Just as we can use the Pole Star in the northern hemisphere to find the direction of north, we can use the Southern Cross in the southern hemisphere to find the direction of south. However, whereas we can use the Pole Star to calculate our latitude, the Southern Cross is too far removed from the south celestial pole to be of any use for that purpose.
The Southern Cross, or Crux, is one of the best known constellations in the southern hemisphere, and is easily recognizable for the cross-shaped asterism formed by its four brightest stars. Crux (Latin for cross) is not visible north of +20° in the northern hemisphere but it is circumpolar south of 34° in the southern hemisphere which means that it never sets below the horizon there.
The cross has four main stars marking the tips (alpha, beta, gamma and delta).
Alpha Crucis is also known as Acrux (a contraction of ‘alpha’ and ‘Crux’). This is the brightest star in the constellation Crux.
Beta Crucis, also known as Mimosa or Becrux is the second brightest star in the constellation.
Gamma Crucis or Gacrux, is the third brightest star in Crux.
Delta Crucis or Palida is the fourth star and has variable levels of brightness .
The Pointers. The constellation Centaurus contains two bright stars: alpha Centauri also known as Rigil Kentaurus and beta Centauri also known as Hadar, which we use as pointers to the Southern Cross.
How to find the direction of south by using the Southern Cross. There are several methods of doing this but the simplest is as follows
- Make an imaginary line between Gacrux and Acrux.
- Extend this line from Acrux (the brightest star) for 4.5 times the length of the Southern Cross, as shown in the diagram below. This will take you to the position of the South Celestial Pole in the sky.
- From the South Celestial Pole, drop a line down to the horizon. Where this line touches the horizon is the direction of south.
Links: Locating Polaris Latitude from Polaris Astro Navigation for Survival
Pingback: The Survival Sundial | Astro Navigation Demystified