incoming object.

crescent phase
The collective term for the phases of a celestial body when it is less than 50% illuminated. It most often refers to the Moon, but is also applied to the inferior planets.

D

dark nebula
A nebula which does not have an embedded star to illuminate the gases. Instead, a dark nebula blocks the light from background stars and creates a dark patch.

declination
A component of the coordinate system used to designate positions on the celestial sphere. Declination is analogous to latitude on the Earth, and ranges from +90 degrees to -90 degrees.

deep sky object
An object in our night sky other than the bodies of the solar system and individual stars. Deep sky objects include nebulae, star clusters, and galaxies.

density
The mass of an object divided by its volume.

differential rotation
A rotational property of gaseous objects, where the equatorial regions rotate at a faster rate than the polar regions.

disk
The flattened structure of a circular collection of material, which often refers to the overall structure of a spiral galaxy. A disk also refers to the apparent circular shape of the Sun and the Moon, or of a planet when viewed through a telescope.

diurnal motion
The apparent daily motion of the objects of the solar system against the background stars.

Dobsonian reflector
A reflecting telescope which has a very simple mount that allows the telescope to move with respect to the horizon and vertically towards the zenith.

Dominion Radio Astrophysical Observatory (DRAO)
A collection of radio telescopes in Penticton, BC. The telescopes are involved in important astronomical research of the universe, including the study of the interstellar medium.

double star
Two stars which are located in the same line of sight from the Earth. They may be binary stars or they may be unrelated and simply lie in the same area of the sky.

Dreyer, John Louis Emil (1852-1926)
Expanded the General Catalogue by John Herschel with the publication of the New Galactic Catalogue in 1888.

dust
The small particles which occupy the interstellar medium.

dust tail
The tail of a comet which is caused by radiation pressure forcing the dust particles away from the coma in a curved arch.

E

Earth
Our home planet and the third from the Sun. The only planet with abundant water in liquid form and a vast biodiversity.

eccentric
Often applied to an orbit which is neither circular nor elliptical.

eccentricity
A value between zero and one which represents the shape of an ellipse or an orbit. A low eccentricity is near zero and is a perfect circle, while a high eccentricity is near one and very oval.

eclipse
The event in which one celestial body passes in front of another, blocking the light from the more distant object. An eclipse can refer to the Moon passing in front of the Sun, but can also be applied to a star in a binary system which passes in front of the other star. An eclipse may also be the passage of one body through the shadow of another, such as a lunar eclipse.

ecliptic
The imaginary line on which the Sun travels through our sky.

ellipse
A flattened circle. Johannes Kepler first postulated that the orbits of celestial bodies are elliptical and not circular.

elliptical galaxy
A type of galaxy which is oval in shape but has no apparent structure which contains old stars with little interstellar matter.

elongation
The measure of the angular distance between the Sun and the inferior planets in our sky. Maximum elongation is the time of greatest separation and is a prime viewing period for the inferior planets.

emission nebula
A glowing cloud of hot interstellar gas which is energized by imbedded hot stars.

equatorial mount
A telescope mount which is aligned to the celestial pole, so that one axis moves the telescope in right ascension and the other moves it in declination. An equatorial mount easily tracks the movement of the sky.

equilibrium
A steady state which will not change unless an external force or event disrupts the stability.

extinction
The dimming effect of the interstellar medium on starlight passing through it.

eyepiece
An optical instrument constructed of lenses which magnifies and focuses the incoming light gathered from a telescope.

F

Far Ultraviolet Spectroscopic Explorer (FUSE)
An orbiting observatory designed to observe radiation in the far ultraviolet portion of the spectrum. It is the most sensitive observatory yet in this portion of the spectrum, and was launched by NASA in 1999 aboard a Delta 2 rocket.

fireball
A very bright meteor which can be much brighter than any star.

finderscope
A small, simple telescope on top of a telescope used to orient the position of the larger telescope, which has a much higher magnification and smaller field of view.

focal length
The length of the path of light from the main mirror or lens to the focal point (the eyepiece).

frequency
A property of a wave that describes how many wave patterns or cycles pass by in a period of time. Frequency is often measured in Hertz (Hz), where a wave with a frequency of 1 Hz will pass by at 1 cycle per second.

full moon
The phase of the Moon in which it is fully illuminated and rises at sunset and sets at sunrise.

fusion crust
The melted exterior of a meteorite caused by the intense heat of entering the Earth's atmosphere.

G

Gagarin, Yuri
The first human in space, a Soviet aboard Vostok 1 in 1961.

galactic bulge
The central bulge of a spiral galaxy, containing a massive amount of interstellar material and most likely a super-massive black hole in the centre.

galactic halo
The circular region surrounding the disk of a spiral galaxy. The halo is thin and is composed mostly of old globular clusters.

galaxy
A system of millions or billions of stars and interstellar gases and material, held together by gravity.

galaxy cluster
A group of galaxies held in proximity due to their mutual gravitational attraction. A galaxy cluster may contain several galaxies up to thousands of galaxies.

Galilei, Galileo (1564 - 1642)
An Italian scientist, Galileo is most well known as the first to construct a telescope and use it to observe the night sky. His observations led to numerous new and important discoveries in astronomy. However, his publications led to his downfall, as he was placed under house arrest for the last several years of his life by the Church.

Galileo
The first space probe to enter the atmosphere of Jupiter in 1995. On its journey to Jupiter, Galileo was also the first probe to flyby an asteroid, flying by both Gaspra in 1991 and by Ida in 1994.

gamma ray
Radiation with the highest amount of energy.

gamma-ray burst
A random burst of energy from distant objects in the form of gamma rays. Gamma-ray bursts release an enormous amount of energy and are not clearly understood.

Garneau, Marc
The first Canadian in space, aboard the Space Shuttle Challenger in 1984.

gaseous planet
A planet composed of thick layers of gases surrounding a dense core.

Gemini telescope
The new state of the art telescope in Chile which was funded in part by the National Research Council of Canada.

geocentric
The theory which stated that the Earth was the centre of the solar system, believed to be an accurate description of the solar system until the 17th century.

geostationary orbit
An orbit which matches the rotation of the Earth, and as a result remains stationary over the same point of the Earth's surface. A geostationary orbit, which can also be called a geosynchronous or synchronous orbit, is located at an altitude of about 35,800 kilometres above the equator.

gibbous phase
The collective term given to the lunar phases between the quarter phases and full moon. A gibbous moon is illuminated by more than 50%.

Giotto
The probe which first visited a comet (Comet Halley in 1986) and took the first detailed images of a cometary nucleus.

global positioning system (GPS)
A navigation system which uses satellites to determine precise locations on the surface of the Earth.

globular cluster
A large collection of millions of old stars that are held in close proximity due to their mutual gravitational attraction. Globular clusters are located in the galactic haloes of galaxies.

gnomon
The central piece of a sundial which casts the shadow required to determine the time of day.

gravitational collapse
When a massive body collapses due to its own mass. Gravitational collapse occurs in stars after the internal pressure cannot support the inward force of gravity.

gravitational field
The area surrounding an object which is under the influence of its gravitational pull. A more massive object will have a larger and stronger gravitational field.

gravitationally bound
Objects held together because of their mutual gravitational attraction.

gravity
The mutual force of attraction between two massive objects.

Great Red Spot
The large storm that has been observed on Jupiter for the last several hundred years.

greenhouse effect
The process in which heat is allowed to enter the atmosphere of a planet but cannot escape.

H

Hadfield, Chris
Canadian astronaut who was a Mission Specialist on Space Shuttle missions in November 1995 and in April 2001, when he installed Canadarm2. He was the first Canadian astronaut to perform a spacewalk.

heliocentric
The true model of the solar system in which the Sun is the centre around which the planets orbit.

helioseismology
The study of the interior of the Sun.

helium
The second most abundant element in the universe. Helium is the by-product of nuclear fusion involving hydrogen.

Herschel, William (1738 - 1822)
An astronomer who continually constructed the world’s largest telescopes in the 18th century. He is most well known for discovering Uranus in 1781.

Hertzsprung-Russell Diagram (H-R Diagram)
A plot of luminosity versus temperature for a group of stars.

Hubble, Edwin (1889 - 1953)
American astronomer who helped prove the true nature of galaxies, previously thought to be nebulae located within the Milky Way. He also discovered the linear relationship between a galaxy's distance and the speed with which it is moving, known as Hubble's Law.

Hubble's constant
A constant which defines the relation between an object's recessional velocity and its distance.

Hubble Space Telescope (HST)
The Hubble Space Telescope is an observatory in orbit around the Earth. HST is named in honour of Edwin Hubble, and has provided astronomers with unprecedented detailed images of our universe.

Hubble tuning fork
The diagram created by Edwin Hubble to classify galaxy types into spiral, barred spiral, elliptical and irregular.

hydrogen
The lightest and most abundant element. Average stars like the Sun are composed mostly of hydrogen, which is being converted into helium.

I

inclination
The angle between the orbital plane of an object and the equatorial plane of the parent object.

inferior planet
A planet located closer to the Sun than the Earth, namely Mercury and Venus.

infrared radiation
Radiation with wavelengths longer than the red end of visible light but shorter than microwaves. Infrared radiation is best observed above the atmosphere, and is a measure of the warmth of gases.

Infrared Space Observatory (ISO)
A space telescope designed to observe in the infrared region of the spectrum. ISO was launched in 1995 and is a project of the European Space Agency.

internal pressure
The outward force generated by heat within stars to resist the collapse of the star. Internal pressure is essential for a star to remain in a state of equilibrium.

International Space Station (ISS)
The major international project which is currently being built by while in orbit around the Earth. The ISS will be home to numerous astronauts and scientists conducting valuable research.

interstellar medium
The space between stars within a galaxy. The interstellar medium is sparsely filled with gas molecules and dust particles.

intrinsic variable star
A star whose apparent brightness will vary with respect to time.

ion
A small particle which has a magnetic charge.

ion tail
The straight tail of a comet which is generated by particles from the solar wind.

irregular galaxy
One of the four categories of galaxies as defined by Hubble. Irregular galaxies have no consistent structure and are large gatherings of young stars and interstellar gases.

island universe
The name given to galaxies before their true nature was understood.

J

Jovian planet
A large and gaseous planet in our solar system, named for its resemblance to Jupiter.

K

Keck telescopes
The world's largest optical telescopes, Keck 1 and Keck 2 both have mirrors 10 metres in diameter and are located on Mauna Kea in Hawaii.

Kelvin
The SI unit of temperature, it is a linear relation to the centigrade scale and refers to the temperature above absolute zero (-273.15 degrees Celsius).

Kepler, Johannes (1571 - 1630)
The German astronomer and mathematician who formulated the three laws of planetary motion, helping prove the heliocentric model of the solar system.

kilogram
The SI unit of mass.

Kuiper Belt
The region beyond the orbit of Neptune that contains thousands of icy bodies thought to become short-period comets.

L

light
The common term given to radiation. Although it generally refers to the visible portion of radiation, radiation of other wavelengths is also referred to as light.

light curves
A plot of the variation of a star's brightness with respect to time.

light year
The distance traveled by light in one year, equal to approximately 9,460,536,000,000 kilometres. A parsec is another common unit of distance in astronomy, and is equal to 3.26 light years.

liquid-mirror telescope
A telescope which uses liquid as a reflecting surface. The liquid is spun at a rate which causes it to become a perfect parabolic surface.

Lagrange Point
A unique point in the plane of two orbiting bodies where a third body experiences no forces and can remain effectively motionless in equilibrium.

Local Group
The galaxy cluster of about 20 galaxies that the Milky Way belongs to.

long-period comet
A comet with an long orbital period around the Sun. The majority of comets discovered each year are long-period comets which had not been previously documented.

Lord Rosse (1800 - 1867)
An Irishman who proposed the theory of island universes in the mid 1800's after observing spiral structure in several deep sky objects though to be nebulae.

luminosity
A measure of the rate at which a star releases energy.

lunar cycle
The cycle of the lunar phases during one synodic month.

lunar eclipse
An eclipse which occurs when the Moon passes through the Earth's shadow.

M

Magellan
The spacecraft which traveled to Venus in the 1990's and accurately imaged the surface using radar.

magnetic field
The region surrounding an object which has a changing electric field. Most stars and planets have magnetic fields which will affect charged particles coming in contact with them.

magnetosphere
The extension of a celestial body's magnetic field into a region of space surrounding the body. Charged particles become trapped within the magnetosphere.

magnification
The degree to which an object is enlarged by an optical instrument such as a telescope.

magnitude
The apparent brightness of an object in our sky. It is a logarithmic scale with negative numbers being the brightest.

maria
The dark smooth "seas" on the surface of the Moon.

Mars Pathfinder
The highly successful NASA probe sent to the surface of Mars, arriving in 1997 and giving scientists valuable information about the planet.

mass
A measure of the amount of material of an object.

matter
A term used to describe anything with mass.

meridian
The imaginary line that runs from the horizon to the zenith while looking directly south. When an object in on the meridian, it is at its highest point in its path across the sky.

Messier, Charles (1730 - 1817)
A French astronomer in the 1700's who generated a list of approximately 100 fuzzy patches of light in the sky. The list was compiled for reference so as to not confuse the objects as comets. This list is called Messier's List and is now the most well known catalogue, containing 110 of the finest deep sky objects.

meteor
A bright streak of light in the sky caused by a dust particle or small meteorite as it enters our upper atmosphere.

meteor shower
An annual display of several to hundreds of meteors visible every hour. A meteor shower occurs because the Earth passes through the debris left by a comet.

meteor storm
An exceptional meteor shower can be termed a storm, often occurring after the comet responsible for the shower has recently passed by the Earth and left a fresh trail of dust particles.

meteorite
A meteor which survives the passage through the atmosphere and falls to the Earth.

meteoroid
A small rocky object in orbit around the Sun.

Microvariability and Oscillations of Stars (MOST)
A small Canadian satellite which will be the most sensitive light gathering telescope when it is launched into Earth orbit.

microwave
Radiation which has a longer wavelength than visible light. Microwaves are often used to communicate with satellites and space probes.

Milky Way
Our home galaxy, seen as a luminous band of cloud stretching across the sky.

Mir
The space station of the former Soviet Union, launched in 1986 and ending service in 2001 after over 86,000 orbits of the Earth.

Mobile Remote Servicer Base System (MBS)
The movable platform on the ISS which supports Canadarm2.

Mobile Servicing System (MSS)
The Canadian contribution to the ISS.

Moon
The Earth's satellite. When visible it is the brightest object in the night sky.

mount
A device used to support a telescope.

N

neap tide
The tide which occurs near a quarter lunar phase, causing the variance between high and low tide to be minimal.

nebula
A collection of interstellar dust and gas which is illuminated by nearby or imbedded stars, or in the case of a dark nebula, blocks the light from background stars.

nebular cloud
A gaseous cloud of interstellar material.

nebular theory
A theory that describes the slow collapse of a nebulous gas cloud into a protostar.

neutron star
The core remnant left by a massive star after a supernova explosion. It is a densely packed ball of neutrons a few kilometres across with a mass greater than the Sun.

new moon
The lunar phase which occurs when the Moon and Sun are in the same location in the sky. The moon is, therefore, not visible as the far side of the Moon is illuminated.

Newton, Isaac (1642 - 1727)
The English scientist who formulated the three classic laws of motion along with the universal law of gravitation.

Newtonian reflector
A reflecting telescope which uses a mirror to focus the incoming light into and eyepiece, and uses an equatorial mount.

Next Generation Space Telescope (NGST)
A large orbital telescope which is currently in the planning phase. It will be larger than the HST and will give unprecedented views of the universe. It has officially been named the James Webb Space Telescope (JWST).

novae
A star that suddenly expels a huge amount of energy, experiencing a rapid increase in its luminosity which slowly fades back to its initial state.

nuclear fusion
The process in which atomic nuclei bond and create heavier elements, which releases a large amount of energy in the form of heat and light. Hydrogen fuses into helium in the core of most stars.

nucleus
The core of an object, which can include a galaxy, star, comet or an element.

O

oblate
A non-circular object, such as the flattening of a gaseous planet due to a rapid rotation rate.

occultation
The passage of an object in front of another, blocking the light from the background object. For example, the Moon can occult a star or planet, or an asteroid can occult a star.

Oort cloud
The theorized cluster of long-period comets located approximately 50,000 AU's from the Sun.

orbit
The elliptical path of an object that is gravitationally bound to another object.

open cluster
A loose collection of hundreds of stars that are gravitationally bound to each other. Also known as a galactic cluster.

opposition
When a superior planet is directly opposite from the Sun in our sky, and is closest to the Earth.

optics
The mirrors and/or lenses used to gather and focus the light inside a telescope.

P

parsec
A unit of distance used in astronomy which is equivalent to 3.26 light years, or more than 30 trillion kilometres.

partial eclipse
An eclipse as seen in the penumbral shadow where the eclipsed object is not completely covered.

penumbra
The outer portion of the shadow during an eclipse. A partial eclipse will occur as the eclipsed object is not completely covered. The penumbra also refers to the outer region of a sunspot.

perigee
The point in its orbit where the Moon is at its closest to the Earth. Opposite of apogee.

perihelion
The point in its orbit where a planet is at its closest to the Sun. Opposite of aphelion.

period-luminosity relationship
The predictable relation between the pulsation period and changes in luminosity of a Cepheid variable star.

perturbation
Small deviances in the location of a star, visible through large telescopes.

photon
A component of light which is considered a particle with zero mass.

photosphere
The surface of the Sun.

Pioneer
Pioneer 10 and 11 were the first probes to explore the outer planets in the 1970's.

Pioneer Venus
The NASA probe to first map the surface of Venus in the late 1970's.

planet
An object which does not produce its own energy and orbits around a star.

planetary conjunction
A close visual grouping of two or more planets in the night sky.

planetary nebula
A nebula which is formed by the expelled gases from a low mass star, being illuminated by the remnant white dwarf.

plate tectonics
The motions of the plates which make up the crust of the Earth.

pointer stars
Known stars which can be used to find other stars or constellations.

polar orbit
An orbital path that will cross both geographic poles with each complete Earth orbit. A polar orbit remains fixed in space as the Earth orbits beneath it, allowing the satellite to eventually scan the entire surface.

Polaris
The star nearest the north celestial pole, also known as the North Star. It is a common misconception that the North Star is the brightest in the sky.

prism
A transparent body with two nonparallel sides which refracts or disperses light into its component colours. White light is separated into a spectrum of colour upon exiting a prism.

prominence
A looping column of gas ejected from the surface of the Sun.

protoplanet
The initial stage of planetary formation. A protoplanet is a small clump of material that may accumulate material to become a planet.

protostar
The initial stage of stellar formation. A protostar generates energy but its core is not hot enough to ignite nuclear fusion.

Ptolemy (87 -150 AD)
An ancient Greek astronomer who believed the planets and Sun orbited the Earth in the order Mercury, Venus, Sun, Mars, Jupiter, Saturn. Also wrote Algamest, the main astronomical handbook for over a thousand years.

pulsar
A star that emits radiation in regular pulses of energy. A pulsar is a rapidly rotating neutron star that is oriented so that the pulses of energy are aligned with the Earth.

Q

quarter moon
The phase of the Moon in which it is illuminated 50%. First quarter occurs between new moon and full moon and is illuminated on the right side, and last quarter occurs between full moon and new moon and is illuminated on the left side (from the northern hemisphere).

quasar
An incredibly luminous object in the distant universe which releases an enormous amount of energy. Because of their immense distances, quasars appear as star-like points of light in the largest telescopes, and they are not fully understood.

R

radar
The process of bouncing radio waves off a body to determine the distance to its surface. Radar is an acronym for RAdio and Detection And Ranging.

RADARSAT
A Canadian built Earth-observation satellite that uses an advanced radar sensor that can image the Earth's surface regardless of weather or light conditions.

RADARSAT-2
An advanced Earth observation satellite, which will give greater resolution and more features than the first RADARSAT. It is set for launch in 2004.

radiation
Energy released as either waves or particles.

radiation pressure
The force exerted on an object by light particles (photons).

radiation zone
The inner region of the solar interior.

radio waves
Radiation with the lowest amount of energy. Because of the long wavelengths, radio waves are not easily scattered and penetrate through clouds and gas.

rainbow
A coloured arc that contains the colours of the spectrum, formed when the Sun's light is refracted and reflected by liquid water in the atmosphere.

red giant
A relatively cool star which is near the end of its life, swelling in size after the inner core has been depleted of hydrogen.

redshift
The shift in light emitted by an object which is receding from the Earth. The radiation is shifted towards the red portion of the spectrum, and a more distant object will have a higher redshift.

reflection nebula
A nebula which is illuminated by reflecting the light of nearby stars.

reflector
A telescope which uses a parabolic mirror at its base to collect light and focus it into an eyepiece for viewing.

refractor
A telescope which uses a lens at the top of the telescopic tube to focus incoming light into a viewable image.

resolution
A measure of the amount of detail visible in an image. A large telescope has a higher resolution than a smaller one, and can, therefore, see greater detail in objects.

retrograde
Motion which is backwards as compared to the standard direction. It refers either to the motion in our sky, or to an object's orbit around the Sun or on its own axis.

right ascension
A component of the coordinate system used to designate positions on the celestial sphere. Right ascension is analogous to longitude on the Earth, and is designated as units of hours, minutes and seconds, the sky being broken up into 24 hours.

Roche limit
The smallest distance from a planet at which an object will not be broken up by tidal forces. An object nearer than the Roche limit will most likely be broken into smaller pieces.

S

satellite
The term given to the moons in orbit around the planets, or any man-made object in orbit around the Earth.

Schmidt-Cassegrain
The most popular recreational telescope because of its compact design and its wide range of applications. It uses two mirrors and a corrector lens which allow the telescope tube to be short for large apertures.

shockwave
A strong wave of gases released by a star which can disturb surrounding regions.

short-period comet
A comet which has a short orbital period around the Sun, allowing its return to our skies to be anticipated.

SI
See Système Internationale d'Unités

sidereal day
The length of time for the Earth to complete one rotation on its own axis, with respect to the stars.

sidereal month
The length of time for the Moon to complete one orbit of the Earth with respect to the background stars.

silicate asteroid
A reflective type of asteroid which inhabits the inner portions of the asteroid belt.

singularity
The hypothetical condition of a black hole. It is a point of mass which has infinite density.

solar eclipse
An eclipse which occurs when the Moon passes in front of the Sun, blocking its light in either a partial or total eclipse.

solar flare
A violent eruption of gases into the solar atmosphere from the solar photosphere.

solar mass
Defined as a unit of mass equal to the mass of the Sun. It is useful to define the mass of stars.

solar nebula
The cloud of nebulous gas that surrounded the early Sun as it developed through the initial stages of its formation.

solar system
Any system of planets and other objects in orbit around a star. Our solar system includes the nine planets along with the numerous asteroids and comets in orbit around the Sun.

solar wind
The constant flow of charged particles from the Sun, extending throughout the solar system.

solid planet
A planet composed of rocky materials with relatively thin or non-existent atmospheres.

space junk
Small inoperable pieces of man-made material in orbit around the Earth. Space junk is the debris left behind by satellites and manned missions.

Space Station Remote Manipulator System (SSRMS)
The major component of the Canadian contribution to the ISS, the SSRMS is a robotic arm used to maneuver heavy equipment around the outside of the Station (also known as Canadarm2).

spacewalk
When an astronaut leaves an orbiting spacecraft and floats free in space while in a special spacesuit.

Special Purpose Dextrous Manipulator (SPDM)
A component of the Canadian contribution to the ISS, the SPDM is an armed robot which is used to maneuver objects too small and delicate for the Canadarm2.

spectral line
The frequency of light emitted by a specific atom or molecule. Each atom or molecule gives off a unique frequency, allowing astronomers to detect the presence of individual elements within a star's atmosphere by observing its spectrum.

spectrometer
An instrument used to separate the incoming light from objects into component frequencies.

spectroscopic parallax
A method of determining distances to stars by comparing their absolute and apparent brightnesses.

spectroscopy
The study of stellar spectra in order to determine the chemical composition of stars.

spectrum
An image of light broken up into its component frequencies, appearing as a rainbow of colours corresponding to the various frequencies.

speed of light
The fastest known speed possible at 299,792.458 kilometres per second.

spiral arms
The curved bands of material spiraling out from the centre of a galaxy, composed of young stars.

spiral galaxy
A galaxy with spiral arms which originate from the central galactic bulge.

spring tide
The tide which occurs near new or full moon, when the Moon and Sun are in alignment, thereby causing the greatest ranges between high and low tide.

Sputnik 1
The first man-made satellite to orbit the Earth, launched by the former Soviet Union in 1957.

standard candle
An object for which the absolute brightness can be determined and then used to determine distance.

star
A massive ball of gas which produces its own energy by means of nuclear fusion occurring within its core.

star cluster
A gathering of stars in a relatively small region, which are gravitationally bound to each other.

star party
A gathering of amateur astronomers at a dark site to observe the night sky with their telescopes.

stellar classification
The classification system of stars based on their temperature, as determined by their spectrum.

stellar parallax
A technique that measures the slight shift in nearby stars to determine their distances. It is often simply termed parallax.

Stonehenge
An ancient architectural design of pillars believed to be connected to the motions of the stars and planets.

summer solstice
The point along the ecliptic when the Sun is furthest north of the celestial equator, giving the northern hemisphere its longest day of the year. It occurs around June 21 and marks the first day of summer in the northern hemisphere.

Sun
The star at the centre of our solar system, providing the light and heat required for life on Earth.

sundial
A device used to determine the time of day by observing shadows cast by a gnomon.

sunspot
A dark blemish on the surface of the sun which is cooler than the surrounding regions. Although not fully understood, sunspots are known to be connected to the solar magnetic field.

super cluster
A huge cluster of galaxies. It can either be a large grouping of thousands of galaxies, or a cluster of individual galaxy clusters.

superior planet
A planet in our solar system that is further from the Sun than the Earth.

supernova
The violent death of a high mass star, occurring when nuclear fusion within the interior of the star can no longer produce the heat required for equilibrium. The star collapses and then rebounds in a massive and luminous explosion.

supernova remnant
The gases left behind after a supernova explosion, which are often illuminated by nearby stars.

synchronous orbit
A stable orbit due to tidal forces, in which the orbiting body has the same orbital period as rotation period, meaning the same face will always be directed towards the parent object. The Moon has a synchronous orbit around the Earth.

synodic month
The length of time for the Moon to go through one complete lunar cycle. It is the orbital period of the Moon with respect to the Sun.

Système Internationale d'Unités (SI)
The international system of units used for measurements of the properties of objects.

T

telescope
An optical device used to gather light and magnify objects in the sky.

terrae
The older regions on the surface of the Moon, covered with rugged terrain pocketed with numerous craters.

terrestrial planet
A planet resembling the Earth; a solid planet.

Thermal Plasma Analyzer
The Canadian instrument aboard a Japanese probe en route to Mars and scheduled to arrive in 2004.

tidal force
The force exerted on an object by the body around which it is orbiting, due to the gravitational pull on the near face of the object being much stronger than the far side. Tidal forces cause the orbiting body to stretch and eventually break apart if it crosses the Roche limit.

tide
The phenomena of varying coastal water levels, caused by the gravitational pull of the Moon and the Sun.

totality
The path along which a solar eclipse is visible, called the path of totality.

transition zone
The layer of the atmosphere of the Sun between the chromosphere and the corona, in which the temperature rises dramatically.

U

ultraviolet (UV)
Radiation just beyond the violet end of the visible spectrum.

umbra
The central region of a shadow during an eclipse, causing a total eclipse. Also the central and cooler region of a sunspot.

universe
The realm of the universe is everything in existence.

V

Van Allen Belts
The zones surrounding the Earth which contain charged particles, trapped by the magnetosphere.

Venera
A program of the former Soviet Union which sent the first probes to the surface of Venus in the 1970's.

vernal equinox
The point along the ecliptic when the Sun crosses the celestial equator moving north. It occurs around March 21 and marks the first day of spring in the northern hemisphere.

Viking
The successful NASA probes sent to Mars in the mid 1970's, each consisting of an orbiter and lander. The landers gave the first close up views of the Martian surface and gave scientists valuable information about the surface conditions.

visible light
Radiation visible to the human eye.

Voyager
The extremely successful NASA spacecrafts sent to the outer solar system, launched in 1977. Voyager 1 passed by Jupiter and Saturn, while Voyager 2 passed by all four jovian planets. The two probes dramatically enhanced our knowledge of the outer solar system, and are still in operation, having begun their second mission called the Voyager Interstellar Mission.

W

wane
To decrease in intensity. In relation to the Moon, it is the progression of phases from full moon to new moon.

wavelength
The distance between successive peaks in a traveling wave. A higher wavelength corresponds to a lower energy.

white dwarf
The small remnant star left after a low mass star explodes and creates a planetary nebula.

winter solstice
The point on the ecliptic when the Sun is furthest south of the equator, giving the southern hemisphere its longest day of the year (and the shortest day of the year in the northern hemisphere). It occurs around December 21 and marks the beginning of winter in the northern hemisphere.

X

X-ray
High energy radiation.

Y

Z

zenith
The point in the sky directly overhead.

zodiac
Twelve constellations through which the Sun travels. These constellations were important to ancient astrology but have no present day significance.