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Circinus

Circinus is a small and faint constellation in the , representing a pair of drawing es used by draftsmen. Introduced in 1756 by French astronomer Nicolas-Louis de Lacaille during his expedition to the , it was created to fill a gap in the southern skies between the constellations and . The name Circinus derives from the Latin word for "," reflecting its depiction as a navigational or drafting tool. Circinus spans 93 square degrees of the sky, ranking as the 85th largest constellation and the fourth smallest overall, located in the third quadrant of the (SQ3). It is bordered by the constellations to the south, Norma to the east, and to the north, and Musca to the west, with also nearby. Visible to observers at latitudes between +30° and -90°, it is best seen in the evening sky during late June, though its stars are dim and require for optimal viewing. The constellation's brightest star is Alpha Circini (Xami), a rapidly oscillating Ap variable star with an of 3.19, located about 54 light-years away. Other notable stars include Beta Circini, a main-sequence star of class A3V with 4.07 at 97 light-years, and Gamma Circini, a consisting of a blue main-sequence star and a yellow . Circinus hosts several deep-sky objects of interest, particularly suited for observation with telescopes. The most prominent is the (ESO 97-G13), a approximately 13 million light-years distant, featuring an powered by a . Discovered in 1977, it was extensively imaged by the , revealing its complex structure and outflowing gases. Other highlights include the NGC 5823, with a of 7.9 and about 3,500 light-years away, containing around 100 stars spanning about 12 light-years; the NGC 5315 (magnitude 9.8); and the Circinus X-1, a system 30,700 light-years distant discovered in 1969. These features make Circinus a rewarding target for astronomers despite its modest brightness.

History and Mythology

Origin and Etymology

Circinus is a modern constellation introduced in the as part of efforts to map the southern celestial hemisphere. It was created by the French astronomer Nicolas-Louis de Lacaille during his expedition to the , where he conducted a comprehensive survey of southern stars from 1751 to 1752. Lacaille first depicted Circinus on a preliminary published in 1756 by the Académie Royale des Sciences, naming it le Compas in French, representing a pair of dividing compasses used in geometric drafting. In the accompanying descriptive text, he referred to it more fully as le Compas du Géomètre, emphasizing its role as a surveyor's tool. This initial designation reflected Lacaille's approach to naming new constellations after scientific instruments, diverging from traditional mythological figures and aligning with the era's emphasis on empirical knowledge. The constellation's name was later Latinized to Circinus in Lacaille's posthumously published catalog, Coelum Australe Stelliferum, issued in 1763. This revision simplified and standardized the nomenclature for international use, with circinus deriving from the Latin term for a pair of compasses or dividers employed in geometry and drafting. Unlike ancient constellations, Circinus lacks associated mythology, serving instead as a testament to 18th-century astronomical innovation.

Cultural and Historical References

Due to its position in the far southern sky and its status as a invention by French astronomer Nicolas-Louis de Lacaille during his expedition to the in the 1750s, Circinus was entirely absent from and Ptolemaic constellations, which focused on northern and equatorial regions visible from the Mediterranean. Similarly, traditional , which divided the sky into four quadrants guarded by mythical creatures, did not incorporate Circinus, as southern constellations beyond the reach of Chinese observers were largely uncharted. This lack of ancient recognition underscores Circinus's role as a product of the Enlightenment-era push to map the hitherto unexplored . Circinus gained formal status as one of the 88 modern constellations when the (IAU) adopted its list at the first General Assembly in in 1922, standardizing boundaries and names for global astronomical use. Prior to this, early European explorers of the southern skies, such as during his 1676–1678 observations from , catalogued faint stars in the region now occupied by Circinus as part of broader southern surveys, though without assigning them to any defined constellation pattern. Halley's Catalogus Stellarum Australium (1679) included positions for over 340 southern stars, some of which lie within modern Circinus borders, highlighting the area's dimness and the challenges of observation with 17th-century telescopes. In the 19th and 20th centuries, Circinus appeared in prominent star atlases, such as Johann Elert Bode's Uranographia (1801), where it was illustrated as a pair of compasses amid other scientific instruments, reflecting its thematic ties to geometry and measurement. Astronomical literature of the period often depicted Circinus as a of precision and the , aligning with Lacaille's intent to honor tools of the and in an era of expanding empirical knowledge. In non-Western contexts, the stars of Circinus occupy a portion of the eastern celestial quadrant associated with the (Qīng Lóng) in traditional stellar divisions, but received no specific , as Chinese astronomers prioritized more northerly asterisms. Today, its modern designation is Yuán guī zuò (圓規座), directly translating to "compass seat," underscoring its Western origins.

Observational Characteristics

Position and Boundaries

Circinus occupies a compact region in the , specifically in the third quadrant (SQ3), with its borders defined by lines of constant and as standardized by the (IAU). The constellation spans from 13h 38.4m to 15h 30.2m and from −70.62° to −55.43°, placing it entirely south of the and limiting its visibility to observers at latitudes south of about 30°N. With an area of 93 square degrees, Circinus ranks 85th in size among the 88 constellations, making it one of the smaller figures in the sky and the fourth-smallest overall. Its boundaries adjoin those of six neighboring constellations: and to the north, Norma to the east, to the south, Musca to the west, and to the southwest, as delineated on official IAU maps. The IAU formalized the list of 88 constellations, including Circinus, at its 1922 in , with precise boundary lines—drawn along and coordinates for the B1875.0—proposed by Belgian Eugène Delporte and approved in 1928 before publication in 1930. Due to the constellation's faint stars, none of which exceed 3.2, it received no or Flamsteed designations upon its creation by Nicolas-Louis de Lacaille in the ; Lacaille instead assigned Greek-letter labels based on apparent . Circinus lies close to the plane of the southern Milky Way, a position that enhances the density of deep-sky objects within its borders by overlaying the galactic disk with interstellar dust and gas lanes. This proximity underscores its value for southern-hemisphere astronomers studying galactic structure, though it also obscures fainter features with foreground material.

Visibility and Observation

Circinus is best observed from locations in the southern hemisphere below 30°S latitude, where it appears high in the sky during its optimal viewing period. The constellation culminates at 21:00 local time in July, reaching its highest point above the horizon for maximum visibility. For observers south of about 35°S, Circinus is circumpolar, remaining visible throughout the night and year without ever setting. Observing Circinus presents challenges due to its faint stars, with none brighter than 3.2, necessitating far from to detect its outline. Mid-southern viewers may find it low on the horizon, further complicating naked-eye observation during non-optimal times. The constellation's position along the adds to the difficulty, as the dense star field obscures its boundaries and makes distinguishing individual features harder without aids. To resolve Circinus effectively, or small telescopes are recommended, particularly for tracing its compact shape against the galactic background. The best season for observation aligns with the southern winter months of to August, when longer nights and clearer atmospheric conditions in the south enhance visibility. Locating it relative to the prominent stars of nearby can aid in initial identification.

Celestial Features

Stars

Circinus contains no stars brighter than third , contributing to the constellation's overall faint appearance, with the brighter members lying at distances ranging from about 50 to 500 light years. The brightest star is Alpha Circini (Xami), with an apparent visual of 3.19. This is a visual comprising a primary of type A7Vp—a rapidly oscillating Ap star with strong and peculiar chemical abundances—and a fainter K0V companion separated by approximately 15 arcseconds. The system lies about 53 light years from Earth. The second-brightest star is Beta Circini, at 4.07 and spectral type A3Va, a white main-sequence star located roughly 97 light years away. It forms a wide with a companion of spectral type L1, discovered through surveys. Gamma Circini, with a combined of about 4.5, is itself a consisting of a B3/4V primary and an F8V companion; the system is situated approximately 450 light years distant. Circini, at 4.85 and spectral type K2III, is an orange giant star about 428 light years from . Circinus harbors numerous stars, though few exhibit large or high prominence. Alpha Circini itself varies slightly due to its pulsations, with a dominant mode period of 6.8 minutes. A representative fainter example is BX Circini, a short-period of 12.6 with an of 0.05 and a period of 0.10658 days, classified as the prototype of its variability type.

Deep-Sky Objects

The constellation Circinus, situated along the southern , hosts a variety of faint deep-sky objects, including galaxies, nebulae, and star clusters obscured by galactic dust and gas. These features are challenging to observe from northern latitudes but reveal active astrophysical processes such as galactic nuclei activity, expansion, and ongoing when viewed with large telescopes. Recent surveys up to 2025 have highlighted the region's role in tracing structure through its molecular clouds and extended objects. The , cataloged as ESO 97-G13, is the nearest Seyfert type II galaxy to the , located approximately 13 million light-years away. It features a powerful powered by a , which drives ionized gas outflows extending up to several kiloparsecs from . Observations of water maser emission have revealed a warped, edge-on around the black hole, with radii spanning 0.1 to 1 , alongside high-velocity outflows indicating dynamical interactions near the . NGC 5315 is a prominent in Circinus, with an of 9.8 surrounding a central Wolf-Rayet star of 14.2. Hubble Space Telescope images disclose its intricate, x-shaped shell structure, formed by bipolar ejections of material from the dying central star in two distinct epochs, creating a chaotic envelope of ionized gas rich in oxygen and nitrogen emissions. Located about 7,000 light-years away, it exemplifies the late evolutionary stages of intermediate-mass stars in the Milky Way's disk. The Circinus West , a dense complex spanning roughly 180 light-years, harbors active low-mass and was the subject of a significant discovery using the Dark Energy Camera on the Victor Blanco 4-meter Telescope. This survey identified 28 new Herbig-Haro objects—glowing knots ejected by young protostars—doubling the previously known count in the region and revealing nine additional protostellar outflows, including some in the adjacent Circinus East. These findings underscore vigorous star-forming activity in embedded clusters of stars younger than 1 million years, with the nebula's filamentary structure pierced by sources indicating nascent stellar nurseries. Among other notable deep-sky objects, the NGC 5823 stands out with an integrated of 7.9 at a distance of about 3,500 light-years, comprising around 100 stars loosely grouped in a field spanning about 12 arcminutes. Another notable is Circinus X-1, an system featuring a orbiting a massive companion star, located about 8 kpc (26,000 light-years) away. Discovered in 1969, it is one of the brightest sources in the sky and exhibits relativistic jets, making it a unique laboratory for studying high-energy . Circinus lacks globular clusters, but its position yields several faint galaxies and emission nebulae visible in long-exposure surveys.

Meteor Showers

The constellation Circinus serves as the radiant for the minor annual known as the Alpha Circinids (ACI). First observed in 1977 from , , this shower is based on limited early records from southern sky surveys. The Alpha Circinids are active in early , with a peak around at solar longitude approximately 74°. The radiant lies near Alpha Circini, the constellation's brightest star, at right ascension 14ʰ 38ᵐ (218.6°), declination -70.2°. Meteors enter Earth's atmosphere at a geocentric velocity of about 27 km/s, producing slow-moving trails. Circinus hosts no major meteor showers, as its small area and faint radiants lack association with prominent parent comets; the Alpha Circinids have no confirmed progenitor. Observation is optimal from sites during local winter, when the radiant rises higher in the evening sky, linking to broader southern debris streams. Detected in 20th-century visual and orbital surveys, the shower has shown no significant outbursts through 2025.

References

  1. [1]
    Circinus - NOIRLab
    Circinus, also known as The Drawing Compass, represents a drafting compass. It is small, with few notable objects, and its brightest stars are Alpha, Beta, and ...
  2. [2]
    Circinus Constellation (the Compass): Stars, Deep Sky Objects ...
    Circinus, meaning 'the compass', is in the southern sky, the fourth smallest constellation, and contains deep sky objects like the Circinus Galaxy.
  3. [3]
    https://www.constellationdirectory.org/constellations/circinus.html
  4. [4]
    The Southern Constellation Circinus - NASA Science
    The southern constellation Circinus contains a galaxy with a black hole core, located 13 million light-years away.
  5. [5]
    Nicolas-Louis de Lacaille - Biography - MacTutor
    On 20 November 1750 he embarked on the vessel Le Glorieux, commanded by d'Après de Mennevillette, a friend of Lacaille's. He headed to the Cape of Good Hope on ...
  6. [6]
    Circinus - Star Tales - Ian Ridpath
    Circinus was introduced in the 1750s by the Frenchman Nicolas Louis de Lacaille, who fitted various figures into gaps between the existing constellations of the ...<|control11|><|separator|>
  7. [7]
    Circinus the Drawing Compass, Alpha Centauri's neighbor - EarthSky
    Jul 28, 2024 · Circinus the Drawing Compass is a dim constellation of the Southern Hemisphere. Its biggest claim to fame might be that it's located next to Alpha Centauri.
  8. [8]
    iMIS
    - **Insufficient relevant content**: The provided URL (https://www.iau.org/public/themes/constellations/) returns a 404 error, indicating the page is unavailable.
  9. [9]
    Star Tales – Edmond Halley's southern star catalogue - Ian Ridpath
    Halley adopted all the 12 new southern constellations invented by Keyser and de Houtman, but only 11 appear in the catalogue because weather prevented him from ...Missing: Circinus region
  10. [10]
    Catalog of Southern Stars | galileo
    Halley recorded the positions of 341 southern stars in this table of the right ascensions and distances of the principal southern stars.Missing: observations sky Circinus
  11. [11]
    Circinus Constellation | Star Map & Facts - Go-Astronomy.com
    Lacaille's constellations, including Circinus, were officially recognized by the International Astronomical Union (IAU) in 1930.Missing: ratification 1922
  12. [12]
    A Guide to the Circinus Constellation and Stars - Centre of Excellence
    Sep 19, 2024 · Circinus is a small, modern southern constellation, named for its compass shape, with three main stars, and is best observed in winter.What is the Circinus... · What is the Meaning of the... · Nebulae in the Circinus...
  13. [13]
    Circinus Constellation - Facts & Features - The Planets
    History and Mythology. The constellation of Circinus was created and first catalogued by the French astronomer Nicolas Louis de Lacaille in the 18th century.
  14. [14]
    Circinus Constellation Map - IAU Office of Astronomy for Education
    Circinus is surrounded by (going clockwise from the top): Lupus, Centaurus, Musca, Apus, Triangulum Australe and Norma.
  15. [15]
    Alpha Circini
    **Key Properties of Alpha Circini (Source: SIMBAD)**
  16. [16]
    Webb Deep-Sky Society: Double Star of the Month: Alpha Cir
    **Summary of Alpha Circini Binary System:**
  17. [17]
    Beta Circini
    ### Key Properties of Beta Circini (from SIMBAD)
  18. [18]
    Discovery of a brown dwarf companion to the A3V star β Circini
    Oct 30, 2015 · We have obtained a medium resolution echelle spectrum of the companion which indicates a spectral type of L1.0 ± 0.5 and lacks the typical ...
  19. [19]
    Gamma Circini
    ### Key Properties of Gamma Circini (from SIMBAD)
  20. [20]
    Epsilon Circini
    ### Key Properties of Epsilon Circini (from SIMBAD)
  21. [21]
    fundamental parameters of the roAp star α Circini - Oxford Academic
    α Circini (HR 5463, HD 128898; V= 3.2) is the brightest of 40 currently known roAp stars. It has one dominant oscillation mode with a period of 6.8 min and a ...Missing: distance | Show results with:distance
  22. [22]
    http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=BX+Cir&submit=SIMBAD+search
  23. [23]
    Active galaxy Circinus - ESA/Hubble
    Circinus is a type 2 Seyfert galaxy, 13 million light-years away, with a black hole-powered core and a powerful Active Galactic Nucleus (AGN).
  24. [24]
    Accretion and Outflow Traced by Water Masers in the Circinus AGN
    Dec 1, 2000 · The first VLBI images of water maser emission in the Circinus Galaxy AGN show both a warped, edge-on accretion disk and an outflow 0.1 to 1 pc from the central ...
  25. [25]
    2 The spectrum of NGC 5315
    The central star is clearly visible, having a visual magnitude of about 14. The star is classified as WC4 and hydrogen deficient (Mendez 1991). A P Cygni ...
  26. [26]
    Hubble Captures Stars Going Out in Style - NASA Science
    NGC 5315 lies in the southern constellation Circinus. IC 4593, at bottom, left, is in the northern constellation Hercules. NGC 5307, at bottom, right ...
  27. [27]
    Circinus West: A Dark Nebula Harboring a Nest of Newly Formed Stars
    Apr 24, 2025 · Another signpost of star formation, of which there is no shortage of in Circinus West, is the presence of Herbig-Haro (HH) objects. HH objects ...
  28. [28]
    [2503.14907] The Herbig-Haro Outflow Content and Star-Forming ...
    Mar 19, 2025 · We have detected 28 new HH objects in Circinus West, doubling the number known in this dark nebula. We have also discovered 9 outflows in ...
  29. [29]
    Variable Showers | SpringerLink
    Geocentric velocity: 17 miles/s (27 km/s). Evidence for the alpha Circinids comes from a single observation made from Queensland, Australia, back in 1977.
  30. [30]
    00162 / ACI / alpha-Circinids / -
    IAU MDC. Shower Database. Home IAU Working Group · Lists of Meteor Showers Report New Showers Nomenclature Rules Shower Status Criteria List of Parent BodiesMissing: details | Show results with:details
  31. [31]
    Meteor Shower Calendar | IMO
    The heart of the Calendar is the Working List of Visual Meteor Showers, thanks to regular updating from analyses using the IMO's Visual Meteor Database, the ...Missing: Circinus | Show results with:Circinus<|control11|><|separator|>