A ring Albedo



Apparent magnitude Asteroid

Asteroid belt

Asteroid number Astronomical unit (AU)



Axial tilt


B ring Big Bang

Black hole

C ring

Cassini division Celestial equator

Celestial poles

Celestial sphere

One of three prominent rings encircling the planet Saturn. A measure of the proportion of light reflected from a planet, asteroid or satellite.

The point in the elliptical orbit of a planet, comet or asteroid that is furthest from the Sun.

The point in the orbit of the Moon or artificial satellite at which it is furthest from Earth.

The visible brightness of a star or planet as seen from Earth. A small rocky and/or metallic object, often irregular in shape, orbiting the Sun in the asteroid belt.

A large group of small bodies orbiting the Sun in a band between the orbits of Mars and Jupiter.

The serial number assigned to an asteroid when it is discovered. The mean distance between Earth and the Sun, about 150 million kilometres.

A layer of gases surrounding a planet or moon, held in place by gravity.

Curtains 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.

The angle between a planet's axis of rotation and the vertical; equal to the angle between a planet's equator and its orbital plane. The imaginary line through the centre of a planet or star around which it rotates.

One of three prominent rings encircling the planet Saturn.

The event that caused the explosive birth of the universe about 13.7

billion years ago.

An object with gravity so strong that no light or other matter can escape it.

One of three prominent rings encircling the planet Saturn. Gap between Saturn's A and B rings.

An imaginary line encircling the sky midway between the celestial poles.

The imaginary points on the sky where Earth's rotation axis points if it is extended indefinitely.

An imaginary sphere surrounding Earth, upon which the stars, galaxies, and other objects all appear to lie.



Coma Comet



Core Corona




Dwarf planet

Dwarf star Earth

Earthquake Eccentricity

Eclipse Ecliptic

Electromagnetic radiation Ellipse

Encke division Equator Escape velocity

The layer of the Sun's atmosphere lying just above the photosphere (visible surface) and below the corona. A large group of stars or galaxies held closely together by gravitational attraction between them. The diffuse, gaseous head of a comet.

A small body composed of ice and dust that orbits the Sun on an often highly elliptical path.

One of 88 officially recognised patterns or groups of stars in the sky as seen from Earth.

A heat-driven process that causes hotter, less dense, material in a fluid to rise while cooler, denser material sinks.

The innermost region or centre of a planet or star.

The tenuous outermost layer of the Sun's atmosphere, visible from

Earth only during a solar eclipse.

The branch of astronomy that deals with the origins, structure and space-time dynamics of the universe.

A circular depression on a planet or moon caused by the impact of a meteor.

The surface layer of a terrestrial planet.

A celestial body that is in orbit around the Sun at a distance less than that of Neptune, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape (that is, it is nearly spherical), has not cleared its neighbourhood around its orbit, and is not a satellite.

Any star of average to low size, mass and luminosity.

Our home planet, one of the major planets of the solar system.

A sudden vibratory motion of Earth's surface.

A measure of how elliptical an orbit is. A perfect circle has an eccentrity of zero, and the more stretched an ellipse becomes the closer its eccentricity approaches a value of 1 (a straight line).

The total or partial disappearance of a celestial body in the shadow of another, such as solar eclipse, lunar eclipse.

The apparent path of the Sun around the celestial sphere; also the plane of the orbit of Earth around the Sun.

The type of radiation energy that includes radio, infrared, visible light, ultraviolet, X-rays and gamma rays.

The oval, closed loop path followed by a celestial body moving around another body under the influence of gravity. One of the narrow bands dividing Saturn's ring system. It is less prominent than the Cassini division.

The imaginary line around the middle of a celestial body, half way between its two poles.

The 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.

Exoplanet A planet beyond the solar system, or belonging to another star. Most exoplanets found to date are massive giant planets like Jupiter. Also known as an extrasolar planet.

Galaxy A huge group of stars, gas and dust held together by gravity and moving through space together.

Galilean moons The four largest moons ofJupiter, discovered by Galileo Galilei in

1610. They are, in order of distance from Jupiter: Io, Europa, Ganymede and Callisto.

Gamma rays Form of electromagnetic radiation with short wavelength and high frequency.

Gas giant A large planet whose composition is dominated by hydrogen and helium. The gas giant planets in our solar system are: Jupiter, Saturn, Uranus and Neptune.

Gravity The force of attraction between two masses.

Habitable zone The region around a star at which a solid planet would be expected to maintain liquid water on its surface.

Heliosphere A bubble blown into the interstellar medium by the pressure of the

Sun's solar wind.

Ice Solid state of water, and by extension methane or ammonia.

Inferior planet A planet that is closer to the Sun than Earth.

Infrared radiation Form of electromagnetic radiation with longer wavelength and lower frequency than the visible light region of the spectrum.

Ion An electrically charged atom or molecule, either positive or negative.

Ionisation The loss or gain by an atom or molecule of one or more electrons, resulting in the atom or molecule having a positive or negative electrical charge.

Ionosphere A layer of Earth's atmosphere between 60 km and 1000 km above the surface, where a percentage of the gases are ionised by solar radiation.

Kepler's laws Three laws discovered by Johannes Kepler that are used to describe the motion of objects in the solar system.

Kuiper belt A region of the solar system beyond Neptune (between 30 AU and

50 AU from the Sun), contains icy and rocky bodies similar to those of the asteroid belt.

Lava Molten rock flowing on the surface of a planet.

Light year The distance light travels in one year, about 9.5 trillion kilometres.

Used to give the distance to stars.

Luminosity The amount of electromagnetic energy radiated by an object, such as a star. Luminosity depends on the temperature and surface area of the object.

Lunar phase The appearance of the illuminated area of the Moon as seen from


Magnetic field A region of force surrounding a magnetic object.

Magnetosphere A region of space surrounding a planet or star that is dominated by the magnetic field of that body.


Major axis





Meteoroids Milky Way


Natural satellite

Nebula Neutron Nuclear fusion

Occultation Oort cloud

Orbit Perigee


Period (of a planet)





A measure of the brightness of a star or planet. The lower the magnitude the brighter the object. The longest diameter of an ellipse.

The inner region of a planet that lies between its crust and its core. A plain of solidified lava on the surface of the Moon; appears darker than the surrounding area.

The amount of matter in a body; usually measured in grams or kilograms.

The 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 Earth's surface are called meteorites.

Any small debris travelling through space; usually from a comet or asteroid.

The 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.

The only natural satellite of the Earth. The Moon takes about 28 days to orbit Earth once.

A smaller heavenly body held in orbit around a bigger body by gravitational attraction. For example, the Moon is a natural satellite of Earth.

A cloud of gas or dust, which may be illuminated by nearby stars. A subatomic particle with no electric charge.

A 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. The apparent disappearance of one celestial body behind another. A sphere of icy bodies surrounding the outer solar system. Much further from the Sun than the Kuiper belt.

The path followed by one celestial body moving around another. The point in the orbit of the Moon or artificial satellite at which it is closest to Earth.

The closest point to the Sun in the elliptical orbit of a comet, asteroid or planet.

The time taken for a planet to orbit the Sun. The visible surface of the Sun or other star.

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 shape (becomes nearly round), and has cleared its neighbourhood around its orbit.

A celestial body in orbit around the Sun at a distance greater than that of Neptune that has sufficient mass for its self-gravity to overcome rigid body forces so that it assumse a hydrostatic equilibrium (near-spherical) shape, and that has not cleared the neighborhood around its orbit.

Flame-like jets of gas thrown outwards from the Sun's chromosphere.

Protostar Protosun Pulsar

Radio telescope

Radio waves Red giant

Retrograde motion

Retrograde orbit


Revolution period



Scattered disc


Semi-major axis

Shepherd satellite

Sidereal time

Solar flare

Solar nebula

Solar system

Solar wind Space probe

Space station

Spectrometer Spectroscopy

A star in its early stages of formation.

The part of the solar nebula that eventually developed into the Sun. A rapidly rotating remnant from a supernova that emits bursts of energy at a regular rate.

A telescope, often in the form of a dish-shaped receiver, designed to detect radio waves.

Electromagnetic waves of low frequency and long wavelength. A large, cool star of high luminosity that is in the later stages of its life.

The apparent westward motion of a planet with respect to background stars.

The orbit of a satellite around a planet that is in the direction opposite to the rotation of the planet.

The orbit of one body about another. One complete orbit is one revolution.

The time taken for one body to orbit another. The spin of a planet, satellite or star on its axis.

Any small object (artificial or natural) orbiting a larger one, such as a moon orbiting a planet.

A distant region of our solar system, thinly populated by icy minor heavenly bodies.

The four divisions of the year of a planet whose axis of rotation is not perpendicular to the plane of its orbit; the distribution of solar radiation over the surface of the planet varies over its year. On Earth, the four seasons are summer, autumn, winter and spring. The semi-major axis of an ellipse is equal to half the length of the long axis of an ellipse. The semi-major axis of a planetary orbit is also the average distance from the planet to its star. A satellite that constrains the extent of a planetary ring through gravitational interactions with the particles in the ring. The orbital period of a planet or satellite as measured with respect to the stars.

A sudden release of energy in or near the Sun's corona, resulting in a burst of radiation and particles being emitted into space. The cloud of gas and dust from which the Sun and solar system formed.

The Sun, planets and their satellites, asteroids, comets and other related objects that orbit the Sun.

A stream of charged particles or ions emitted by the Sun. A 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. A craft or vehicle that is in a stable orbit around Earth or other planet and is the temporary home of astronauts. A device used to analyse the light from stars. The analysis of light from a planet or star to determine the composition and condition of the planet or star.

Star A self-luminous sphere of gas.

Sunspot A 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 planet A planet that is more distant from the Sun than Earth.

Supernova An exploding star, which briefly emits large amounts of light.

Synchronous orbit A condition in which a moon's rotation rate and revolution rate are equal.

Tectonic forces Forces within a planet or moon that lead to the deformation of the crust of the body.

Terrestrial planet A planet whose composition is mainly rock (Mercury, Venus, Earth and Mars).

Transit The 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.

Weight The force of gravity acting on an object.

White dwarf A small, dense, very hot but faint star. The final state of all but the most massive stars before they fade out.

X-rays Electromagnetic radiation with short wavelength and high frequency (between ultraviolet and gamma rays).

Zodiac Name given to a group of twelve constellations that lie along the path followed by the Sun across the sky.

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