The Andromeda Observatory

Asterism

A group of stars that form a pattern within a constellation rather than being a complete constellation in their own right. For example the Plough (or Big Dipper) is a group of seven bright stars within the much larger constellation of Ursa Major.

Astronomical Twilight

A situation where the Sun is between 12° and 18° below the horizon. The night sky is not considered truly dark until the Sun is lower than this.
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Astronomical Unit

A unit of distance, equal to the mean distance of the Earth to the Sun (149,597,870 kilometres).
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Atmospheric Absorption

Also known as atmospheric extinction, as light travels through the Earth's atmosphere, some of it is absorbed and scattered, making the source appear fainter. The more atmosphere that the light travels through, the more pronounced the effect. For objects seen on the horizon, the light is travelling through much more atmosphere than for those seen at the zenith. As a result of this the object at the horizon appears fainter than it would at the zenith by up to several magnitudes.

C

Celestial Equator

This is an imaginary line around on the sky. It is a projection of the plane of the Earth's equator onto the Celestial Sphere.
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Celestial Poles

This is a projection of the Earth's poles onto the Celestial Sphere.
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Celestial Sphere

A representation of the sky much as the ancients believed it to be, as a sphere surrounding the Earth rotating once in 23 hours and 56 minutes. It is useful to imagine the sky in this way for the purposes of identifying the location of objects.
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Circumpolar

If over the course of a day an object remains constantly above the horizon it is said to be circumpolar.

Civil Twilight

A situation where the Sun is less than 6° below the horizon.
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Conjunction

Conjunction is a term used to describe the situation where two or more objects are close to each other in the sky. More specifically, in the case of the inner planets, Mercury and Venus, when they lie between the Sun and Earth they are at Inferior Conjuntion. If the Sun lies between an inner planet and the Earth, then that planet is at Superior Conjunction.

Culminate

In the daily rotation of the celestial sphere, 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 the situation is reversed.

D

Descending Node

Usually the point in the orbit of a solar system object, at which it intersects the ecliptic whilst travelling southwards.
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Declination

An object's declination refers to its angle either above or below the celestial equator. It is the equivalent of latitude on the Earth.
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E

Eclipse

An eclipse occurs when an object moves into the shadow of another. A lunar eclipse occurs when the Moon moves into the shadow of the Earth. Since this can only occur when the Moon is opposite the Sun in the sky, it can only occur at a Full Moon. This does not happen every month because of the tilt of the Moon's orbit. A solar eclipse is not really an eclipse but an occultation. Sometimes the Sun is seen as a bright ring around the Moon because the Moon is too far away to completely obscure it and this is known as an annular eclipse.
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Ecliptic

The ecliptic is an imaginary line which traces the path of the Sun around the sky over the period of a year. Since this apparent motion is due to the Earth orbiting the Sun and the paths of all the planets in the solar system (with the exception of Pluto) lie within a few degrees of the plane of the Earth's orbit, they can be found somewhere close to this line in the sky.

Elongation

This term is used to describe the distance between objects in the sky, usually in degrees. For the inner planets, Mercury and Venus, their maximum elongation from the Sun is called Greatest Elongation. This is further described as Greatest Eastern or Greatest Western Elongation depending on which side of the Sun the planet lies. When at Greatest Eastern Elongation, the planet will be found in the evening sky and when at Greatest Western Elongation, in the morning sky. Also at this time, the planet will appear at half phase through a telescope.

Equinox

The Equinox marks the time when the Sun crosses the celestial equator. This occurs twice each year around March 21st (Vernal) and September 21st (Autumn). On these dates, day and night around the world are of 12 hours duration each.
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L

Light Year

A unit of distance, equal to the distance travelled by light in one year (9,460,700,000,000 kilometres).
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Limiting Magnitude

The magnitude of the faintest object visible to a given imaging system. For the human eye this is around magnitude 5 or 6. Light pollution often results in only brighter objects than these being visible since it increases the brightness of the sky background.
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M

Magnitude

Magnitude describes the brightness of an object. Lower numbers refer to brighter objects so that a magnitude 1 star is brighter than a magnitude 5 star. in fact a difference of 5 magnitudes is equal to a difference of 100 times.
Normally we refer to the apparent magnitude of an object, i.e. its brightness as seen from Earth. Absolute magnitude is the brightness of an object as it would appear at a fixed distance. In the case of stars this distance is 10 parsecs. This allows a direct comparison of their actual luminosity.

Main Sequence

Main sequence stars are those stars which are in their stable hydrogen burning phase of their lives. Such stars can differ markedly both in colour, temperature and size but is ultimately a result of their mass. The Hertsprung-Russell diagram is used to depict these relationships. The massive stars appear at one side of the main sequence and are hot blue-white stars and the least massive appear at the opposite end and are cool and red. The Sun is a main-sequence star that falls between these two extremes.
As stars run out of hydrogen fuel they move off the main sequence and appear elsewhere on the diagram. Such stars include red-giants and white-dwarfs.

Messier Object

A deep sky object catalogued by the French comet hunter Charles Messier. These objects are given Messier Numbers, a number preceeded by the letter M. This catalogue of 103 objects, (extended later by other astronomers) comprises of the more prominent deep sky objects visible from Northern latitudes.
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N

Nautical Twilight

A situation where the Sun is between 6° and 12° below the horizon.
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Nebula

Nebula (plural nebulae) , which means cloud, is used to describe many "fuzzy" deep sky objects other than galaxies, although these too were once classed as nebulae before their true nature was discovered.

Newtonian Telescope

A telescope design attributed to Sir Isaac Newton which uses a concave mirror as its main light collector.
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NGC

NGC is an abbreviation for the New General Catalogue, a listing of several thousand deep sky objects drawn up by J.L.E. Dreyer.
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O

Occultation

An occultation occurs when an object passes in front of another, thus obscuring it from view.
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Opposition

Opposition is a term used to describe the situation where an outer planet appears opposite the Sun in the sky. This means that it will appear due south at midnight for northern hemisphere observers and due north at midnight for southern hemisphere observers. The planet is also usually (but not always) at its closest to the Earth.

P

Parsec

A measure of distance equal to 3.26 light years. It is the distance at which an object appears to move by 1 arc second due to parallax caused by the Earth's orbit around the Sun, when viewed first at the appropriate time, and again six months later.

Penumbra

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Perihelion

The point in an orbit around the Sun at which an object is closest to the Sun.
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Planetary Nebula

A planetary nebula has no relation to planets other than many may appear to look like planets at first glance when seen through a telescope. It is actually the outer layers of a star that have been blown off as the star changes from a red giant to a white dwarf. Most stars end this way but larger ones can, instead, explode as a supernova.
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Precession

The tilt of the Earth's axis is not fixed in space. It wobbles slightly, like a spinning top, taking 25600 years to make a full circle. As a result, over this period, several bright stars will be recognised as the Pole Star. 3000 years ago it was Thuban in the constellation Draco. In about 12000 years time Vega in Lyra will lie in this notable position.
Another effect is that the position marking the Vernal Equinox also changes. Known as the First Point of Aries, it now lies in Pisces and in 600 years or so will move into Aquarius.
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R

Radiant

The point in the sky from which all meteors of a given stream appear to have originated. Meteor showers are usually named after the constellation in which the radiant lies, e.g. The Geminid's radiant lies in Gemini. One notable exception is the Quadrantids which are named after the constellation Quadrans Muralis which has fallen into disuse. The area of sky containing the radiant now belongs to Boötes.
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Refracting Telescope

A telescope design of a tube with a lens at each end. A large lens acts as its main light collector, which focuses the light ahead of a smaller eyepiece.
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Right Ascension

An object's right ascension (RA) refers to its angle around the celestial sphere from a fixed point in the sky. This point has been chosen as the position of the Sun's Ascending Node which is the position of the Sun at the Vernal (Spring) Equinox. RA is the equivalent of longitude on the Earth, where angular distances are measured from Greenwich, England.
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S

SCT

SCT is an abbreviation for Schmidt Cassegrain Telescope. This telescope incorporates both mirrors and lenses, a design known as catadioptric.
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Seeing

Seeing is a term used to define how the atmosphere affects the quality of an astronomical image. Using the scale devised by Eugenios Antoniadi. 1 represents good seeing where an image appears steady and detail is readily apparent and 5 represents the situation where conditions are so bad it is difficult to discern any detail. These numbers are often represented as Roman numerals I - V.

Solstice

Solstice means "Sun standing still" and refers to the two occasions each year when the Sun reaches its most northerly (around June 21st) or southerly (around December 21st) declination. In the northern hemisphere these mark the beginning of Summer and Winter respectively. In the southern hemisphere, the reverse is true.
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Supernova

A star of sufficient mass that is no longer able to produce energy from nuclear fusion can collapse in on itself and then explode as a supernova. So much energy is released that, for a while, it may appear brighter than the host galaxy.
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T

Terminator

This is the line between the sunlit and dark side of a celestial object. This marks the places where the object is experiencing sunrise or sunset. As on Earth, at these times long shadows are cast and these can be well seen on the Moon, showing the landforms much more clearly.

U

Umbral and Penumbral Shadow

These are the shadows caused by light emitted from an object larger than a point source. The umbral shadow is the main shadow and the penumbral shadow is a much fainter one. If you were stood in an umbral shadow, for instance during a total eclipse of the Sun, the Sun would be completely obscured. If you could see a partial eclipse of the Sun you would be standing in the Moon's penumbral shadow.
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Universal Time

Equivalent to Greenwich Mean Time, it is used as a standard reference when measuring the timing of celestial events,

Z

Zenith

The point in the sky directly above the observer's head, i.e. perpendicular to the horizon.

ZHR

ZHR is an abbreviation for Zenith Hourly Rate. This is a figure used to estimate the activity of a meteor shower. It is the number of meteors that would be seen per hour under excellent sky conditions if the radiant was directly overhead. Since this is not normally the case an observer will usually see less than this figure.