Octans
What is Octans?
Constellations· Southern sky, all year · holds the south celestial pole. Octans contains the south celestial pole. It has nothing like Polaris, so the faint 5.4-magnitude star Sigma Octantis serves as the southern pole star. Lacaille named it after the octant, a navigator's instrument.
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Constellation basics
| Season | circumpolar in the south; never visible from London or New York |
|---|---|
| Brightest star | ν Octantis, mag 3.7 |
| How to find | extend the long axis of Crux 4.5 times southward |
| Area rank | 50th of 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
The faint southern pole star on Brazil's flag
Sigma Octantis, right beside the south celestial pole, is a faint star that the naked eye can barely see. Yet it appears on the flag of Brazil. The blue disc on the flag holds 27 stars, and each stands for one of Brazil's states or its Federal District. Sigma Octantis represents the Federal District, where the capital, Brasília, lies.
A star close to the celestial pole barely moves as the sky turns, while the other stars circle around it. That is why it is often described as a fitting symbol for the capital at the heart of the country. The other stars on the flag were also chosen from southern constellations, such as the Southern Cross and Scorpius.
That there is no bright star near the south pole is simply chance. The odds of a bright star sitting right next to any one point in the sky are low, and Polaris lying so near the north pole is actually the unusual case. So in the southern hemisphere people have found the pole using the Southern Cross, or by picking the midpoint of a line between the bright stars Achernar and Hadar.
The constellation in depth
Octans was drawn by Lacaille in the 1750s to fill the empty sky around the south celestial pole. An octant was an instrument sailors used to measure the height of the Sun or stars and so work out their position. Octans sits in the same spot in the south that Ursa Minor, home of Polaris, holds in the north, but it has no bright star. So southern navigators used the Southern Cross to find their way instead.
Sigma Octantis, about one degree from the pole, plays the role of the southern pole star. At magnitude 5.4, though, it can only just be seen under a clear, dark sky. Even the brightest star, Nu Octantis, is only a magnitude 3.7 orange giant. Nu Octantis is a pair of stars, and a planet may orbit there, according to some reports.
The celestial poles do not stay in one place forever. Earth's axis slowly swings around in a circle, like a spinning top, changing the direction it points. This motion is called precession, and one full circle takes about 26,000 years. So over long ages the south celestial pole gradually drifts out of Octans.
Because Octans holds the south celestial pole, it never sets from the southern hemisphere and can never be seen from the northern hemisphere. Extend the long axis of the Southern Cross about 4.5 times southward to land near the pole. In the Sky view, set Sydney or Cape Town and look south. Sigma Octantis only appears once you raise the setting to show fainter stars.
Frequently asked
- When can I see Octans?
- Octans is best seen in circumpolar in the south; never visible from London or New York.
- How do I find Octans?
- To find Octans, look for extend the long axis of Crux 4.5 times southward.
- In one line, what is Octans?
- Constellations — Octans contains the south celestial pole.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.