Glossary

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Aphelion
The point at which a celestial body is furthest away from the sun.

Arcminute (or Minute of Arc)
An arcminute is a unit of angular measurement, equal to one sixtieth (1/60) of one degree.

Arcsecond (or Second of Arc)
An arcsecond is a unit of angular measurement, equal to one sixtieth (1/60) of one arcminute. There are 60 arcseconds in an arcminute. The arcsecond is 1/1296000 of a circle.

Asterism
A pattern of stars seen in the sky which is not an official constellation. An example is the saucepan shape called the Big Dipper (or the Plough) which is a small part of the constellation Ursa Major.

Circumpolar
A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets (that is, never disappears below the horizon), due to its proximity to one of the celestial poles. Circumpolar stars are therefore visible (from said location) for the entire night on every night of the year (and would be continuously visible throughout the day too, were they not overwhelmed by the Sun’s glare).

Constellation
A pattern of stars in the sky that are connected together in some arrangement; typically to form a visible figure or picture.

Declination (abbrev. Dec)
Declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension (or hour angle). Dec is comparable to latitude, projected onto the celestial sphere, and is measured in degrees north and south of the celestial equator.

Light Year
A light year is a unit of length, equal to just under ten trillion kilometres (or just under 6 trillion miles). As defined by the International Astronomical Union (which is the body which has the authority to promulgate the definition), a light year is the distance that light travels in a vacuum in one year.

Magnitude (Absolute)
Absolute magnitude is the apparent magnitude an object would have if it were at a standard luminosity distance (10 parsecs, 1 AU, or 100 km depending on object type) away from the observer. It allows the overall brightnesses of objects to be compared without regard to distance.

Magnitude (Apparent)
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, normalised to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower the value of its magnitude.

Messier Catalogue
The Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in his “Catalogue des Nébuleuses et des Amas d’Étoiles” (“Catalogue of Nebulae and Star Clusters”) included in the Connaissance des Temps for 1774 (published in 1771). The original motivation of the catalogue was that Messier was a comet hunter, and was frustrated by objects which resembled but were not comets. He therefore compiled a list of these objects, in collaboration with his assistant Pierre Méchain.

Nebula (plural: nebulae)
A nebula is an interstellar cloud of dust, gas and plasma. There are different types of nebula: planetary nebulae, protoplanetary nebulae, supernova remnants, emission nebulae and reflection nebulae

Parsec
The parsec (“parallax of one arcsecond“) is a unit of length, equal to just over 30 trillion kilometres, or about 3.26 light years.

Perihelion
The point at which a celestial body is closest to the sun.

Planet
A planet is defined by the International Astronomical Union as 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 (nearly round) shape, and
• has cleared the neighbourhood around its orbit.

Plasma
A plasma is a gas in which the atoms of the gas have had some of their electrons stripped off, making the gas electrically conductive so that it responds to electromagnetic fields.

Precession
The Earth ‘wobbles’ as it travels through space, tracing out a conical shape in a cycle of approximately 25,765 years. This movement is caused by the gravitational forces of the Sun and the Moon, and to a lesser extent other bodies, on the spinning Earth. This causes the positions of the stars to slowly move in a westerly direction over time.

Right ascension (abbrev. RA)
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere equatorial coordinate system. The other coordinate is declination

Setting circles
Setting circles consist of two marked disks attached to the right ascension (RA) and declination (DEC) axis of an equatorial mount. The right ascension disk is graduated into hours, minutes, and seconds. The declination disk is graduated into degrees, minutes, and seconds. Since right ascension coordinates are fixed to the celestial sphere the RA disk is usually driven by a clock mechanism in sync with sidereal time. Locating an object on the celestial sphere with settings circles is similar to finding a location on a terrestrial map using latitude and longitude.

Star Cluster
Star clusters are groups of stars which are gravitationally bound. Two distinct types of star cluster can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars, while open clusters generally contain less than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.

Supernova (plural: supernovae or supernovas)
A supernova (from the Latin nova “new”) occurs when a massive star reaches the end of it’s life. When nuclear fusion ceases in the core of the star, the star collapses inward on itself. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode.