h2g2 - Constellations: Circinus 'the Geometer's Compass'

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Constellations: Circinus 'the Geometer's Compass'

To the pure geometer the radius of curvature is an incidental characteristic – like the grin of the Cheshire Cat.
– Sir Arthur Stanley Eddington

Name:	Circinus
Genitive:	Circini
Short form:	Cir
Area:	93 sq deg
Co-ordinates1:	Right Ascension 15^h, Declination − 60°
Origin:	Modern

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.

Stars

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'.

Star	Bayer designation	Catalogue No	Brightness (^m)	Distance (light years)	Spectral classification
α Cir	alpha	HR5463	+3.20 var	53.5	F0
β Cir	beta	HR5670	+4.07	97	A3
γ Cir	gamma	HR5704	+4.51	509	B5
δ Cir	delta	HR5664	+5.09	6,400	O8
ε Cir	epsilon	HR5666	+4.85	373	K2
θ Cir	theta	HR5551	+5.11	834	B4
η Cir	eta	HR5593	+5.17	100	G8

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

Catalogue	Name	Type	Mag	Distance (light years)
NGC 5315	ESO 97-9	Planetary nebula	+14.4	7,000
NGC 5715	Dun 333	Open cluster	+9.8	4,842
NGC 5823	Caldwell 88	Open cluster	+7.9	5,000
RCW 86	G315.4-2.1	Supernova	+13.4	8,200
ESO 97-G13	Circinus Galaxy	Seyfert galaxy	+12.1	13M

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.

Geometric Compass Entries in the Edited Guide

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

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Exoplanet added to Circinus (Last Posting: Jan 27, 2010)

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Entry Data

Entry ID: A39250703 (Edited)

Written and Researched by:
shagbark

Edited by:
Galaxy Babe - eclectic editor

Date: 18 August 2008

Referenced Guide Entries

Constellations: Orion 'the Hunter'

Triangles

Constellations: Ursa Major 'the Great Bear'

Constellations: Taurus 'the Bull'

Constellations: Cassiopeia 'the Boastful Queen'

Constellations: Andromeda 'the Chained Maiden'

Constellations: Cygnus 'the Swan'