In the northern hemisphere, the bright and easy-to-find star Polaris marks the position of the north celestial pole. This makes it easy for stargazers and navigators to find north, and get oriented in the night sky.
But in the southern hemisphere, it is different.
By sheer bad luck, the south celestial pole (SCP), the imaginary point in the sky directly above the Earth’s south pole, coincides with no bright star, and the southern polar sky has few other guides to find the pole (see image at top of page). But using the Southern Cross and a couple of well-known stars in the constellation Centaurus, it’s possible to roughly locate the SCP. Here’s how it works…
The SCP lies in the dim constellation Octans, the Octant, one of the handful of constellations laid out by the diligent astronomer Lacaille. Octans has no bright stars, although the dim 5th-magnitude sigma Octanis lies within a degree of the pole. But this star is not easy to find with the unaided eye.
A better way to find the pole uses the constellation Crux and the two brightest stars in Centaurus, Rigil Kent and Hadar. These stars are sometimes called the “Southern Pointers”. Extend an imaginary line through the long axis of the cross, from the star at the top (Gacrux) through the star at the bottom (Acrux). Now extend another line perpendicular to the Southern Pointers which is perpendicular to the line connecting them. The line from Crux intersects the line perpendicular to the pointers about 5 degrees NW of the south celestial pole.
As a double check, draw an imaginary line from Hadar in Centaurus to Achernar in the constellation Eridanus. The SCP lies just south of the halfway point between these two bright stars.
A diagram showing how to find the south celestial pole (SCP), which is at the “bullseye” in the celestial coordinates on this image. Click to enlarge.
These methods are good enough to get you looking south. Next time, you’ll learn how to use the rotation of Crux about the south celestial pole as a clock to help determine the approximate local time. But for now, that’s it.