Constellations: Circinus 'the Geometer's Compass' Content from the guide to life, the universe and everything

Constellations: Circinus 'the Geometer's Compass'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
Camelopardalis | Cancer | Canes Venatici | Canis Major | Canis Minor | Capricornus | Carina | Cassiopeia | Centaurus
Cepheus | Cetus | Chamæleon | Circinus | Columba | Coma Berenices | Corona Australis | Corona Borealis | Corvus
Crater | Crux | Cygnus | Delphinus | Dorado | Draco | Equuleus | Eridanus | Fornax | Gemini | Grus | Hercules | Horologium
Hydra | Hydrus | Indus | Lacerta | Leo | Leo Minor | Lepus | Libra | Lupus | Lynx | Lyra | Mensa | Microscopium | Monoceros
Musca | Norma | Octans | Ophiuchus | Orion | Pavo | Pegasus | Perseus | Phoenix | Pictor | Pisces | Piscis Austrinus
Puppis | Pyxis | Reticulum | Sagitta | Sagittarius | Scorpius | Sculptor | Scutum | Serpens | Sextans | Taurus
Telescopium | Triangulum | Triangulum Australe | Tucana | Ursa Major | Ursa Minor | Vela | Virgo | Volans | Vulpecula
To the pure geometer the radius of curvature is an incidental characteristic – like the grin of the Cheshire Cat.
– Sir Arthur Stanley Eddington

Short form:Cir
Area:93 sq deg
Co-ordinates1: Right Ascension 15h, Declination −60°

The faint constellation Circinus is located next to Centaurus, the Centaur. Also called 'a pair of compasses', this should not be confused with a 'magnetic compass'. In Latin Pyxis and Circinus are never confused. In English, however, both are referred to as a compass.

The first, Pyxis, is the instrument that tells the navigator which direction the boat is going: the 'mariner's compass'. The second item has two arms, with a sharp point on one arm and a pen or pencil lead at the end of the other, in order to trace out a circle.

For many years cartographers and draughtsmen routinely used such a compass with a triangle and parallel straight edge to earn a living. Now such instruments have been relegated to the secondary school geometry class. Yet in 1751, when Nicolas Louis de Lacaille first laid out the southern sky, such instruments were essential tools.

Seen best by those in the southern hemisphere in late evening during May, Circinus borders Triangulum Australe, Norma, Lupus, Centaurus, Apus and Musca. Located at declination 60° south, it would be almost overhead in Tierra Del Fuego but low in the southern sky at places like Nairobi, Kenya, and nearly invisible from Cairo, Egypt.


About 400 years ago, Johann Bayer first started putting lower case Greek letters to the brightest stars. Teamed with the genitive of the constellation, this became known as the 'Bayer designation'.

Catalogue NoBrightness (m)Distance
(light years)
α CiralphaHR5463+3.20 var53.5F0
β Cirbeta HR5670+4.0797A3
γ CirgammaHR5704+4.51509B5
δ CirdeltaHR5664+5.096,400O8
ε CirepsilonHR5666+4.85373K2
θ CirthetaHR5551+5.11834B4
η CiretaHR5593+5.17100G8

With alpha Circini at the vertex of the two arms and beta and gamma at the tips, this compass would be set to inscribe a very small circle.

Deep Space Objects

The New General Catalogue (NGC) of Nebulae and Star Clusters is continually updated by the NGC/IC Project. Other deep space objects are detected by space telescopes like the Chandra and Spitzer.

The Circinus Galaxy is a spiral galaxy with an active nucleus, known as a Seyfert2 galaxy. It has been imaged by the Hubble Space Telescope and shows gas being expelled, which astronomers think is evidence of black hole activity.

Open cluster NGC 5823 features in Sir Patrick Moore's catalogue for backyard astronomers as Caldwell 88.

Planetary nebula NGC 5315 gives a striking impersonation of a flower in bud in this Hubble image.

Supernova remnant RCW 86 was imaged by orbiting X-ray telescope Chandra in the 21st Century. Light from the initial stellar explosion first reached Earth in 185 AD, and the 'guest star' was recorded by Chinese astronomers. This makes RCW 86 the oldest recorded supernova.

Deep Space Objects Table

(light years)
NGC 5315ESO 97-9Planetary nebula+14.47,000
NGC 5715Dun 333Open cluster+9.84,842
NGC 5823Caldwell 88Open cluster+7.95,000
RCW 86G315.4-2.1Supernova+13.48,200
ESO 97-G13 Circinus Galaxy Seyfert galaxy+12.113M

Extrasolar Planets

In 2010 an extrasolar planet was detected in this constellation. HD 129445 b is a gas giant orbiting a yellow dwarf star approximately the same mass as our own Sun. In 2011 two planets were detected in orbit around HD 134060. Both worlds are unsuitable for life as we know it to exist there.

Geometric Compass Entries in the Edited Guide

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2Named after their discoverer, the US astronomer Carl Seyfert.

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