Circinus: The Compasses

Right next to one of the most famous stars in the sky sits one of the least famous constellations. Circinus is small enough that you could miss it entirely while your attention is on its far more celebrated neighbor. That's more or less the point of it — it was never meant to compete.

A draftsman's tool, drawn next to a legend

Nicolas-Louis de Lacaille created Circinus in the 1750s to represent a draftsman's dividing compasses, the kind used for measuring and drawing circles and arcs. Like the rest of Lacaille's inventions, it carries no ancient mythology. It's simply a small triangle of stars, placed next to Alpha Centauri in Centaurus, one of the closest and brightest stars visible from Earth — an odd bit of company for a constellation this modest, but that's where the available faint stars happened to sit.

A rare kind of star hiding in a small shape

Circinus's brightest star, Alpha Circini, is described as the nearest and brightest of the rapidly-oscillating peculiar stars, sitting at magnitude 3.19. That's a genuinely unusual designation — these are stars with a specific, fast pattern of brightness variation that sets them apart from ordinary stars, and Alpha Circini is the standard example, the closest one of its kind to Earth. Beta Circini, a white star at magnitude 4.07, is the constellation's other listed star, considerably fainter and far less remarkable by comparison.

An active galaxy seen through the Milky Way's glow

The Circinus Galaxy, cataloged as ESO 97-G13, is a relatively nearby active spiral galaxy — but it's seen through the haze of the Milky Way itself, since Circinus sits low against our own galaxy's disk from Earth's vantage point. That haze makes it a harder target than its relative closeness would suggest; you're not just fighting distance, you're fighting the dust and light of our own galactic neighborhood sitting between you and it.

A small triangle, best seen in June

Circinus is a southern-hemisphere constellation with a best month of June, wedged between Centaurus and the small triangle of Triangulum Australe. Given its size and faintness, it's easy to overlook even when it's well placed — this is a constellation you find on purpose, not one you notice in passing.

That's true of most of Lacaille's inventions. He wasn't working with the sky's leftover bright stars — those had mostly been claimed by older, ancient figures long before he got to the southern hemisphere. He was drawing shapes out of whatever was left, which is exactly why a tool as small as a pair of compasses ended up represented by a constellation this modest in size. The compensation, in Circinus's case, is that its brightest star happens to be one of the most scientifically interesting ones in this stretch of sky.

What this means for you tonight

If your Real Sky map location is set in the southern hemisphere, June is your window for Circinus, and the easiest way in is to first find Alpha Centauri — bright and unmistakable — and then look for the small, faint triangle sitting right beside it. Alpha Circini at magnitude 3.19 is worth a second look once you've found it, since it's not just the brightest star in a faint constellation, it's specifically noted as the nearest example of a rare class of rapidly-oscillating star, which is a more interesting fact than most stars this dim ever get to carry.

Use Alpha Centauri's brightness as your anchor and work outward slowly rather than scanning the whole sky at once. Once your eyes have locked onto the bright pair, the faint triangle of Circinus should sit close enough in your field of view that a small, deliberate shift is all it takes, rather than a fresh search from scratch.

How star brightness works is a useful read before judging how hard a magnitude 3.19 star should be to spot next to something as bright as Alpha Centauri, and what a constellation really is covers how a shape this small and recent earned its place among the 88.