The constellation Octans

The constellation Octans

Octans is a constellation of the southern hemisphere. It is circumpolar around the South Pole. This means that its stars are so close to the celestial pole that they never set as seen from Sydney. The constellation is therefore above the horizon all year round there. The constellation is the southern counterpart of the constellation Ursa Minor in the northern hemisphere.

The southern pole star is σ Octantis. Because it is so close to the south celestial pole, it is the only star of the southern sky that always stays in the same place in the sky. It lies almost exactly above the Earth's axis, around which the Earth turns once every 24 hours, and like its northern counterpart it hardly takes part in this rotation. However, it can only be seen with the naked eye under the best conditions, because its brightness is more than twenty times weaker than that of the Pole Star in the northern hemisphere. The constellation Crux is therefore much better suited for finding the south celestial pole.

However, because of the wobble of the Earth's axis, σ Octantis will not keep its special position for ever. In about 3,000 to 7,000 years, the pole will move through the constellations Carina and Vela. Then the people of the southern hemisphere will also have a pole star that is visible to the naked eye.

History

The French theologian Nicolas-Louis de Lacaille, who devoted his life to astronomy, geodesy and mathematics, introduced 14 new constellations of the southern hemisphere. In 1750, he set off on a four-year journey to the Cape of Good Hope in order to observe, among other things, the constellations of the southern sky. During this time, he catalogued almost 10,000 stars. Nicolas-Louis de Lacaille named most of his new constellations after technical instruments. In this way, the constellations Sculptor, Fornax, Caelum, Antlia, Pictor, Microscopium, Reticulum, Octans, Horologium, Pyxis, Mensa, Telescopium, Norma and Circinus came into being. He also divided the large ancient constellation Argo Navis into the three constellations Carina, Puppis and Vela. His star catalogue of the southern sky was published in 1763, one year after his death, under the title “Coelum Australe Stelliferum”. What his 14 new constellations have in common is that they consist of relatively faint stars.

With Octans, Nicolas-Louis de Lacaille immortalised an important instrument for measuring angles, the octant, the forerunner of the sextant, which is also immortalised in the sky. Sailors used it to measure the angular height of stars above the horizon and therefore, like the sextant later, for navigation and for determining coordinates.