Crux, being Latin for cross, commonly known as the Southern Cross (in contrast to the Northern Cross), is the smallest of the 88 modern constellations, but also one of the most famous. It is surrounded on three sides by the constellation Centaurus while to the south lies the Fly (Musca).
Notable features
With the lack of a significant pole star in the southern sky (σ Octantis is closest to the pole, but is so faint as to be useless for the purpose), two of the stars of Crux (Alpha and Gamma, Acrux and Gacrux respectively) are commonly used to mark south. Following the line defined by the two stars for approximately 4.5 times the distance between them leads to a point close to the Southern Celestial Pole.
Alternatively, if a line is constructed perpendicularly between α Centauri (Toliman) and β Centauri, the point where the above line and this line intersect marks the Southern Celestial Pole.
Unlike what some think it is not opposite Ursa Major, however in tropical regions both Crux (low in the South) and Ursa Major (low in the North) can be both in the sky during April-June. It is exactly opposite Cassiopeia in the sky and is therefore cannot be together with the latter in the sky at a time. On locations south of 34° southern latitude it is always completely in the sky.
Notable deep sky objects
The Coalsack Nebula is the most prominent dark nebula in the skies, well visible to the naked eye as big dark patch in the southern Milky Way.
Another deep sky object within Crux is the Open Cluster NGC 4755, better known as the Jewel Box or Kappa Crucis Cluster, that was discovered by Nicolas Louis de Lacaille in 1751-1752. It lies at a distance of about 7,500 light years and consists of approximately 100 stars spread across an area of about 20 ly.
History
Due to precession of the equinox the stars comprising Crux were visible from the Mediterranean area in antiquity, so their stars had to be known by Greek astronomers. However, it was not regarded as a constellation of its own, but rather as part of Centaurus.
The invention of Crux as a separate constellation is generally attributed to the French astronomer Augustin Royer in 1679. It was known in that shape well before that, however.
The five brightest stars of Crux (α, β, γ, δ and ε Crucis) appear on the flags of Australia, Brazil, New Zealand (epsilon omitted), Papua New Guinea and Samoa, and the Australian States and Territories of Victoria, the Australian Capital Territory, the Northern Territory as well as the flag of Magallanes Region of Chile. The flag of the Mercosur trading zone displays the four brightest stars (epsilon omited). Crux also appears on the Brazilian coat of arms. A stylised version of Crux appears on the Eureka Flag.
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