Absolute Zero The coldest possible temperature in the Universe which is 0 kelvin (0 K) or -273.15 degrees Celsius. It is the temperature at which all atoms cease to move.

Acceleration The rate of change of velocity. It is measured in m/s2. The approximate value for the acceleration due to gravity at the Earth’s surface is 10 m/s2 which means an increase in velocity of 10 m/s every second for a freely falling body ignoring air resistance.

Active Galactic Nuclei (AGN) A collective name for very strong sources from active galaxies such as Quasars, Seyfert Galaxies, BL Lacertae objects. These compact regions of active galaxies are thought to be powered by accretion onto supermassive black holes (more than a million solar masses).

Albedo A measure of how reflective a body is which is the percentage of incident light that is reflected back into space. The Earth has an albedo of 39% while the Moon’s albedo is only 7% similar to that of coal. Venus has a very high albedo of 65% due to its cloudy atmosphere.


The angle of an object above the observer’s horizon. An object on the horizon has an altitude of 0°, while at the zenith its altitude is 90°.


The diameter of a telescope’s objective lens or primary mirror usually measured in centimeters or meters. At present (2014) the largest optical telescope is 11.8 metres.

Arcminute A measure of angle equal to 1/60 of a degree.

Arcsecond A measure of angle equal to 1/60 of an arcminute or 1/3600 of a degree.

Astrolabe An ancient astronomical instrument that combines a planisphere with sights to enable one to compute astronomical problems and measure time. The instrument may have been invented by Hipparchus in the second century BCE.

Astronomical Unit (A.U.) The unit of distance equal to the mean distance of the Earth as it orbits the Sun (149 598 000 kilometres).

Aurora Also known as the Northern or Southern Lights is a phenomena seen at high latitudes close to the magnetic North and South poles and are due to energetic charged particles trapped in the Earth’s magnetic field exciting oxygen and nitrogen atoms high up in the Earth’s atmosphere (50 to 80 km).

Baryonic Matter A class of subatomic matter which includes neutrons and protons. You, me, the hundreds of billions of stars and planets are all made of baryonic matter, but this makes up just 4% of the total mass of the Universe!

Black Body A hypothetical body which absorbs all incident radiation and which emits radiation that is a function of its temperature only. The Sun is approximately a black body!

Black Hole A body whose gravitational field is so strong as to stop all electromagnetic radiation, including light, from escaping.

Brown Dwarf An object that has insufficent mass to be a star and cannot produce energy through nuclear fusion reactions. It will have a mass between 13 and 75 times the mass of the planet Jupiter.

Celestial Sphere An imaginary sphere centred on the Earth on which the Sun, stars, planets and the Moon are located.

Centripetal Force Any force which makes a body move in a circular orbit. The centripetal force always acts at right angles to the velocity of a body and towards the centre of the circle.

Cepheid A type of variable star named after the prototype δ Cephei which pulsates with a period of 5.33 days. Cepheid variable stars have periods between 1 and 100 days. Henrietta Leavitt in 1912 discovered that the luminosity of a Cepheid is linked to its period - the longer the period the more luminous the star.

Cherenkov Radiation Radiation produced when charged particles with mass move faster than the speed of light in that medium.

Conjunction The close position as seen in the sky of two or more astronomical objects.

Cosmic Microwave Background This radiation is the leftover radiation from the Big Bang which decoupled from the matter in the Universe after 380 000 years when the Universe had cooled to 3 000 K. The radiation now has a temperature close 2.73 K and has been red shifted by the expansion of the Universe into the microwave region. It was first discovered by Arno Penzias and Robert Wilson in 1965.

Cosmic Rays These are particles that have been accelerated to enormous energy of between 1 MeV (low energy cosmic rays from our Sun) up to a staggering 100 million TeV the highest known cosmic ray energy. They were discovered by Victor Hess in 1912.

Dark Matter A mysterious and unknown form of matter which does not emit radiation, but is observed through its gravitational effects. It makes up 26 - 27% of the Universe. The idea was first mooted by  Zwicky in 1933 after studying the dynamics of the Coma Cluster. More evidence came from Vera Rubin and Kent Ford in 1970 from studying the rotational curve of the Andromeda Galaxy M31.

Dark Energy A form energy that fills the Universe and is responsible for the acceleration of its expansion. It makes up 75% of the total mass content of the Universe.

Electromagnetic Radiation Radiation that can travel through the vacuum of space and consists of an oscillating electric field at right angles to an oscillating magnetic field. Light is a form of electromagnetic radiation.

Electromagnetic Spectrum There are 7 named regions of the Electromagnetic Spectrum which in order of wavelength from longest to shortest are:

Radio Waves / Microwaves /Infrared / Light / Ultraviolet / X-ray / Gamma-Rays.

Electronvolt (eV) A unit of energy used by Physicists at the atomic level. A photon of light has a typical energy in the range 1.5 eV (red light) to 3 eV (violet light).

1 eV = 1.60 * 10-19 joules.


The apparent angular distance of an object from the Sun, measured between 0 to 180° east or west of the Sun. For example, the First Quarter Moon has an eastern elongation of 90°; Venus has a maximum possible elongation of 47°.

Escape velocity This is the velocity of an object that is required to leave the gravitational field of a large body like a planet or a star. The escape velocity required to leave the Earth is 11.2 km/s.

Exoplanet A planet that orbits a star which is not our Sun. Since 1990 there have been more than 3 413 exoplanets discovered (28 MAY 2022).

Galaxy A huge conglomeration of stars that are held together by mutual gravity and range in size from dwarf galaxies, a few million suns, to giant elliptical galaxies that are ten times the mass of our Milky Way Galaxy.

Galilean Moons The four satellites seen by Galileo in January 1610 have been subsequently named Io, Europa, Ganymede and Callisto.

Gas Giant A planet that is at least 10 times the mass of the Earth and is mostly composed of gas (mostly the lightest elements: hydrogen and helium).

Geocentric Universe The theory that was adopted by Ptolemy in his great work the Almagest that all bodies orbit the Earth including the 5 planets, the Sun, the Moon and all the stars.

Giant Molecular Cloud Huge galactic clouds of mostly molecular hydrogen (H2) up to 100 000 solar masses that are very cold ~ 10 - 60 K. Some parts of giant molecular clouds can be regions of star formation in a galaxy.

The nearest star formation region to the Earth is the Orion Molecular Cloud 1500 light years away.

Gravitational Wave A ripple in space-time predicted by the General Theory of Relativity which travels at the speed of light and will affect the dimensions of a body extremely slightly when it passes through the body.

HARPS  An acronym for an instrument called the High Accuracy Radial Velocity Planet Searcher which can detect changes of radial velocity of a star to better than 1 m s-1 ! It is used in the La Silla Observatory in Chile and has discovered more than 50 planets the latest one orbits Alpha Centauri B which is only 4.37 light years away.

Heliocentric Universe A view of our Solar System in which all the planets including the Earth revolve around the Sun. It was the Polish astronomer, Nicolaus Copernicus who expounded this idea in his great book ‘de Revolutionibis Orbium Coelestium’ published in 1543 on his deathbed.

Herschel Space Observatory An ESA Infrared observatory with a 3.5 m mirror (The largest IR telecope ever launched). It was launched in May 2009 and was placed at the L2 Lagrangian point, 1.5 million km from Earth. Its last observation was made 4 years later in April 2013 when the liquid helium coolant was exhausted.

Hertzsprung-Russell Diagram A graph which plots the luminosity of stars (y axis) against their surface temperature (x axis) devised independently by two astronomers: Ejnar Hertzsprung and Henry Russell. The diagram is invaluable in explaining stellar evolution.

Hot Jupiters  extrasolar planets that are similar in mass to our own Jupiter but orbit very close to their parent star (0.015 AU to 0.5 AU cf Jupiter orbits our SUN at an average distance of 5.2 AU).

Hubble Space Telescope HST

Interferometry The technique by which the two or more telescopes can be combined such that their spatial resolution is equivalent to a telescope whose diameter would be equal to the maximum distance between the telescopes.

kelvin The SI unit of temperature. The link between temperature in degrees Celsius and kelvin is approximately T (in K) = t (in °C ) + 273. So room temperature of 20 °C would be 293 K.

Kepler Satellite  This satellite was launched on 7 March 2009 for a nominal 5 year mission to detect planets in particular Earth sized planets by observing 150 000 stars

in a 115 square degree area Cygnus and observing TRANSITS. The 0.95 m mirror can detect a dimming by a planet crossing a star by 0.01% or a photometric accuracy of 1 part in 100 000. Imagine looking down on the city of Canterbury in Kent at night and someone either turning a light on or off and detecting the change in brightness!

KOI  Kepler Object of Interest

Kuiper Belt Object  These objects are small icy planetisimals that orbit the Sun beyond the orbit of Neptune between 30 and 55 astronomical units. This large region of space may have over 100 000 planetisimals some of which become short period comet (period less than 200 years) if their orbit is perturbed by Neptune. The material in the Kuiper Belt is extremely primitive and represents the material in the early protoplanetary nebula.

Lagrangian Point There are 2 Lagrangian points in which small bodies may orbit about the Sun without being perturbed by a planet and they are at 60 degrees ahead and behind a planet in its orbit around the Sun.

Light Year The distance light or any other form of electromagnetic radiation travels in the vacuum of space in 1 year. The distance is about 9.5 trillion kilometers or 63 200 A.U. The nearest star besides the Sun is Proxima Centauri at 4.23 light years.

LISA  Laser Interferometer Space Antenna - now known as eLISA. Planned launch date now 2034. The three satellites will be around 1 million km apart operating Michelson Morley Interferometers they should be able to detect gravitational waves.

Luminosity This is a measure of the power output of a star and can measured in watts (W). The Sun’s luminosity is 4 * 1026 W which is enough energy produced in 1 second to last the Earth’s human population for a 100 000 years! There are some very rare stars that are a million times more luminous than the Sun! The lowest luminosity stars have a luminosity that is 1/100 000 the luminosity of the Sun.

Magnitude A unit of brightness of an astronomical object which has a logarithmic scale such that a difference of 5 magnitudes represents a change of 100 times in light intensity. The Sun is the brightest object with magnitude -26.7 while the full Moon is -13. The dimmest stars that can be seen by the naked eye are magnitude +6.

Main Sequence Star Stars on the Main Sequence of the Hertzsprung-Russell Diagram are ones which derive their energy from nuclear fusion of Hydrogen into Helium. Ninety percent of a stars lifetime is spent on the Main Sequence. The Sun is a main sequence star and is halfway through its lifetime of 9 billion years.

Maser Cosmic masers were first discovered in 1965 and are the astronomical counterpart to laboratory masers. MASER is an acronym for Microwave Amplification by Stimulated Emission of Radiation and work on the principle of the laser, but in the microwave region.

Milky Way  The name of our Galaxy and our Solar System lies some 30 000 l.y from the centre. Our galaxy consists of some 100 billion stars and in terms of ordinary matter: 90% Stars, 9% Gas mostly in the form of hydrogen and 1% dust grains.

Muon (μ) A muon is an elementary particle similar to the electron and with negative charge, but it is unstable and only lasts a few millionths of a second. Cosmic rays produce huge numbers of muons and several pass through your head every second! They were discovered by Carl Anderson in 1936.

Mural Quadrant An ancient astronomical instrument that was used to measure the altitude of celestial objects that crossed the observer’s meridian.

Neutrino A neutrino is an uncharged lepton with a mass less than one ten millionth of the mass of an electron that only interacts very weakly with matter. They were postulated by Enrico Fermi in 1944 and discovered in 1956 by Reines and Cowan. It is now known that there are 3 flavours of neutrino: electron neutrino; muon neutrino; and the tau neutrino.

Neutron Star At the end of a massive stars life (mass > 10 Mo) the star will explode as a supernova and the remnant core can form a fast spinning neutron star composed mainly of neutrons and with a size approximately 10 km in diameter. The density of a neutron star is similar to that of an atomic nucleus so 1 cm3 of neutron star will have a mass of a billion tons!

Nova Novae are very old stars (white dwarfs) that accrete material onto their surfaces from binary companions that can lead to a runaway thermonuclear explosion producing up to a million fold increase in brightness for a few days.

Oort Cloud A large spherical cloud of material that may lie some 50 000 to 100 000 AU from the Sun and contain many billions of comets. The idea was first suggested by Jan H. Oort in 1950. It is believed that the Oort cloud is the source of long period comets whose orbital period around the Sun may be measured in thousands of years.

Parallax The apparent position of a star changes as the Earth orbits the Sun and astronomical parallax is the measure of this angle in arcseconds. The nearest star, Proxima Centauri, exhibits a parallax of 0.76 arcseconds.

Parsec A unit of distance equivalent to a parallax of 1 arcsecond. The parsec or pc is equivalent to 3.26 light years or 206 265 astronomical units.

Photon A particle of electromagnetic radiation whose energy is specified by Plancks equation E = hf where h = Plancks constant 6.63 * 10-34 J s and f = frequency of radiation in hertz (Hz).

Planet A body that orbits a star like the Sun which is large enough to have become spherical through its own gravity and to have cleared its neighbourhood of objects close to its orbit.

Planck Surveyor Following on the success of COBE 1992 and WMAP 2001 for exploring the Universe in microwaves, ESA launched the Planck Surveyor in April 2009 (on the same rocket with the Herschel space observatory. Observations of the Cosmic Microwave Background were made and the satellite finished observing in October 2013.

Ptolemaic System The Ptolemaic system was based on the Geocentric theory as described in the famous book ‘The Almagest’ written by Ptolemy in 150 AD and was adopted by the Roman Catholic Church.

Pulsar Pulsars are fast spinning neutron stars that can emit radiation that appears as short pulses if the beam of radiation is in our line of sight (like a lighthouse beam). The pulses are extremely regular and range from milliseconds to several seconds. There are more than 1600 pulsars that have been observed in our Galaxy, the first was discovered by Jocelyn Bell in 1967. The Crab Nebula which is the supernova remnant of the supenovae seen in 1054 by the Chinese has a 33 millisecond pulsar that has been observed at all wavelengths from radio to gamma-rays.

Quadrant An astronomical quadrant was an ancient instrument used to measure the altitude of celestial objects above the horizon used up until the 17th century.


This is a measure of the ability of an optical instrument to resolve fine detail such as two close stars as separate entities rather than one star. Resolution is dependent on the aperture of the optical instrument and the wavelength of the radiation.

Retrograde Motion 

The outer planets exhibit retrograde motion as seen from the Earth when they are seen to move in the sky from East to West rather than the normal West to East motion. This is due to the Earth, which has a faster speed than the outer planets, catching up with the outer planet and then pulling away as it orbits the Sun.

Roche Limit The strong tidal forces of a large planet can disrupt a smaller orbiting body such as a satellite if it orbits at a distance less than the Roche limit which was devised by Edouard Roche in 1848. For two bodies of similar density the critical distance is less than about 2.45 times the radius of the larger body. Examples of bodies orbiting a large planet less than the Roche limit are the ring systems of Jupiter, Saturn, Uranus and Neptune.

Solar Mass A unit of mass used by astronomers equivalent to 1.99 * 1030 kg. Stars range in mass from 0.06 of a solar mass to 100 solar masses. The largest galaxies are over a trillion solar masses.

Spectroscope An instrument that splits up electromagnetic radiation into its different wavelengths and is used to analyze how the intensity of radiation varies with wavelength. With a spectroscope, the astronomer, Sir Norman Lockyer, discovered the element Helium in the Sun in 1868 before it was found in the Earth.

Spectroscopic Parallax

By analysing the spectrum of a star one can determine its surface temperature and luminosity and knowing its apparent brightness this in turn enables one to determine its distance. This technique works for stars out to about 10 kiloparsecs.


The splitting of electromagnetic waves into its various component wavelengths. In terms of Light one is splitting white light into its component colours. A spectrum can be analyzed to reveal many properties including: surface temperature, radial velocity, composition, and magnetic field strength.

Star Formation Region

A region of space where stars have recently or are being formed from gas and dust in a part of a giant molecular cloud. The Orion Molecular Cloud 1500 light years from Earth is the nearest star formation region.


A cataclysmic explosion at the end of a stars life which can release as much energy per second as the output of a whole galaxy i.e. 100 billion stars. There have been 6 observed supernovae by the naked eye in the last 2 000 years.

Synchrotron radiation When charged particles traveling close to the speed of light are accelerated in magnetic fields then synchrotron radiation is released. The radiation was first postulated by I. S. Shklovski in 1953.


The line separating the illuminated and unilluminated hemispheres of a planet or satellite.

Thermal Radiation This is electromagnetic radiation due to a bodies thermal energy and is often similar to Black Body radiation in its spectrum.

Thermodynamic Temperature

Temperature which is measured in kelvin formerly known as absolute temperature.


Transient Lunar Phenomena. A rarely observed, short-lived anomalous coloured glow, flash or obscuration of local surface detail, whose causes are poorly-understood.

Universal Time (U.T.)

The standard measurement of time used by astronomers over the world. UT is the same as Greenwich Mean Time, and it differs from local time according to the observer’s position on the Earth and the time conventions adopted in that country.


Our Universe started in a Big Bang explosion in which matter and space expanded from a singularity some 13.7 billion years ago. Recent measurements show that the expansion of Universe is accelerating due to a mysterious form of matter called Dark Energy.

Variable Star

Any star which varies its light output is deemed a variable star. Also classed as variable stars are binary stars which eclipse each other and thus their variability is a line of sight phenomenon. Our Sun during its 11 year sunspot cycle will only vary by 0.1%, but some stars vary by as much as 10 magnitudes or by a factor of 10 000 times.

There are many different classes of variable star with some varying like clockwork like Cepheids, the prototype being the star Delta Cephei discovered by John Goodricke in 1784. Others like Mira (Omicron Ceti) which was first noticed by Fabricus in 1596 are long-period variable stars varying over months to years.


The point in the sky directly above the observer it is a point that has an altitude of 90°.