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Astronomical Glossary

Definitions for some of the most common terms used in astronomy and space exploration.

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 process by where dust and gas accumulated into larger bodies such as stars and planets.
Achondrite
A stone meteorite which contains no chondrules.
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.
Altitude
The angular distance of an object above the horizon.
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. A higher number represents a smaller opening while a lower number represents a larger opening.
Aphelion
The point in the orbit of a planet or other celestial body where it is farthest from the Sun.
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.
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.
Axis
Also known as the poles, this is an imaginary line through the center of rotation of an object.
Azimuth
The angular distance of an object around or parallel to the horizon from a predefined zero point.

B

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 theory that suggests that the universe was formed from a single point in space during a cataclysmic explosion about 18 billion years ago. The force of the explosion accounts for the current expansion of the universe.
Binary
A system of two stars that revolve around a common center 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.
Bolide
A term used to describe an exceptionally bright meteor. Bolides typically will produce a sonic boom.

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.
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 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.
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.
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.
Coma
An area of dust or gas surrounding the nucleus of a comet.
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.
Conjunction
An event that occurs when two or more celestial objects appear close together in the sky.
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.

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.
Declination
The angular distance of an object in the sky from the celestial equator.
Density
The amount of matter contained within a given volume. Density is measured in grams per cubic centimeter (or kilograms per liter). The density of water is 1.0, iron is 7.9, and lead is 11.3.
Disk
The surface of the Sun or other celestial body projected against the sky.
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 color 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.

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.

G

Galactic Halo
The name given to the spherical region surrounding the center, 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 center 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 frequently the sites of new star formation.
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 vapor are two of the major gases responsible for this effect.

H

Heliopause
The point at which the solar wind meets the interstellar medium or solar wind from other stars.
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.
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.

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 semimajor axis; 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
A large ring of icy, primitive objects beyond the orbit of Neptune. Kupier 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.

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.

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.
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.
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-colored 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 to1/1000 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.

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.
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 star that flares up to several times its original brightness for some time before returning to its original state.
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

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. 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. The gas is usually illuminated by the star, producing a variety of colors and shapes.
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
An unusually bright object found in the remote areas of the universe. Quasars release incredible amounts of energy and are among the oldest and farthest objects in the known universe. They may be the nuclei of ancient, active 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.
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.
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.
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.
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.
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 erosion or by the action of faults.
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 center 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 5 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
Grass-like patterns of gas seen in the atmosphere of the Sun.
Spicules
The range of colors produced when visible light passes through a prism.
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.
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.
Steady State Theory
The theory that suggests the universe is expanding but exists in a constant, unchanging state in the large scale. The theory states that new matter is being continually being created to fill the gaps left by expansion. This theory has been abandoned by most astronomers in favor of the big bang theory.
Stellar Wind
The ejection of gas from the surface of a star. Many different types of stars, including our Sun, have stellar winds. The stellar wind of our Sun is also known as the Solar wind. A star's stellar wind is strongest near the end of its life when it has consumed most of its fuel.
Stone Meteorite
A meteorite which resembles a terrestrial rock and is composed of similar materials.
Stony Iron
A meteorite which contains regions resembling both a stone meteorite and an iron meteorite.
Sunspot
Areas of the Sun's surface that are cooler than surrounding areas. The usually appear black on visible light photographs of the Sun. Sunspots are usually associated disturbances in the Sun's electromagnetic field.
Supergiant
The stage in a star's evolution where the core contracts and the star swells to about five hundreds times its original size. The star's temperature drops, giving it a red color.
Superior Conjunction
A conjunction that occurs when a planet passes behind the Sun and is on the opposite side of the Sun from the Earth.
Superior Planet
A planet that exists outside the orbit of the Earth. All of the planets in our solar system are superior except for Mercury and Venus. These two planets are inferior planets.
Supernova
A supernova is a cataclysmic explosion caused when a star exhausts its fuel and ends its life. Supernovae are the most powerful forces in the universe. All of the heavy elements were created in supernova explosions.
Supernova Remnant
An expanding shell of gas ejected at high speeds by a supernova explosion. Supernova remnants are often visible as diffuse gaseous nebulae usually with a shell-like structure. Many resemble "bubbles" in space.
Synchronous Rotation
A period of rotation of a satellite about its axis that is the same as the period of its orbit around its primary. This causes the satellite to always keep the same face to the primary. Our Moon in synchronous rotation about the Earth.
Synodic Period
The interval between points of opposition of a superior planet.

T

Tektite
A small, glassy material formed by the impact of a large body, usually a meteor or asteroid. Tektites are commonly found at the sites of meteor craters.
Telescope
An instrument used to collect large amounts of light from far away objects and increase their visibility to the naked eye. Telescopes can also enlarge objects that are relatively close to the Earth.
Terminator
The boundary between the light side and the dark side of a planet or other body.
Terrestrial
A term used to describe anything originating on the planet Earth.
Terrestrial Planet
A name given to a planet composed mainly of rock and iron, similar to that of Earth.
Tidal Force
The differential gravitational pull exerted on any extended body within the gravitational field of another body.
Tidal Heating
Frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and/or other neighboring satellites.
Transit
The passage of a celestial body across an observer's meridian; also the passage of a celestial body across the disk of a larger one.
Trojan
An object orbiting in the Lagrange points of another (larger) object. This name derives from a generalization of the names of some of the largest asteroids in Jupiter's Lagrange points. Saturn's moons Helene, Calypso and Telesto are also sometimes called Trojans.

U

Ultraviolet
Electromagnetic radiation at wavelengths shorter than the violet end of visible light. The atmosphere of the Earth effectively blocks the transmission of most ultraviolet light, which can be deadly to many forms of life.
Umbra
The area of total darkness in the shadow caused by an eclipse.
Universal Time (UT)
Also known as Greenwich Mean Time, this is local time on the Greenwich meridian. Universal time is used by astronomers as a standard measure of time.

V

Van Allen Belts
Radiation zones of charged particles that surround the Earth. The shape of the Van Allen belts is determined by the Earth's magnetic field.
Variable Star
A star that fluctuates in brightness. These include eclipsing binaries.
Visible Light
Wavelengths of electromagnetic radiation that are visible to the human eye.
Virgo Cluster
A gigantic cluster of over 2000 galaxies that is located mainly within the constellation of Virgo. This cluster is located about 60 million light years from Earth.
Visual Magnitude
A scale used by astronomers to measure the brightness of a star or other celestial object. Visual magnitude measures only the visible light from the object. On this scale, bright objects have a lower number than dim objects.

W

Wavelength
The distance between consecutive crests of a wave. This serves as a unit of measure of electromagnetic radiation.
White Dwarf
A very small, white star formed when an average sized star uses up its fuel supply and collapses. This process often produces a planetary nebula, with the white dwarf star at its center.

X

X-ray
Electromagnetic radiation of a very short wavelength and very high-energy. X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays.
X-ray Astronomy
The field of astronomy that studies celestial objects by the x-rays the emit.
X-ray Star
A bright celestial object that gives off x-rays as a major portion of its radiation.

Y

Yellow Dwarf
An ordinary star such as the Sun at a stable point in its evolution.

Z

Zenith
A point directly overhead from an observer.
Zodiac
An imaginary belt across the sky in which the Sun, Moon, and all of the planets can always be found.
Zodiacal Light
A faint cone of light that can sometimes be seen above the horizon after sunset or before sunrise. Zodiacal light is caused by sunlight reflecting off small particles of material in the plane of the solar system.
- Prepared by Jim Mayberry