Crux Crux Camp David Category="Crux constellation"

Crux
[image] click for larger image
Abbreviation	Cru
Genitive	Crucis
Symbology	Cross
Right ascension	12 h
Declination	−60°
Area	68 sq. deg. Ranked 88th
Number of stars(magnitude < 3)	4
Brightest star	β Cru (Becrux or Mimosa) (App. magnitude 1.25)
Meteor showers	Crucids
Borderingconstellations	Centaurus Musca
Visible at latitudes between + 20° and − 90° Best visible at 21:00 (9 p.m.) during the month of May

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 nevertheless 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 (Sigma 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 Alpha Centauri and Beta Centauri, the point where the above line and this line intersect marks the Southern Celestial Pole.

Contrary to some people's belief, it is not opposite Ursa Major. In fact, in tropical regions both Crux (low in the South) and Ursa Major (low in the North) can be in the sky from April to June. It is exactly opposite Cassiopeia on the celestial sphere, and therefore cannot be in the sky with the latter at the same time. For locations south of 34° southern latitude Crux 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

[image] 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 omitted). Crux also appears on the Brazilian coat of arms. A stylised version of Crux appears on the Eureka Flag. The constellation was also used on the dark blue, shield-like patch worn by personnel of the U.S. Army's Americal Division, as well as the blue diamond of the 1st Marine Division.

Stars

: Stars with proper names:

(α^{1,2^{Cru) - double 0.77, 1.73 Acrux}}
(β Cru) 1.25 Becrux or Mimosa
(γ Cru) - double 1.59, 6.42 Gacrux
(δ Cru) 2.79; Decrux [Delcrux]

Stars with Bayer designations:

: ε Cru 3.59; ζ Cru 4.06; θ¹ Cru 4.32; θ² Cru 4.72; η Cru 4.14; ι Cru 4.69; κ Cru 5.89; λ Cru 4.62; μ¹ Cru 4.03; μ² Cru 5.08

Other notable stars:

HD 108147 7.00 - has a planet

External links

Letter of Andrea Corsali 1516-1989: with additional material ("the first description and illustration of the Southern Cross, with speculations about Australia ...") digitised by the National Library of Australia.
The Deep Photographic Guide to the Constellations: Crux