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Octans was one of 14 new southern constellations introduced in the 1750s by the French astronomer 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 several stars that had previously been regarded as part of Hydrus, the lesser water snake, one of the constellations formed at the end of the 16th century by Dutch navigators.

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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.

Octans is another example of a constellation in which the star labelled Alpha is not the brightest. In this case, Alpha Octantis, magnitude 5.2, has the distinction of being the faintest star labelled Alpha in any constellation. Its closest rival, Alpha Mensae, is about 0.1 of a magnitude brighter.



© Ian Ridpath. All rights reserved


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