Star Tales – Octans

Octans was one of 14 new constellations introduced in the 1750s by Nicolas Louis de Lacaille. It represents a navigational instrument known as a reflecting octant, invented in 1730 by the Englishman John Hadley (1682–1744). Lacaille originally named it l’Octans de Reflexion on his chart published in 1756, but changed this simply to Octans on the second edition in 1763. Fittingly enough for a navigational instrument, Octans encompasses the south celestial pole, but despite this privileged position it contains little of note, consisting of no stars brighter than fourth magnitude.

When constructing the constellation, Lacaille annexed three stars that had formed part of Keyser and de Houtman’s Hydrus. Nu Octantis, for example, was originally the tip of the tail of Hydrus. Beta Octantis was another star in the tail of Hydrus and was the most southerly star catalogued by de Houtman: he gave its declination as –83° 40’.

Octans encompasses the south celestial pole, as shown in the Uranographia of Johann Bode where it was called Octans Nautica. The octant was the forerunner of the modern sextant. For Lacaille’s original depiction, see here.

An octant consisted of an arc of 45 degrees, i.e. an eighth of a circle, hence the name. The navigator sighted the horizon through a telescope and adjusted a movable arm until the reflected image of the Sun or a star overlay the direct view of the horizon. In later designs the arc was extended from one eighth of a circle to one-sixth and the instrument became the modern sextant.

There is, unfortunately, no southern equivalent of the bright northern pole star, Polaris. The nearest naked-eye star to the south celestial pole is Sigma Octantis, a degree away from the pole, although at magnitude 5.4 it is far from prominent.