AphelionThe point in the elliptical orbit of a planet, comet or asteroid that is furthest from the Sun.
ApogeeThe point in the orbit of the Moon or artificial satellite at which it is furthest from Earth.
Apparent magnitudeThe visible brightness of a star or planet as seen from Earth.
AsteroidA small rocky and/or metallic object with a small size, often irregular in shape, orbiting the Sun in the asteroid Belt.
Asteroid beltA large group of small bodies orbiting the Sun in a band between the orbits of Mars and Jupiter.
Astronomical unit (AU)The mean distance between the Earth and the Sun, about 150 million km.
AtmosphereA layer of gases surrounding a planet or moon, held in place by gravity.
AuroraCurtains or arcs of light in the sky, usually in polar regions, caused by particles from the Sun interacting with Earth’s or other planet’s magnetic field.
Axial tiltThe angle between a planet’s axis of rotation and the vertical; equal to the angle between a planet’s equator and its orbital plane.
AxisThe imaginary line through the centre of a planet or star around which it rotates.
Big BangAn explosion that caused the birth of the universe about 13.7 billion years ago.
Black holeAn object with gravity so strong that no light or other matter can escape it.
Cassini divisionGap between Saturn’s A and B rings.
Celestial equatorAn imaginary line encircling the sky midway between the celestial poles.
Celestial polesThe imaginary points on the sky where Earth’s rotation axis points if is extended indefinitely.
Celestial sphereThe imaginary sphere surrounding the Earth, upon which the stars, galaxies, and other objects all appear to lie.
ChromosphereThe layer of the Sun’s atmosphere lying just above the photosphere (visible surface) and below the corona.
ComaThe diffuse, gaseous head of a comet.
CometA small body composed of ice, rock and dust that orbits the Sun on an elliptical path.
ConstellationOne of 88 officially recognised patterns or groups of stars in the sky as seen from Earth.
ConvectionA heat-driven process that causes hotter, less dense, material in the Sun’s interior to rise while cooler, denser material sinks.
CoreThe innermost region or centre of a planet or star.
CoronaThe tenuous outermost layer of the Sun’s atmosphere, visible from Earth only during a solar eclipse.
CosmologyThe branch of astronomy that deals with the origins, structure and space-time dynamics of the universe.
CraterA circular depression on a planet or moon caused by the impact of a meteor.
CrustThe surface-layer of a terrestrial planet.
Dwarf planetIs a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, has not cleared its neighbourhood around its orbit, and is not a satellite.
EarthOur home planet, one of the major planets of the Solar System.
EarthquakeA sudden vibratory motion of the Earth’s surface.
EccentricityA measure of how elliptical an orbit is. A perfect circle has an eccentricity of zero, and the more stretched an ellipse becomes the closer its eccentricity approaches a value of 1 (a straight line).
EclipseThe total or partial disappearance of a celestial body in the shadow of another, such as a solar eclipse or lunar eclipse.
EclipticThe apparent path of the Sun around the celestial sphere; also the plane of the orbit of the Earth around the Sun.
Electromagnetic radiationThe name given to a range of radiations that travel at the speed of light. Includes infrared rays, ultraviolet rays, visible light, X-rays and gamma rays.
EllipseThe oval path, closed loop followed by a celestial body moving around another body under the influence of gravity.
Encke’s divisionOne of the narrow bands dividing Saturn’s ring system. It is less prominent than the Cassini division.
EquatorThe imaginary line around the middle of a celestial body, half way between its two poles.
Escape velocityThe minimum speed an object (such as a rocket) must attain in order to travel from the surface of a planet, moon or other body and into space.
GalaxyA huge group of stars, gas and dust held together by gravity and moving through space together.
Galilean moonsThe four largest moons of Jupiter discovered by Galileo Galilei in 1610. They are in order of distance from Jupiter, Io, Europa, Ganymede, and Callisto.
Gamma raysForm of electromagnetic radiation with short wavelength and high frequency.
Gas giantA large planet whose composition is dominated by hydrogen and helium. The gas giants planets in our solar system are: Jupiter, Saturn, Uranus and Neptune.
GravityA force of attraction that exists between two masses.
HeliosphereA bubble blown into the interstellar medium by the pressure of the Sun’s solar wind.
IceRefers to solid states of water, methane, or ammonia which occur on planets or asteroids.
Inferior planetA planet that is closer to the Sun than Earth.
Infrared radiationForm of electromagnetic radiation with longer wavelength and lower frequency than the visible light region of the spectrum.
IonAn electrically charged atom or molecule, either positive or negative.
IonosphereA layer of the Earth’s atmosphere between 60 and 1000 km above the surface, where a percentage of the gases are ionised by solar radiation.
Kepler’s lawsThree laws discovered by Kepler that are used to describe the motion of objects in the solar system.
Kuiper beltA region of the solar system beyond Neptune (between 30 and 50 AU from the Sun), contains icy and rocky bodies similar to the asteroid belt.
LavaMolten rock flowing on the surface of a planet.
Lunar phaseThe appearance of the illuminated area of the Moon as seen from Earth.
Magnetic fieldA region of force surrounding a magnetic object.
MagnetosphereA region of space surrounding a planet or star that is dominated by the magnetic field of that body.
MantleAn inner region of a planet that lies between its crust and core.
MareA plain of solidified lava on the surface of the Moon; appears darker than the surrounding area.
MassThe amount of material in a body; usually measured in grams or kilograms.
MeteorThe bright streak of light that is seen when a rock or piece of space debris burns up as it enters Earth’s atmosphere at high speed. Meteors that hit the Earth’s surface are called meteorites.
Milky WayThe galaxy of stars and gas clouds that our solar system belongs to, seen as a luminous band of stars across the night sky. It is a spiral galaxy.
MoonThe only natural satellite of the Earth. The moon takes about 28 days to orbit the Earth once.
NebulaA cloud of gas or dust in the universe that may be illuminated by nearby stars.
Nuclear fusionA process whereby light atomic nuclei (such as hydrogen or helium) combine to produce heavier nuclei, with the release of energy; often called ‘burning’. Occurs in stars but not planets.
NucleusA collection of ices and dust that makes up the solid part of a comet.
OccultationThe apparent disappearance of one celestial body behind another.
Oort cloudA sphere of icy bodies surrounding the outer solar system. Much further from the Sun than the Kuiper belt.
OrbitPath of one celestial body when moving around another.
PerigeeThe point in the orbit of the Moon or artificial satellite at which it is closest to Earth.
PerihelionThe closest distance to the Sun in the elliptical orbit of a comet, asteroid or planet.
Period (of a planet)The time taken for a planet to orbit the Sun.
PhotosphereThe visible surface of the Sun or other star.
PlanetA celestial body that is in orbit around the Sun, has sufficient mass for its self-gra
vity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape (becomes nearly round), and has cleared its neighbourhood around its orbit.
PlanetesimalA small rocky or icy body, one of the small bodies that coalesced to form the planets.
ProminencesFlame-like jets of gas thrown outwards from the Sun’s chromosphere.
ProtosunThe part of the solar nebula that eventually developed into the Sun.
Radio telescopeA telescope, often in the form of a dish-shaped receiver, designed to detect radio waves.
Radio wavesElectromagnetic waves of low frequency and long wavelength.
Retrograde motionThe apparent westward motion of a planet with respect to background stars.
Retrograde orbitAn orbit of a satellite around a planet that is in the direction opposite to which the planet rotates.
RevolutionThe orbit of one body about another. One complete orbit is one revolution.
RotationThe spin of a planet, satellite or star on its axis.
SatelliteAny small object (artificial or natural) orbiting a larger one, such as a moon orbiting a planet.
Scattered discA distant region of our solar system, thinly populated by icy minor heavenly bodies.
SeasonsThe four divisions of the year of a planet whose axis of rotation is not perpendicular to the planet of its orbit. On Earth, the four seasons are: summer, autumn, winter and spring.
Shepherd satelliteA satellite that constrains the extent of a planetary ring through gravitational interactions with the particles in the ring.
Sidereal timeThe orbital period of a planet or satellite as measured with respect to the stars.
Solar flareA sudden release of energy in or near the Sun’s corona resulting in radiation being emitted into space.
Solar nebulaThe cloud of gas and dust from which the Sun and solar system formed.
Solar systemThe Sun, planets and their satellites, asteroids, comets, and related objects that orbit the Sun.
Solar windA stream of charged particles or ions emitted by the Sun.
Space probeA spacecraft or artificial satellite used to explore other bodies (such as the planets or Moon) in the solar system. Such a craft contains instruments to record and send back data to scientists on Earth.
Space stationA craft or vehicle that is in stable orbit around the Earth or other planet and is the temporary home of astronauts.
StarA self-luminous sphere of gas.
SunspotA highly magnetic storm on the Sun’s surface that is cooler than the surrounding area and so appears dark compared to the rest of the Sun.
Superior planetA planet that is more distant from the Sun than Earth is.
SupernovaAn exploding star, which briefly emits large amounts of light.
Tectonic forcesForces within a planet or moon that lead to the deformation of the crust of the body.
Terrestrial planetA planet whose composition is mainly rock (Mercury, Venus, Earth and Mars).
TransitThe passage of one astronomical body in front of another, for example, when a planet passes in front of the Sun’s disc as seen from Earth.
WeightThe force of gravity acting on an object.
X-raysElectromagnetic radiation with short wavelength and high frequency (between ultraviolet and gamma rays).
ZodiacThe name given to a group of twelve constellations that lie along the path followed by the Sun across the sky.
Index
A
Apollo program
Ariel
Asteroid belt
Asteroids
Auroras
C
Callisto
Cassini probe
Ceres
Chandra space telescope
Chariklo
Chromosphere
Comets
Copernicus
Corona
Coronal hole
Coronal mass ejection
D
Dawn probe
Dione
Dwarf planet
E
Earth
atmosphere
composition
magnetic field
surface
temperature
Enceladus
Epimetheus
Eris
Eros
Europa
G
Galileo
Ganymede
Gaspra
Grand Tack hypothesis
Great red spot
H
Haumea
Herschel
Hubble space telescope
Hyperion
I
Iapetus
International Space Station
Io
Ixion
J
James Webb Space Telescope
Janus
Juno probe
Jupiter
atmosphere
composition
interior
magnetic field
moons
rings
surface
temperature
K
Kepler
Kepler’s Laws
Kepler space telescope
Kuiper belt
Kuiper belt objects
L
Law of Gravitation
M
Magnetic fields
Makemake
Mars
atmosphere
composition
interior
magnetic field
moons
probes
surface
temperature
Mars Science Laboratory
Mercury
atmosphere
composition
interior
magnetic field
surface
temperature
Messenger probe
Meteoroids
Milky Way
Mimas
Miranda
Modern Laplacian theory
Modern Nebula theory
Moon
atmosphere
composition
magnetic field
phases
surface
temperature
N
Neptune
atmosphere
composition
magnetic field
moons
rings
The Solar System in Close-Up Page 25