Octans (the Octant) is a faint southern constellation best known for containing the south celestial pole, the point around which the stars of the southern sky appear to rotate. Unlike the Northern Hemisphere, where the bright star Polaris provides a convenient guide to true north, the Southern Hemisphere has no comparably bright pole star. Sigma Octantis, also called Polaris Australis, lies close to the south celestial pole but is barely visible to the naked eye under ordinary conditions.
Where Is Octans and When Can You See It?
Octans is the 50th-largest constellation, covering approximately 291 square degrees of the southern sky. It surrounds the south celestial pole and borders Apus, Chamaeleon, Hydrus, Indus, Mensa, Pavo, and Tucana.
Because Octans contains the south celestial pole, it is visible throughout the year from much of the Southern Hemisphere. From sufficiently southern latitudes, the entire constellation is circumpolar, meaning it never sets below the horizon. Unlike most constellations, Octans has no particular observing season from these locations. Its stars circle the pole throughout the night as Earth rotates.
Finding Octans can be challenging because its stars are relatively faint. A practical method is to use the Southern Cross, or Crux, to estimate the direction of the south celestial pole. Extending the long axis of the Cross approximately four and a half times its length points toward the pole, which lies within Octans. A star chart is helpful for identifying Sigma Octantis and the other faint stars surrounding it.
Who Created the Octans Constellation?
Octans was introduced by Nicolas-Louis de Lacaille, who surveyed the southern sky from the Cape of Good Hope between 1751 and 1753. He named it after the octant, an instrument used by navigators to measure the angular distance between the horizon and celestial objects such as the Sun or stars. The octant was an important predecessor of the sextant and helped sailors determine their latitude while traveling at sea.
Lacaille originally used the name Octans Hadleianus, honoring the instrument associated with English mathematician John Hadley. The name was subsequently shortened to Octans. Like many of Lacaille’s southern constellations, Octans represents a scientific instrument rather than a figure from ancient mythology.
Notable Stars in Octans
- Nu Octantis – The constellation’s brightest star, an orange giant approximately 69 light-years away, shining at magnitude 3.8. It belongs to a binary system whose unusual radial-velocity measurements have prompted investigations into a possible planetary companion.
- Beta Octantis – A white star approximately 140 light-years away, with an apparent magnitude of 4.1. It is one of the constellation’s more easily identified stars under dark skies.
- Delta Octantis – An orange giant approximately 280 light-years away, shining at magnitude 4.3. It has expanded considerably as it has evolved beyond the main sequence.
- Sigma Octantis – The southern pole star, approximately 270 light-years away, with an apparent magnitude of 5.4. It is a small-amplitude variable star and is much fainter than the northern pole star Polaris.
- Theta Octantis – An orange giant approximately 220 light-years away, with an apparent magnitude of 4.8.
- Alpha Octantis – An eclipsing binary system approximately 150 light-years away. Its brightness changes as the two stars periodically pass in front of one another.
Sigma Octantis: The Southern Pole Star
Sigma Octantis, also known as Polaris Australis, is the star most closely associated with the south celestial pole. It lies approximately one degree from the pole, making it a useful reference point for astronomers who need to align telescopes with Earth’s rotational axis.
However, Sigma Octantis is much fainter than Polaris, the northern pole star in Ursa Minor. At magnitude 5.4, it can be difficult to see without a dark sky and good observing conditions. This makes it less convenient for navigation than its northern counterpart.
Southern Hemisphere observers often use the Southern Cross and the bright stars Alpha and Beta Centauri to estimate the direction of the south celestial pole instead.
Notable Deep-Sky Objects in Octans
Octans contains no Messier objects and relatively few bright deep-sky targets. Its galaxies and star clusters are generally challenging objects for amateur telescopes.
NGC 2573 — Nebula Polarissima Australis
NGC 2573 is a faint galaxy situated close to the south celestial pole. It was observed by astronomer John Herschel, who gave it the historical name Nebula Polarissima Australis, meaning the southernmost polar nebula. Although originally classified as a nebula, NGC 2573 is now recognized as a distant galaxy. Its position near the pole makes it a distinctive object in the southern sky, although it is too faint for casual observation.
NGC 6438 and NGC 6438A
NGC 6438 and NGC 6438A are two galaxies that appear close together in the constellation. Their overlapping appearance makes them an interesting target for astronomers studying galaxy structure and possible interactions. Both are faint objects requiring a telescope and favorable observing conditions.
Melotte 227
Melotte 227 is a sparse open star cluster in Octans. Open clusters contain groups of stars that formed from the same original cloud of gas and dust. Unlike globular clusters, they are generally less densely packed and more loosely bound by gravity. Melotte 227 is not among the sky’s most prominent clusters, but it provides another deep-sky target within this otherwise inconspicuous constellation.