Astronomical Dictionary


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This is by no means a comprehensive glossary of astronomical terms however it should provide some assistance to novices in particular. Click on the symbol for a diagram or picture associated with that word. Cross references within the glossary are indicated by hyperlink text. Click on the letters below to go to the first entry of alphabetical sections.

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Click here to visit Aries on line, the Derby and District Astronomical Society magazine. It contains a variety of articles including beginners materials. The following is an excellent reference book - Space Encyclopedia (Heather Couper(Editor), Nigel Henbest(Editor)), as is Philip's Atlas of the Universe.

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The axis of the Earth's rotation slowly changes its direction. It makes a complete revolution once every 25,800 years. As a consequence, the position of the equinoxes on the ecliptic slowly drift westward.

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Precession also describes the general change in direction of any planet or axis of an orbit. A famed case of precession is that of the precession of the planet Mercury. Ordinary mechanics could not explain the amount of precession of the orbit, it needed Einstein's General theory of Relativity to do this because the gravitational influence of the Sun is so high, even at the distance of Mercury.

Primary This describes the larger body of a pair that are orbiting (e.g. the Earth is the Moon's primary) or the brighter member of a binary star. It is also a term that is applied to the main mirror of reflecting telescopes.
Prime Focus This is the point at which the light entering a telescope is focussed by the main mirror or lens, independent of any other optics of the system.


A cloud of gas ejected from the surface of the Sun. Prominences can take a variety of forms for example loops where an arc of gas is seen.
Proper motion This is the actual motion of a star as seen from Earth, as opposed to the apparent motion caused by the rotation of the Earth. The proper motion is caused by the actual movement of the star and the Sun through space. I believe that it was first discovered by Edmund Halley. Eventually, the proper motion of the stars will cause the shapes of the constellations to slowly change but the effect is imperceptible in a human lifetime. The star with the largest proper motion is Barnard's star which changes its position relative to the other stars in the sky at a rate of 10.3 arcseconds per year.
Protostar A star is thought to have been formed from a cloud of gas and dust that has collapsed under the influence of gravity. When a gas is compressed, it heats up. Since a star is formed from a huge mass of gas, it becomes very hot indeed. When the temperature has become high enough, the mass of gas and dust begins to glow. This is a protostar. Eventually, if the mass is great enough, the temperature rises enough to cause nuclear reactions at the centre. When this happens, the protostar becomes a star. If the mass is less than one tenth of that of the Sun, the star will never become hot enough to become a star.
Pulsar Short for pulsating star. First recognised by Jocelyn Bell Burnell. Originally thought to be artificial in origin because a pulsar gives out a regular flash of energy at radio and other wavelengths every few seconds or less. They are believed to be rapidly rotating neutron stars that flash each time they rotate, rather like the beam of a lighthouse. Possibly the best known is the pulsar at the centre of M1 - the crab nebula.
Purkinje effect When the human eye becomes adapted to the dark, the sensitivity changes. In daylight, the main sensitivity is in the yellow-green region of the spectrum. At night, the dark adapted eye becomes loses red sensitivity and the main sensitivity moves to the green part of the spectrum. This change of sensitivity is called the purkinje effect and explains why the moon looks bluer than direct sunlight.
QSO Same as quasar. When first discovered, quasars were called QSO's which apparently stood for quasi stellar radio source. When it became evident that not all QSO's were strong radio sources, they were renamed quasars.
Quadrature A planet is at quadrature when the angle between a superior planet differs from that of the Sun by 90 degrees when measured by an observer. At quadrature, small phase effcts can be seen, Mars is the most noticeable, looking definitely gibbous at such times.
Radiant The point from which the meteors of a meteor shower seem to come. Meteors travelling in parallel paths enter the atmosphere, perspective makes them seem to come from more or less the same point, rather like looking along a railway track.
Radiation belts An alternative name for the Van Allen belts, or, more generally, the equivalent of the Van Allen belts around other planets.
Radio astronomy The study of radio waves from space rather than light waves, which is referred to as "optical astronomy". Radio waves are the longest wavelength radiation of the electromagnetic spectrum, with wavelengths greater than 1 millimetre. The first really serious radio astronomy was carried out between the two wars when Karl Jansky discovered radio waves were being received from space. After the Second world war, Sir Bernard Lovell built the first serious radio telescope at Jodrell Bank in Cheshire. This was constructed with a fully steerable receiving disc 250 feet (76m) in diameter. It was the first, much to the annoyance of the Russians) to track and record the signals from Sputnik 1. In the 1960's, Jocelyn Bell Burnell discovered the first known pulsar. This telescope is currently undergoing an upgrade to replace the reflecting surface in order to increase its sensitivity. Pairs or multiple radio telescopes are used as interferometers which greatly increases their resolving power. The largest single radio telescope is built into a hollow in the mountains near Aricebo in Puerto Rica. This is non steerable and central to the SETI project.
Radius vector The imaginary line joining an orbiting body and the object it orbits. According to Kepler's Laws of planetary motion, the radius vector of a planet sweeps out equal areas in equal times. Thus, when the planet is closer to the Sun it must be travelling faster since the radius vector is shorter.
Red shift If an astronomical body is moving away from the observer, the light will seem to be shifted to the red end of the spectrum. The faster the movement, the greater the red shift. It occurs because the wavelength of light is slightly stretched as the body moves away from the observer by the Doppler effect. Red shift is measured by looking at the key spectral lines. For an object moving away from the Solar System, they will appear closer to the red end than normal. The faster the object is receding, the greater the red shift will be.
Red Dwarf Apart from being one of the best comedy series ever, a red dwarf is a type of star. These stars are much cooler and smaller than our Sun, being about one tenth of the mass and diameter.
Red Giant This is almost the final stage in the evolution of many stars. Late in its life, a star such as the Sun will cool and swell in size because its nuclear fuel in the core (hydrogen) has been used up. The star is still able to generate energy by burning the hydrogen in a shell around the core. This is not as concentrated as in the core (less pressure) so the temperature of the star drops. What happens after the Red Giant stage depends upon the original mass of the star. A Red Giant is at least 10 times the diameter of the Sun.
Refraction The bending of light as it passes through a transparent object such as a lens or the atmosphere.
Regiones Highland areas on the planet Venus, regiones are smaller than the two main 'terrae'. Click here for a summary of named planetary features.
Regolith The 'soil' found on the surface of the Moon.

Retrograde motion

Westward motion of a planet. The planet does not actually change its direction of motion, it is an effect created when the Earth overtakes the planet.
Revolution Not to be confused with rotation. This is the term used to describe the orbital motion of a body around its primary.
Rift An apparent division of the Milky Way. It is caused by dust clouds between the stars of the Milky Way and the observer. See also dark nebulae.
Right Ascension Abbreviated to RA. It is used in place of longitude for measuring the position of astronomical objects. It is the angle that is measured westward from the vernal equinox to the foot of the star's hour circle. The Earth turns more or less 1 degree every 4 minutes so 1 hour of Right Ascension corresponds to about 15 degrees.
Rotation Not to be confused with revolution. This is the term applied to the turning of a body around its axis.
Saltation This is really a geological term. It describes the process in which particles are bounced along by the wind or moving water. On Mars, the process of saltation is responsible for the blanking out of the surface during dust storms. When the winds move at speeds in excess of about 100 metres per second, saltation of the sand grains occurs. As the grains bounce along, they send even smaller dust particles into the air. It is these tiny particles, raised by saltation, that form the planet wide dust clouds.
Satellite An object (artificial or natural) that orbits a planet. Artificial satellites are used for a variety of applications such as communications, Earth observation (remote sensing of resources, climate etc.) and outside of the Earths atmosphere to observe deep into space as they avoid the distortion and pollution of the atmosphere. Some observations of objects in space can only be achieved from space telescopes. Artificial satellites are placed into two main types of orbit - polar to cover as much of the surface as possible each day and geostationary or geosynchronous (geo = Earth, synchronous = in the same relative place) so they stay over the same location all of the time. Remote sensing satellites are usually in polar orbits whilst communications satellites are in geosynchronous orbits.
Schwarzchild radius The German astronomer Schwarzchild investigated black holes mathematically as long ago as 1916. He worked out the critical radius of non rotating black holes, the event horizon as we now call it. For a body the size of the Sun, if it were compressed into a sphere of less than 3km, it would become a black hole. The Earth would need to be compressed down to less than 1cm!
Scintillation Twinkling i.e. the apparent changes in brightness of a star due to the Earth's atmosphere. When a star is viewed low in the sky, the light has to pass through a greater thickness of atmosphere than when it is at the zenith. The brighter the star and the lower the angle, the greater the scintillation. In extreme cases, the scintillation makes it seem like the star is changing colour. Sirius (the 'Dog Star') is particularly affected. On nights of bad seeing conditions, even the stars at the zenith are affected. Air currents are caused by masses of air at different temperatures. Thes refract (bend) light by different amounts and disturb the normally straight lines in which light travels. Since air currents are on the move, the path of the light through the atmosphere is also wobbling about.
Seeing A term used to describe the steadiness of the atmosphere as judged by looking at the telescopic image of a star. The seeing is affected by air currents at different altitudes in the atmosphere. High seeing is affected by air movements higher than 1km. Low seeing is affected by air currents near the ground. The observer has a little control over these. The telescope should be sited away from buildings because heat escaping from these warms up the surrounding air and causes it to rise. Avoid observing over built up areas since these are warmer and also lead to rising air currents. Warm air trapped in an observatory or in the telescope tube has the same effect. Always leave a telescope and observatory to cool off to the surrounding temperature (1-2 hours) before observing, it really does make a difference. Taking a telescope outside from a warm room and immediately using it to observe is not good. Large telescopes are affected by seeing much more than small ones. The Hubble Space Telescope has no seeing problems, it is located in the perfect observing place - above the atmosphere. Earth bound telescopes have been developed with 'adaptive optics' which compensate for the seeing and give images of very high quality without the problems of Hubble. The accepted scale for describing the seeing is Antoniadi's scale.
Seyfert Galaxy Carl Seyfert first drew attention to tis type of galaxy which now bears his name, I believe that the classic example is M87 in the Virgo cluster. He noticed that some galaxies had particularly small but extremely bright nuclei. In addition, these galaxies had rather inconspicuous spiral arms and were strong radio sources.
Shooting star The popular name for a meteor or fireball.
Sidereal time The passage of time as measured using the stars. It differs from Solar time by about 4 minutes per day on average which is why the stars seem to gradually drift across the sky.
Sirius The brightest star in the Northern (winter) sky. It is the main star in the constellation of Canis Minor (the lesser dog). It was an important star to the ancient Egyptians because when it appeared in the autumn, it signaled that the Nile floods were about to begin. The silt left behind when the waters subsided was fertile and provided excellent agricultural land.
Solar wind The Sun is constantly sending out a variety of gases and other particles. These constitute the Solar Wind. Luckily for us, the Earth has a strong magnetic field which shields us from the worst of the Solar wind. This deflects the particles of the wind into regions known as the 'Van Allen belts'. when these become overloaded, the particles spill into the atmosphere and cause aurorae. See also stellar wind.
Solstice The position of the Sun when at its highest or lowest point in the sky. The summer solstice represents the longest day and vice versa for the winter solstice.
Spectra The plural of spectrum.
Spectral lines Fairly obviously, the lines found on a spectrum! There are bright lines and there are dark lines. The bright lines are created by glowing objects such a a sodium lamp , emission nebula or a fluorescent tube. Stars however tend to show dark lines in their spectra. This is because of the complexities of their atmospheres or intervening materials. Where the light shines through a cooler layer of gas, the bright lines are reversed by a process of absorption, thus appearing dark. Where the light of a star shines through a dark nebula, absorption will cause the lines to appear dark. The positions of the lines depend on the chemicals present in the star or nebula. Each element has its own characteristic sets of spectral lines. These can be very complex and take a great deal of experience and expertise to analyse.
Spectral type A way of classifying a star by taking a spectrum of its light. This can give a great deal of information about the chemical composition and temperature of stars. The main divisions are, from hottest to coolest are O, B, A, F, G, K, M, R, N, S. The hottest stars are blue/white whilst the coolest are red. The Sun is a G type star, glowing yellow hot. Each division can be subdivided further, giving ever more information about a star. The spectral type of a star can also be determined from its colour index.

Light is made from many different colours mixed together. Each colour is a different wavelength. The colours that are in the light of a star depend upon the chemicals present and the conditions of the star. The light from a star can be split into the component colours and analysed to give this information. It is split using a spectroscope.

There are a variety of clearly defined lines on the spectra from astronomical objects, these are naturally enough called spectral lines. They give a good indication of the temperature and chemical nature of the object under study. Click for a diagram of the electromagnetic spectrum

Spectroscope An instrument used to analyse the light from a star. It works by splitting starlight into its component colours using a prism (triangular block of glass) or a device called a diffraction grating.
Star A star is a luminous object. In other words, they shine by their own light. Planets and other bodies that form part of the Solar System shine only because they reflect the light from the Sun. Stars are spheres of glowing gases moving through space. The pull of gravity is constantly trying to collapse the material of a star into the smallest volume possible. In the stable main sequence stage of a star's life, the inward pull of gravity is exactly balanced by the outward flow of energy. This keeps the star more or less the same size. Larger stars 'burn' more rapidly, live shorter lives and meet more violent ends than smaller stars like the Sun. See also supernova. They are powered by nuclear fusion reactions so they do not burn in the everyday sense of the word. Old stars, nearing the ends of their lives (white dwarf stars, red dwarf stars) have no internal energy source since their nuclear fuel is exhausted. These are cooling down to become invisible (black dwarf stars) since they will emit no light when cool. They glow only because they are so massive and will take millions of years to dissipate their heat into space. For excellent detail about formation and the life of stars, go to the stellar evolution website.
Stellar wind The flow of particles from the surface of a star. The stellar wind flows outwards at high speed into space. It is very low in density but nevertheless will interact with anything that it comes into contact with. It has a similar effect to radioactivity so is harmful to life. We call the stellar wind in our own solar system the 'Solar wind'.
Sunspot An area of the Sun's surface that is cooler than the surroundings. The Sun's surface is about 6000 degrees C, a sunspot is about half of this value. Because it is cooler, it gives out less light and appears dark by contrast. The number of sunspots is not constant, they vary on an 11 year cycle.
Superior A term that indicates an astronomical body has an position that is is further from the Sun than the Earth. It is applied in several different situations (see also conjunction)
Supernova The end of the line for a large star. It rips itself apart in a massive explosion when the nuclear fuel is used up. The heavier elements are made in this explosion and are spread through space to be incorporated into the dust and gas clouds that form younger stars.
Synchronous rotation The Moon rotates once on its axis per orbit of the Earth thus we see the same side of the Moon whenever we observe it. This is a good example of synchronous rotation. Synchronous rotation therefore means that there is a relationship between the orbital period and the rotation period. Synchronisation of these two periods is not necessarily one rotation to one orbit, the planet Mercury rotates three times for each two orbits of the Sun. Gravity is the cause of this phenomenon.
Synodic period The time taken for an astronomical object, most usually a planet, to return to the same position in the sky as seen from Earth. One convenient way of thinking about it is the time between successive conjunctions or oppositions. The synodic period is different to the length of the year of a planet because the Earth moves in its orbit. This means the planet under observation will have to move further in order to reach the same relative position as when it was first observed. The synodic period of a moon is the time taken for it to go through one cycle of phases as seen from its parent planet.. to synodic period diag.

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