Circinus · Star Almanack

{% capture constellation_story %} ## Which kind of compass? The English word compass can suggest a needle pointing north. That is not the instrument represented by Circinus. Its subject is a pair of drawing compasses: two hinged legs used to mark a circle or transfer a distance. The distinction separates Circinus from Pyxis, whose name refers to a mariner's compass. Both instruments belong in the southern atlas, but they perform different tasks. The small figure of Circinus is especially suitable for a simple line drawing. A few joined stars can suggest two legs opening from a hinge, without requiring an elaborate animal or a crowded mythological scene. ## A working object becomes a constellation Nicolas-Louis de Lacaille introduced Circinus through his eighteenth-century work at the Cape of Good Hope. Its original French name, Le Compas, preceded the Latin Circinus. It belongs to the equipment of arts and sciences that distinguishes many of Lacaille's additions. Telescopes and clocks may seem obvious astronomical subjects. A drawing compass makes the broader point that observation also depends on calculation, construction, and the accurate making of diagrams. Historical globes preserve these choices. A Smithsonian celestial globe includes the group of southern instruments, showing how proposed names and pictures passed into objects used to represent the sky. The constellation is therefore a memorial to a tool and part of a record of publishing and teaching. It does not need an ancient legend to justify its inclusion. ## Holding one point while another moves To draw a circle, one point stays at the center while the other moves at a set distance. The simple action produces a precise relationship all around the circumference. A divider can also carry a distance from one part of a drawing to another. The instrument's usefulness comes from preserving a relationship while changing location or direction. Those ordinary operations provide a way to understand its appeal as a celestial emblem. A star map turns positions and angles into a surface that can be consulted. Tools for drawing and comparing distances belong naturally to that process. This explanation concerns what the instrument does and why its subject is meaningful. It should not be read as a quotation of Lacaille's private motive. ## Beyond the modest figure Circinus is a small southern constellation, and its outline is only the first scale at which it can be examined. In its direction lies the Circinus Galaxy, one of the nearby galaxies with an active nucleus. An active nucleus is powered by processes around a central supermassive black hole as material is drawn inward. Observations at wavelengths beyond ordinary visible light help investigate regions affected by dust and intense activity. The name of the galaxy identifies the constellation in which it appears. It does not claim that the galaxy resembles a pair of compasses or has a physical connection with the foreground stars of the figure. That difference is useful for an observer learning the atlas. Circinus first supplies a memorable shape and a historical object. A closer investigation then leads to something on a vastly different scale, whose name preserves the address while its behavior requires a very different explanation. ## Sources - [Chandra X-ray Observatory — Circinus: Lacaille and the drawing compass](https://chandra.harvard.edu/photo/constellations/circinus.html) - [SkyEye — Circinus: the Compasses and its original name](https://obliquity.com/skyeye/88const/Cir.html) - [Smithsonian National Museum of American History — Celestial Globe with Lacaille's southern constellations](https://americanhistory.si.edu/collections/object/nmah_1063819) - [IAU Working Group on Star Names — Circinus](https://exopla.net/circinus/) - [ESO — SINFONI Observations of the Circinus Galaxy](https://www.eso.org/public/images/eso0426l/) - [NASA JPL — A Super Special Galaxy: Circinus and its active nucleus](https://www.jpl.nasa.gov/images/pia13942-a-super-special-galaxy/) {% endcapture %}{{ constellation_story | markdownify }}