Knowing how to find the Southern Celestial Pole can be very useful in astrophotography.
In the northern hemisphere the star Polaris sits right on the northern axis, however in the southern hemisphere we don't have such a well defined marker.
Of course we can use a compass, but it can be just as easy to find when you know how, and this is what I am going to share below... But first,
How many South Poles are there?
The simple answer is three. We can work out were south on based on either the spin or the earth, or where the magnet points south, and this is not necessarily the same place.
The Geographic South Pole
The Geographic South Pole is the southern terminus of the axis around which earth spins. It is the 90th parallel, the point most distant from the equator.
The Magnetic South Pole
This refers to the orientation of the earths magnetic field, the southern point where the magnetic field lines are perpendicular to the earths surface.
This pole actually moves around due to perturbations in the flow of our planets core. The Magnetic South Pole is currently at about 80 degrees south and longitude is 107. Given that Perth is about 110 degrees this runs in our favour. Essentially the Magnetic South Pole is about 10 degrees closer to us than our geographic latitude suggests, increasing our chances of seeing the aurora.
Not to alarm you or anything but north hasn't always been north, and south hasn't always been south.
In fact our magnetic field appears to flip disarmingly regularly. We cans from the magnetic orientation locked in magnetic minerals in oceanic crust that the earths magnetic field has flipped back and forward many times.
Our planet doesn't even have a pure north or south. We have patches of the southern hemisphere that show a northern inclination in a compass!
You can read more about this from the experts at NASA here.
As you can see the magnetic field is not simple, this is why we have...
The Geomagnetic South Pole
This is an idealised dipole model of our earths magnetic field.
What is the Southern Celestial Pole
This is the point in space that corresponds to our southern axis. As we watch the stars at night we will notice their position in the sky changes over a few hours. This is because our earth is spinning and therefore our view of the Milky Way changes.
If we take a long exposure on the camera we can actually see this in motion. The image below is a stack of images totalling around two hours.
How to find the Southern Celestial Pole
We can use constellations to find the SCP. The first is one dear to all Aussies; the Southern Cross, also known as the Crux.
Close to the Southern Cross you will see two bright stars Alpha and Beta Centauri. We can use these in combination to find the southern axis.
Now we draw some imaginary lines.
In your mind extend the long limb of the Southern Cross...
Then draw a line between Alpha and Beta Centauri to become the short limb. At 90 degrees to this draw what will become the long limb and extend that out also.
The point at which these two long lines cross is the Southern Celestial Pole.
Now if we take a long exposure we can see the stars rotating around his central point.
This is called a 'star trail'.
And that is how we find the geographic south.
Some additional thoughts
Apparent Polar Wander
Apparent Polar Wander (APW) is actually a technique used to calculate the position of continents in the past. The surface of our planet is constantly moving around thanks to plate tectonics, old crust being destroyed at subduction zones and new crust being formed at mid ocean ridges. It can be hard to imagine how different the map of our world has looked throughout geological history, but by calculating how far north or south a particular piece of rock was when it cooled we know its rough latitude. Geologists use this to help work out the positions of continents in the past, and how ancient super continents looked. More on APW here.
