Octans

What is Octans?

Constellations· Southern sky, all year · holds the south celestial pole. The constellation that contains the south celestial pole. Lacking anything like Polaris, the faint 5.4-magnitude sigma Octantis serves as the southern pole star. Lacaille named it after the navigator's octant.

Where Octans is in your sky right now: https://nowsky.org/solar/sky?sel=con:Octans — location and time set automatically, no login.

Constellation basics

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Frequently asked

When can I see Octans?

circumpolar in the south; never visible from Korea.

How do I find Octans?

extend the long axis of Crux 4.5 times southward.

In one line, what is Octans?

Constellations — The constellation that contains the south celestial pole.

Where is Octans in the sky right now?

Open the sky view for your location and time and it shows the position in the sky right now.

Why there is no southern pole star, and where the pole is heading

That there is no bright star at the south celestial pole is chance. Even with bright stars spread evenly across the sky, the odds of one brighter than 5th magnitude falling within a degree of any given point are low, and Polaris is the rare stroke of luck. Earth's axis precesses with a period of 26,000 years, so the south pole traces a great circle across the sky, and at present it passes through the empty sky of Octans.

The pole will move toward Carina and in about 12,000 years will lie a few degrees from Canopus, when the second brightest star of the night will serve as the southern pole star; the north pole will then be near Vega. That southern navigators today extend the Southern Cross 4.5 times to point at empty sky is an inconvenience handed out by one phase of precession.

The constellation in depth

Octans was drawn by Lacaille in the 1750s to fill the empty sky around the south celestial pole, honouring the octant, the angle-measuring instrument navigators used at the time. It is the southern counterpart of Ursa Minor in position, but with no bright star, southern sailors steered by the Southern Cross instead. Its faint stars have no traditional names, only Greek letters assigned in the 18th century.

Sigma Octantis, about one degree from the pole, plays the part of a south pole star, yet at magnitude 5.4 it is barely visible even under dark skies. The brightest star, nu Octantis, is an orange giant of magnitude 3.7 in a binary system with a reported planet candidate. Precession is slowly carrying the pole out of the constellation.

Because it holds the south celestial pole it never sets from the southern hemisphere and can never be seen from the north. Extend the long axis of the Southern Cross about 4.5 times to land near the pole. In the Sky view, set Sydney or Cape Town and look just above the southern horizon. Sigma Octantis only appears once you raise the object level to show fainter stars. A dark site helps, as its stars are all faint.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.