- Written by Nadia O’Carroll
The constellation consists of five stars/star clusters called Alpha Crucis (Acrux), Beta Crucis (Mimosa), Gamma Crucis (Gacrux), Delta Crucis and Epsilon Crucis. Trailing below the Southern Cross are two bright stars known as the pointer stars, they are Alpha Centauri and Beta Centauri.
Although it is the smallest constellation, the distances between the stars of the Southern Cross are mind boggling for humans. The estimated distance between the top star and the lower star of the constellation is 232.77 light years, or expressed in more familiar terms, if an astronaut travelled between these two stars at a constant speed of 100 km/hour, it would take about 2.5 billion years to complete the journey.
The Southern Cross features in the cultures and legends of many people who inhabited the Southern Hemisphere and was used by seafarers and travellers in celestial navigation. It is a significant symbol for Australia and New Zealand and features in both their flags.
The Ancient Greeks knew of the Southern Cross constellation and considered it to be the hind part of the large constellation called Centaurus, named after the Centaur, a mythical creature that was half man half horse.
It is now impossible to see the Southern Cross from Greece, what has changed? The answer is a process called axial precession, this is a slow, steady, continuos change of Earth’s rotational orientation. The best way to understand it, is to think of the Earth as a spinning top, the angle of axis stays constant but the top of the Earth moves around in an arc like a wobbling top. A complete 360 degree cycle takes 25,765 years, one degree every 71.6 years, and it is this slow alteration in the orientation of the Earth which has caused the Southern Cross to recede below the horizon in Greece. Axial precession was first observed and understood by the great Greek astronomer, Hipparcus, between 127 BC and 147BC.
There are some interesting astronomical features within the Southern Cross, The Coal Sack is a dark nebula visible with the naked eye and The Jewel Box is a beautiful star cluster that can be seen well with binoculars. Unfortunately light pollution is masking our view of the night sky, the smallest Southern Cross star, Epsilon Crucis, has already disappeared from the skies of large Australian cities. Even on Tamborine Mt sky glow and local bright, poorly designed street, commercial and gate lighting are steadily reducing our spectacular view of the starry night sky.