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Finding The South Celestial Pole (SCP)

Finding The South Celestial Pole (SCP)

I suppose we have always taken the south of the planet or where we are on it at any given time for granted. Let’s face it – 180o is south, we all know that! But, that is not much good to an equatorial telescope mount. They need to be on celestial south and that at this time is in the constellation of ‘Octans’ about 1o from a 5.5 magnitude star  called ‘Sigma Octanis’. A very dark sky (and some luck) is needed to view this star, something we just don’t have in the coastal north of Australia. Let’s now have a look at 3 or 4 methods of finding ‘it’, (SCP).

Method 1:

If you cast your eyes southward you will see the constellation ‘Crux’ or ‘Southern Cross’ and the two (2) pointers ‘Alpha’ and ‘Beta’ Centaurus. Directly below Crux is an asterism that looks like a crushed box (a little), that is ‘Musca’ (The Fly) and below that again is our target – ‘Octans’. It is very difficult to see in our northern position at Townsville on Latitude 19o south.

Now to find the SCP I like to use a nice straight piece of dowel about 90cm long which I hold up towards ‘Crux’. The object of the exercise is to align the dowel along the axis of the Southern Cross. Now measure down the dowel 4½ times the length of ‘Crux’. This will put you very close to the SCP. Again a nice dark sky would simplify the process but alas, we have what we have!

Method 2:

The second method is to (using your dowel) imagine another line drawn at right-angle mid-way between the two (2) pointers ‘Alpha’ and ‘Beta’ Centaurus straight down to dissect the line we had through ‘Crux’ in Method One (1). The point where these two (2) lines cross is very close to the SCP.

Method 3:

This method is some what more involved and requires that you have vision of ‘Centaurus B’, ‘Pavo’ and ‘Archernar’ a star easily found on a southern hemisphere map. Using our dowel once again take an imaginary line from ‘Centaurus B’ straight down to ‘Archernar’. Now imagine another line drawn from ‘Alfa’ Pavonis straight through between ‘Beta’ and ‘Delta’ Pavonis, to dissect our first line, carry through just a tiny bit (about a degree). This can also be used in conjunction with the line through ‘Crux’ from Method One (1). The SCP will be between this and the line from ‘Archernar’ within a couple of degrees. I realise this can seem quite daunting but it is really simple with a little practice.

Method 4:

This method involves using a compass to set your mount on South Celestial Pole. Knowing the variation between magnetic and true south is necessary and can be difficult to obtain (maybe from a topographic map or a surveyor perhaps?) We in Townsville (Australia) can set our mounts to around 172.5o and the latitude to 19.11o south. Now the setting scale on most mounts is just not that good so just set it a ‘teensy’!!! bit over 19o. A good latitude reference map for us in the north is the Cyclone Warning Map issued every year.

There are a couple of simple methods involving the formation of imaginary triangles given that all the celestial points are visible.

  1. Using the Small Magallenic Cloud (SMC) and Large Magallenic Cloud (LMC) as two (2) points of our triangle should put the third point close to the SCP.
  2. This method is very similar to Method (1) with the only difference being our reference points. In this case make your imaginary triangle base between ‘Archernar’ (Alfa Eridani) and ‘Canopus’ (Alfa Carinea) which will put the third point within a couple of degrees of the SCP.

I hasten to add that the latitude setting on your mount must be applied for your area in every case. The SCP is found at the same number of degrees above due south as the observers latitude and if you plan your viewing session well you will have plenty of time to set your scope on a star and do a minor drift correction or two (2) if need be. Once correct take note of your compass bearing for future reference. These settings can be a little haphazard (rough) for just normal viewing but need to be spot on for imaging. A compass that is or can be illuminated is very handy for that late set-up so have a look for a good one. The one I have is a military compass (or similar) obtained from Anaconda Sports Warehouse in conjunction with an Ultra Violet Torch to light it up at night. Cost was around $160AUD.

It is also a really good idea to get to know your viewing spot from a direction point of view. I reckon some of you think that is a bit ‘corny’ but believe me if you are lucky enough to focus on something important and the old adrenalin kicks in you wont know up from down let alone north, south, east or west. So check it all out so you know these directions well at night, you never know!

The Sun rises and sets in slightly different places (Winter and Summer) across the year so have a check. Get yourself a copy of ‘The Atlas of the Southern Night Sky’ by Steve Massey & Steve Quirk, it’s all in there folks!

Hope this has helped some.

Sue.
(Bertha)

Diagram below is a sketch only and may not be totally accurate.

FINDING SOUTH CELESTIAL POLE

polar-alignment

  • Method 1 - Solid Black Line
  • Method 2 - Solid Red Line (used in conjunction with Method 1)
  • Method 3 - Dotted Red Line