Octans is a faint constellation located in the deep southern sky. Its name is Latin for the eighth part of a circle, but it is named after the octant, a navigational instrument. The constellation was devised by French astronomer Nicolas Louis de Lacaille in 1752, and it remains one of the 88 modern constellations.
Octans is bordered by seven different constellations, most of which are far more prominent than itself: Apus (the bird-of-paradise), Mensa (the table), Chamaeleon (the chamaeleon), Pavo (the peacock), Indus (the Indian), Tucana (the toucan), and Hydrus (the male water snake). Covering 291 square degrees, Octans ranks 50th in area out of the 88 modern constellations.
Because the constellation is circumpolar to the South Celestial Pole, it can be seen throughout the entire Southern Hemisphere at any given time of the year. The three brightest stars, (Nu, Beta, and Delta) are at their highest point in the sky during October and early November.
Octans encompasses the south celestial pole, as shown on Chart XX of Johann Bode’s Uranographia star atlas, where it was called Octans Nautica. The octant was the forerunner of the modern sextant.
[http://www.ianridpath.com/startales/octans.htm]
Octans was one of 14 constellations created by Nicolas Louis de Lacaille during his expedition to the Cape of Good Hope, and was originally named ‘l’Octans de Reflexion,’ French for ‘the reflecting octant.’ It was part of his catalogue of the southern sky, the Coelum Australe Stelliferum, which was published posthumously in 1763. In Europe, it became more widely known as ‘Octans Hadleianus,’ in honor of English mathematician John Hadley, who invented the octant in 1730. There is no real mythology related to Octans, partially due to its faintness and relative recentness, but mostly because of its extreme southerly latitude.
This map shows the sky from Sydney, Australia looking due south around 1:30-2 a.m. in late February. Southern observers can use the axis of the Southern Cross (Crux) to point them to the southern polestar. LMC and SMC stand for the Large and Small Magellanic Clouds, small satellite galaxies of the Milky Way.
[http://astrobob.areavoices.com/2010/02/24/polar-inequities/]
Octans and the South Celestial Pole
[https://www.davidmalin.com/fujii/source/Oct.html]
Nu Octantis is the brightest star in the constellation. It has an apparent visual magnitude of 3.76 and is approximately 69 light years distant from the solar system. The star is an orange giant with the stellar classification K1III. It is one of the least luminous giant stars known, with a luminosity only 16 times that of the Sun. It is also a relatively small giant, with a radius only 5.9 times solar and a mass 1.4 times that of the Sun. The star will eventually expand and, in about 100 million years, it will become 15 times larger and 60 times brighter than it is now. The star’s estimated age is about 12.1 billion years. Nu Octantis has a binary companion, a K7-M1 class dwarf with a mass 0.5 times solar. The two are separated by 2.55 astronomical units and orbit each other with a period of 2.9 years. The primary star, Nu Octantis A, has an unconfirmed planet with a mass at least 2.5 times that of Jupiter in its orbit.
Beta Octantis is a white star halfway between the dwarf and subgiant stage of evolution. It has the stellar classification of A9IV-V. It has an apparent visual magnitude of 4.14 and is approximately 140 light years distant from the Sun. It is the second brightest star in Octans constellation.
Delta Octantis is an orange giant star belonging to the stellar class K2III. It has an apparent visual magnitude of 4.31 and is approximately 279 light years distant from the solar system. The star has a mass 1.2 times that of the Sun and a radius 25 times solar. Its estimated age is the same as the Sun’s, about 4.3 billion years.
[http://www.constellation-guide.com/constellation-list/octans-constellation/]
South Celestial Pole: the trapezoidal asterism at the center includes Sigma Octantis
Sigma Octantis is a magnitude 5.42 star in the constellation Octans, notable for being the current South Star. It is approximately 270 light years from Earth, and is classified as a giant, with a spectral type of F0 III. It is also a Delta Scuti variable, with magnitude varying by about 0.03 magnitudes over 2.3 hours. At magnitude +5.42, Sigma Octantis is barely visible to the naked eye, making it unusable for navigation, especially by comparison with the much brighter and more easily visible Polaris. Because of this, the Crux constellation is often preferred for determining the position of the South Celestial Pole.
Its position near the southern celestial pole makes it the southern hemisphere’s pole star, hence it is occasionally called Polaris Australis. Its counterpart is Polaris, the current North Star. To an observer in the southern hemisphere, Sigma Octantis appears almost motionless and all the other stars in the Southern sky appear to rotate around it. Once Sigma Octantis’ approximate position has been determined, either by the major stars in Octans or using the Southern Cross method, it can be positively verified using an asterism: Sigma, Chi, Tau, and Upsilon Octantis are all stars of around magnitude 5.6, and form the distinctive shape of a trapezoid.
[https://en.wikipedia.org/wiki/Sigma_Octantis]
South Pole star trails
No star dips below the horizon and the Sun never climbs above it in this remarkable image of 24 hour long star trails. Showing all the trails as complete circles, such an image could be achieved only from two places on planet Earth. This example was recorded during the course of May 1, 2012, the digital camera in a heated box on the roof of MAPO, the Martin A. Pomerantz Observatory at the South Pole. Directly overhead in the faint constellation Octans is the projection of Earth’s rotational axis, the South Celestial Pole, at the center of all the star trail circles. Not so well placed as Polaris and the North Celestial Pole, the star leaving the small but still relatively bright circle around the South Celestial Pole is Beta Hydri. The inverted umbrella structure on the horizon at the right of the allsky field of view is the ground shield for the SPUD telescope. A shimmering apparition of the aurora australis also visited on this 24 hour night.
[https://apod.nasa.gov/apod/ap120802.html]
[https://en.wikipedia.org/wiki/Octans]
No comments:
Post a Comment