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Constellation in the southern celestial hemisphere

Bordered by Centaurus, Musca, Apus, Triangulum Australe, Norma and Lupus, Circinus lies adjacent to the Alpha and Beta Centauri stars. As it is at declination −50° to −70°, the whole constellation is only visible south of latitude 30° N. The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a polygon of 14 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between and , and the declination coordinates are between −55.43° and −70.62°. Circinus culminates each year at 9 p. m. on 30 July. The recommended three letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is "Cir".

Circinus is a faint constellation, with only one star brighter than fourth magnitude. Alpha Circini, a white main sequence star with an apparent magnitude of 3.19, is 54 light years away and 4° south of Alpha Centauri. Not only the brightest star in the constellation, it is also the brightest example of a rapidly oscillating Ap (RoAp) star in the night sky. It has the unusual spectral type A7 Vp SrCrEu, showing increased emissions of strontium, chromium and europium. Stars of this type have oddly localised magnetic fields and are slightly variable. Alpha Circini forms a binary star system with an orange dwarf companion of spectral type K5 and magnitude 8.5, The distance between the two stars is 260 AU and they take 2600 years to rotate around a common centre of gravity. It has around 1.8 times the diameter of the Sun. Gamma Circini is a binary star 450 light years away, while the dimmer component is a yellow star of magnitude 5.5. They orbit each other every 180 years. Over 3600 light years away, this system would outshine Venus at magnitude −4.8 if it were 32 light years (10 parsecs) distant. and Zeta Circini is a blue white main sequence star of spectral type B3V and magnitude 6.09, located around 1273 light years away. 493 variable stars have been recorded in Circinus, but most have a very small range or are quite dim. Three prominent examples are Theta Circini, T Circini, and AX Circini. AX is a Cepheid variable that varies between magnitudes 5.6 and 6.19 over 5.3 days. It is a yellow white supergiant of spectral type F8II+, 1600 light years away. BP Circini is another Cepheid variable with an apparent magnitude ranging from 7.37 to 7.71 over 2.4 days. Both cepheids are spectroscopic binaries, with companions that are blue white stars of spectral type B6 and 5 and 4.7 solar masses, respectively. BX Circini is a faint star that fluctuates between magnitudes 12.57 and 12.62 over a period of 2 hours 33 minutes. Over 99% of its composition appears to be helium. Its origin is unclear, but thought to be the result of the merger of a helium and a carbon/oxygen white dwarf. Several stars with planetary systems lie within the borders of Circinus, although none of the host stars are particularly prominent. HD 134060 is a sun like yellow dwarf star of spectral type G0VFe+0.4 and magnitude 6.29, around 79 light years away. Its two planets were discovered in 2011 through the radial velocity method: the smaller, HD 134060 b, has a mass of 0.0351 MJ (Jupiter masses) and orbits its star every 3.27 days, at 0.0444 AU; the larger, HD 134060 c (0.15 MJ), orbits farther out at 2.226 AU, with a period of approximately 1161 days. Even fainter, at magnitude 8.8, HD 129445 is 220 light years away and has 99% of the Sun's mass and a similar spectral type of G8V. HD 129445 b, a Jupiter like planet (1.6 MJ) discovered in 2010 via the radial velocity method, orbits this star at a distance of 2.9 AU, approximately every 1840 days. As this constellation intersects the plane of the Milky Way, there are many massive stars located in this constellation, including GKF2010 MN18 (or simply MN18), a blue supergiant located in a bipolar nebula, as well as 9 Wolf Rayet stars, a very high number for such a small constellation.

Three open clusters and a planetary nebulae are found within the borders of Circinus, all visible with amateur telescopes of varying sizes. NGC 5823, also called Caldwell 88, NGC 5823 appears distinct to the observer, sometimes seen as a reversed "S", as described by John Herschel, although it has also been described as "tulip shaped" and "boxy". Comparatively, open cluster NGC 5715 is fainter (integrated magnitude of 9.8)—its brightest star is only 11th magnitude—and smaller (7.0 arcminutes), comprising only 30 stars. The third open cluster, Pismis 20, contains 12 stars in a diameter of 4.5 arcseconds but exhibits a magnitude similar to NGC 5823 (7.8). At 8270 light years, it requires an amateur telescope with an aperture over 300 mm to be easily discerned. Bernes 145 is a dark and reflection nebula first listed in the 1971 Bernes Catalog. The dark nebula component is easily visible in a large amateur telescope, and it measures 12 by 5 arcminutes. The smaller reflection nebula component requires a larger instrument and averted vision to be seen. Circinus also houses ESO 97 G13, commonly known as the Circinus Galaxy. Discovered in 1977, and therefore hosts an active galactic nucleus. Circinus X 1 is an X ray binary star system that includes a neutron star. Observations of Circinus X 1 in July 2007 revealed the presence of X ray jets normally found in black hole systems. Located at 19,000 light years, the pulsar PSR B1509 58, also called the Circinus Pulsar, has expelled a 20 light year long jet of material from its southern pole, clearly visible in the X ray spectrum. Another supernova remnant in Circinus is that of SN 185. Recorded by Chinese observers in 185 AD, SN 185 was visible in the night sky for around eight months; its remnants, known as RCW 86, cover an area larger than the typical full moon. A white dwarf star in a close binary system can accumulate material from its companion until it ignites and blows off in a thermonuclear explosion, known as a nova. Nova Circini 1926, also known as X Circini, was observed at magnitude 6.5 on 3 September 1926, before fading and fluctuating between magnitudes 11.7 and 12.5, during 1928, and magnitude 13, in 1929. Nova Circini 1995 (BY Circini) reached a maximum apparent magnitude of 7.2 in January 1995. BW Circini is a low mass X ray binary system, comprising a black hole of around 8 solar masses and a yellow G0III G5III subgiant star. X ray outbursts were recorded in 1987 and 1997, and possibly 1971–72.

Circinus is the radiant of an annual meteor shower, the Alpha Circinids (ACI). First observed in Queensland in 1977, the meteors have an average velocity of 27.1 km/s and are thought to be associated with a long period comet. In 2011, Peter Jenniskens proposed that the debris trail of comet C/1969 T1 could intersect with the Earth's orbit and generate a meteor outburst coming from a radiant close to Beta Circini. The ACI shower peaks on 4 June, the day it was first observed.