Astronomical Terms Database

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Absolute Magnitude

The apparent magnitude that a star would possess if it were a distance of 10 parsecs from Earth. In this way, absolute magnitude provides a direct comparison of the brightness of stars. A star’s luminosity and its distance from Earth provide the basis for its apparent magnitude. If all stars were the same distance from Earth, then their apparent magnitudes would only be dependent on their luminosities. Therefore, absolute magnitudes are true indicators of the amount of light each star emits. The value is lower, even into the negatives, for objects that appear brighter, and higher for objects that appear dimmer. Absolute magnitude is usually measured through the visible spectrum. However, it can be measured through the entire spectrum. Absolute visual magnitude, through the visible spectrum, is abbreviated as Mv. Compare “Apparent Magnitude,” below.


An accumulation of dust and gas into larger bodies such as stars, planets and moons, or as discs around existing bodies.


A measure of the reflectivity of an object and is expressed as the ratio of the amount of light reflected by an object to that of the amount of light incident upon it. A value of 1 represents a perfectly reflecting (white) surface, whilst a value of zero represents a perfectly absorbing (black) surface. Some typical albedos are: The Earth - 0.39; The Moon - 0.07; Venus - 0.59.


The point in an orbit around the Sun at which an object is at its greatest distance from the Sun (Opposite of perihelion).


Similar to aphelion The point in an orbit when a body orbiting the Earth, (eg Moon or artificial satellite.) is farthest from the Earth (opposite of perigee).


The point in an orbit when a planet is farthest from any body other than the Sun or the Earth.

Apparent Magnitude

A measure of how bright an astronomical object, as viewed from Earth, appears to be to the naked Human eye, but calculated to a value that negates any interference from Earth's atmosphere. The value is lower, even into the negatives, for objects that appear brighter, and higher for objects that appear dimmer. Unless otherwise indicated, apparent magnitude is usually measured through the visible spectrum. However, it can be measured through specific wavelengths, such as near infra-red. Apparent magnitude is abbreviated as m. Compare “Absolute Magnitude,” above.

Arc Minute

A measure of angular separation, - one sixtieth of a degree.

Arc Second

Another measure of angular separation, - one sixtieth of an arc minute. (1/3600th of a degree.)

Ascending Node

The point in the orbit of an object, when it crosses the ecliptic, (or celestial equator) whilst moving south to north


(Also "planetoid") These are rocky bodies, the vast majority of which orbit the Sun between Mars and Jupiter. It is thought that there must be around 100,000 in all. The largest asteroid is Ceres which has a diameter of 579 miles. The smallest detected asteroids have diameters of several hundred feet. Together with comets and meteoroids, asteroids make up the minor bodies of the solar system. They are considered to be left over planetesimals from the formation of our solar system. The gravitational pull of Jupiter is thought to have stopped the members of the asteroid belt from forming a planet.

Astronomical Object

Naturally occurring structures that exist in observable space. Asteroids, moons, planets, and stars are examples of single astronomical objects. Nebulae, star systems, star clusters, and galaxies are examples of compound astronomical objects.

Astronomical Unit

Abbreviated as AU, it is the mean distance from the Earth to the Sun, 149,597,871 kilometers (92,955,807.3 miles).


A glow in the Earth's ionosphere caused by the interaction between the Earth's magnetic field and charged particles from the Sun (The Solar Wind). It gives rise to the "Northern Lights", or Aurora Borealis, in the Northern Hemisphere, and the Aurora Australis in the Southern Hemisphere.


Binary Star

A system of two stars orbiting around a common centre of mass due to their mutual gravity. Binary stars are twins in the sense that they formed together out of the same interstellar cloud. See also “Star System,” below; compare “Optical Binary,” below.

Blue Moon

The second full moon in a calendar month, or the third full moon in a season containing four.


Celestial Equator

The projection of the Earth's equator upon the celestial sphere.

Celestial Sphere

The projection of space onto the night sky, an imaginary hollow sphere of infinite radius surrounding the Earth but centred on the observer. (First postulated by Ptolemy.) It is the basis of sky charts, and the celestial co-ordinate system. The coordinate system most commonly used is right ascension and declination. The sphere itself is split up into arbitrary areas known as constellations.

Celestial Poles

The projection of the Earth's poles onto the celestial sphere.


The layer between the photosphere and the corona in the atmosphere of the Sun, or any other star, mainly composed of excited hydrogen atoms.


(1) The dust and gas surrounding the nucleus of a comet.

(2) A defect in an optical system which gives rise to a blurred, pear shaped, comet-like image. Comet: An icy object in independent orbit about the Sun; smaller than a planet, usually having a highly elliptical orbit extending out to beyond Jupiter.


When two bodies appear to close together in the sky, i.e. they have the same Right Ascension. Mercury & Venus are said to be at Superior Conjunction when they are behind the Sun, and at Inferior Conjunction when they are in front of it. The outer planets are simply said to be at conjunction when they pass behind the Sun.


An arbitrary grouping of stars which form a pattern. The sky is divided into 88 constellations. These vary in size and shape from Hydra, the sea monster, which is the largest at 1,303 square degrees, to Crux, the cross, which is the smallest at 68 square degrees.


The outer layer, and hottest part, of the Sun's atmosphere


A special telescope which blocks light from the Sun's disc, thus creating an artificial eclipse, in order to study its atmosphere.

Cosmic Ray

An extremely fast, energetic and relativistic (high speed) charged particle.


The Universe: the word is derived from the Greek, meaning 'everything'.


An object is said to culminate when it reaches its highest point in the sky. For northern observers, this occurs when the object is due South. For southern observers when it is due North.



A system for measuring the altitude of a celestial object, expressed as degrees north, or south, of the celestial equator. Angles are positive if a point is North of the celestial equator, and negative if South. It is used, in conjunction with Right Ascension, to locate celestial objects.

Descending Node

The point in the orbit of an object, when it crosses the ecliptic whilst traveling north to south.

Direct Motion (Prograde Motion)

(1) Rotation or orbital motion in a anticlockwise direction when viewed from the north pole of the Sun (i.e. in the same sense as the Earth); the opposite of retrograde.

(2) The East-West motion of the planets, relative to the background of stars, as seen from the Earth.

Dwarf Star

A star that lies on the main sequence and is too small to be classified as a giant star or a supergiant star. For example, the Sun is a yellow dwarf star. See also the Star Classification Table.



The eccentricity of an ellipse (orbit) is the ratio of the distance between its focii and the major axis. The greater the eccentricity, the more 'flattened' is the ellipse.


A chance alignment between the Sun, or any other celestial object, and two other celestial objects in which one body blocks the light of the Sun, or other body, from the other. In effect, the outer object moves through the shadow of the inner object.


The apparent path the Sun (and, approximately that of the planets) as seen against the stars. Since the plane of the Earth's equator is inclined at 23.5 degrees to that of its orbit, the ecliptic is inclined to the celestial equator by the same angle. The ecliptic intersects the celestial equator at the two equinoxes.


The angular distance between the Sun and any other solar system body, usually the Earth, expressed in degrees. The term Greatest Elongation is applied to the inner planets, Mercury and Venus. It is the maximum elongation from the Sun. At Greatest Elongation, the planet will appear 50% phase.

Equatorial Mount

A telescope mount so designed so that the two axes, which support it, are aligned, one to the polar axis and the other to the Earth's equator. Once an object is centred in the telescope's field of view, only the polar axis need be adjusted to keep the object in view. If the polar axis is driven at Sidereal rate, it will counteract the rotation of the Earth, keeping the object (except the Moon) stationary in the field of view.


This is the time when the Sun crosses the celestial equator. There are two equinoxes; Vernal (Spring), around March 21st and Autumnal (Autumn) around September 23rd. On these dates, day and night are equal. Actual dates and times vary due to the Earth's precession.



Unusually bright spots, or patches, on the Sun's surface. They precede the appearance of sunspots and can remain for some months afterwards.


A strand of (relatively) cool gas suspended over the Sun (or star) by magnetic fields, which appears dark against the disc of the Sun. A filament on the limb of the Sun seen in emission against the dark sky is called a solar prominence.



Vast star systems containing thousands of billions of stars, dust and gas, held together by gravity. Galaxies are the basic building blocks of the Universe. There are three main classes, Elliptical, Spiral and Barred, named after their appearance.

Galilean Moons

Jupiter's four largest moons: Io, Europa, Ganymede and Callisto. First discovered by Gallileo.

Geosynchronous Orbit

Sometimes known as a geostationary orbit, in which a satellite's orbital velocity is matched to the rotational velocity of the planet, and as such, a geostationary satellite would appear to be stationary relative to the Earth.

Globular Cluster

A spherical cluster of older stars, often found in galaxies.


The mottled, orange peel, appearance of the Sun's surface; caused by convection within the Sun.






(1) The angle between the orbital plane of the orbit of a planet, and the ecliptic.

(2) The angle between the orbital plane of a satellite and the equatorial plane of the body it orbits.

Inferior Conjunction

When Mercury, or Venus, are directly between the Sun and Earth

Inferior Planets

These are the planets Mercury and Venus. They are called inferior planets because their orbits lie between that of the Earth and the Sun.


Light Year

The distance traveled by light in one year, equal to 9.4607E12 km (5.88 * 10E12 miles or 63,240 AU).


The outer edge of the disc of a celestial body


Absolute brightness. The total energy radiated into space, per second, by an astronomical object, such as a star. Compare “Absolute Magnitude,” above.


The period between successive New Moons.



A magnetosphere is a region of space surrounding an astronomical object, in which charged particles are affected by that object's magnetic field.
Planets that have an active magnetospheres are capable of mitigating or blocking the effects of solar radiation or cosmic radiation. This protects all living organisms from potentially detrimental and dangerous consequences.


The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest star visible to the unaided eye, has magnitude 6. A decrease of one unit represents an increase in apparent brightness by a factor of 2.512. Apparent magnitude of a star is the brightness as we see it from Earth, whilst absolute magnitude is a measure of its intrinsic luminosity. Lower numbers represent brighter objects.


Also known as a "shooting star" or "falling star", is a bright streak of light in the sky caused by a meteorite as it burns up in the Earth's atmosphere.


A rock of extraterrestrial origin, found on Earth.

Minor Planets

Another term for asteroids.


A naturally occurring satellite, or relatively large body, orbiting a planet.

Multiple Star System

See “Star System,” below.



A term used to describe celestial objects which have a fuzzy, or nebulous, appearance (from the Latin for cloud.), such as gas, or dust, clouds. Galaxies were once described thus.


An existing star which suddenly increases its brightness by more than 10 magnitudes and then slowly fades.



This is when one celestial body, passes in front of, and obscures, another.

Open Cluster

A group of young stars, possibly bound together by gravity, that formed together.


A planet is said to be "in opposition" when it appears opposite the Sun in the sky. For the outer planets, this is generally the closest they come to the Earth, hence when they are most easily visible.

Optical Binary

Also known as “visual binary,” it is a pair of stars that happen to lie close to one another on the celestial sphere because of a chance alignment. The stars are not physically associated with one another and lie at vastly different distances. Compare “Binary Star,” above.


The path of one body around another due to the influence of gravity.



The angular difference in apparent direction of an object seen from two different viewpoints.


Abbreviated as pc, it is a unit of measurement used to express large distances to astronomical objects beyond Earth’s Solar System. It is the distance at which a star would have a parallax of one arc second, equal to 3.26163344 light years, 206,264.806 astronomical units, or 30,856,775,800,000 kilometers (19,173,511,600,000 miles).


Means, literally, "dim light". It most often refers to the outer shadow cast during eclipses, and defines the region of shadow which gives rise to a partial eclipse. It is also the lighter area surrounding the central region of a sunspot.


The point in an orbit closest to a body other than the Sun or the Earth.


The point in its orbit where the Moon, or planet is closest to the Earth.


The point in its orbit when an object is closest to the Sun.


To cause a celestial body, to deviate from its predicted orbit, usually under the gravitational influence of another celestial object.


The visible surface of the Sun.


Bright regions in the Sun's chromosphere.

Planetary System

A set of gravitationally bound non-stellar astronomical objects that orbit a star or a star system. Compare “Star System,” below.


An aid to locating stars and constellations in the night sky. It consists of two discs. One with the entire night sky, and the other, which covers the first, having a window through which a portion of the sky can be seen. The second disc is set according to the date and time.


Circular motion about the axis of rotation of a body; fixed with respect to the stars. The Earth is a giant gyroscope whose axis passes through the North and South Poles and this axis precesses with a period of 27,700 years.


A cloud, or plume, of hot, luminous gas in the Sun's corona. It appears bright when seen against the cool blackness of space. When they are in silhouette against the disc they are known as filaments. They are mainly composed of hydrogen, helium and calcium.



When a superior planet; Jupiter, Saturn etc.; is at right angles to the Sun, as seen from Earth.


Compact, extra galactic, objects at extreme distances, which are highly luminous. They are thought to be active galactic nuclei. The name is an acronym for quasi-stellar radio source. A quasar is very similar to a QSO (quasi-stellar object) but gives out radio waves also.



The part of the sky from which a particular meteor stream appears to come from. Meteor showers are usually named after the constellation in which the radiant originates.

Red Giant

A spectral type K or M star nearing the end of its life having a low surface temperature and large diameter eg Betelgeuse in Orion.

Red Shift

The lengthening of the wavelength of electromagnetic radiation caused by relative motion between source and observer. Spectral lines are red-shifted from distant galaxies, indicating that the galaxies are moving away from us due to the expansion of the Universe.


The amount of small detail visible in an image (usually telescopic); low resolution shows only large features, high resolution shows many small details.


Rotation of a planet, or orbit, opposite to that normally seen.

Right Ascension (RA)

The angular distance, measured Eastwards, from the Vernal Equinox. It is one of the ordinates used to reference objects on the celestial sphere. It is the equivalent to a longitude reference on the Earth. There are 24 hours of right ascension within 360 degrees, so one hour is equivalent to 15 degrees. Together with declination, it represents the most commonly used co-ordinate system in modern astronomy.


Semi-major Axis

The semi-major axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing through the foci of the ellipse with end points on the ellipse itself. The semi-major axis of a planetary orbit is also the average distance from the planet to its primary.

Shepherd Moon

Is a moon which constrains the extent of a planetary ring by means of gravitational forces. It is believed that shepherd moons are responsible for Saturn's rings.

Sidereal Time

Star time; the hour angle of the vernal equinox. Time measured with respect to the fixed stars rather than the Sun or body orbiter.

Sidereal Month

The 27.32166 day period of the Moon's orbit.

Solar Cycle

The 11-year variation in sunspot activity.

Solar Flare

A sudden, short lived, burst of energy on the Sun's surface, lasting from minutes to hours.

Solar Wind

A stream of charged particles emitted from the Sun which travel into space along lines of magnetic flux.


This is the time when the Sun reaches its most northerly or southerly point (around June 21st & December 22nd. respectively.). It marks the beginning of Summer and Winter in the Northern Hemisphere, and the opposite in the Southern Hemisphere.

Spectral Classification

A method of classifying stars which is based upon the appearance of the absorption lines in their spectra.

Star Cluster

A loose association of stars within the the Milky Way Galaxy. Examples include the Pleiades, also known as the Seven Sisters, the Hyades, and the Beehive Clusters, each visible from Earth, to the naked Human eye.

Star System

Also referred to as “multiple star system,” it is a set of a few gravitationally bound stellar objects, such as stars, that orbit each other. A set of many such stellar objects, or stars, is a star cluster. See also “Binary Star,” above. Compare star system; and “Planetary System,” above.


A cooler region of the Sun's photosphere (which, thus, appears dark) seen as a spot, on the Sun's disc. They are caused by concentrations of magnetic flux, typically occurring in groups or clusters. The number of sunspots varies according to the Sun's 11 year cycle. More sunspots are seen at the Maxima of solar cycles, with few being observed during the Minima between.

Superior Conjunction

This is when Mercury, or Venus, are behind the Sun

Superior Planets

Also known as the outer planets. These are the planets beyond the Earth's orbit. They are, in order: Mars; Jupiter; Saturn; Uranus; Neptune; Pluto


An exploding star.



The boundary between day and night regions of the moon's, or a planet's, disc.


The apparent journey of Mercury or Venus across the Sun's disc, or of a planet's moon across the disc of its parent.



From the Latin for shade, it is the shadow area defining a total eclipse. or the dark central region of a sunspot.


Variable Star

Any star whose brightness or magnitude varies with time. The variations can be intrinsic because of internal processes or extrinsic, due to eclipses, dust and other phenomena. Variations can also be irregular or periodic.


White Dwarf

A whitish star, of up to 1.4 Solar masses, and about the size of the Earth with consequential very high density, characterized by a high surface temperature and low brightness.

Worm Hole

A hypothetical shortcut through the space time continuum.



The point on the celestial sphere directly above an observer, or the highest point in the sky reached by a celestial body.

Zenithal Hourly Rate - (ZHR)

It is the number of meteors per hour, for a particular stream, that is estimated will be seen under favorable seeing conditions if the radiant were directly overhead the observer. Usually the actual figure is less than this.


The apparent path, in the sky, followed by the sun, moon and most planets, lying within 10 degrees of the celestial equator. Ancient Astrologers (nothing to do with modern astronomy! ) divided it into 12 groups, the Signs of the Zodiac, though there are actually 13 astronomical constellations which lie on the zodiac, since the Sun passes through Ophiuchus each December. Ophiuchus is not recognized by astrologers.

Zodiacal Light

A faint glow from light scattered off interplanetary dust in the plane of the ecliptic.