Finding south using the Southern Cross – an essential skill

Finding south using the Southern Cross – an essential skill

Published by Nick Lomb on July 21, 2008 30 Comments

Southern Cross - finding south

Finding south using the Southern Cross, drawing Nick Lomb

Lesley asks how do you find north using the Southern Cross? Well, you find south as the Cross is in the southern sky and then it is trivial to find north as it is exactly in the opposite direction to south.

How do you find south? The first step is to identify the Southern Cross – it is a compact group of bright stars close together in the sky with the two Pointer stars always pointing to them from nearby. Then extend the main axis of the Cross from and in the direction of its brightest star by four and a half times its length. You have now reached the South Celestial Pole – the point about which the Cross and all stars turn in the sky. From the Pole drop a line straight down to the horizon – that is south.

It is worth practicing this from your backyard as such knowledge will be most helpful if you are ever lost in the bush!

Leave a Reply

30 Responses to “Finding south using the Southern Cross – an essential skill”

  1. May 07, 2014 at 3:09 pm, Toby said:

    This is so cool but it’s still confusing

    Reply

  2. August 18, 2013 at 10:50 am, Robyn said:

    I still cannot figure out how to find south using the southern cross. Are you able to describe it in more simple terms? Thanks

    Reply

    • August 19, 2013 at 12:57 pm, Nick Lomb said:

      Hello Robyn. I would have thought that the diagram in the post would make it clear. Still, once you have found the Southern Cross – in August it is leaning on its side in the south-west – in your imagination extend the line between the two furthest apart stars to the left for a distance four and a half times their separation. You have now reached the south pole in the sky and if you drop an imaginary line to the horizon from there the point where the line reaches the horizon is due south. I hope that helps.

      Reply

  3. June 21, 2013 at 11:50 am, Michael Edleston said:

    When I was first taught to find south using the Southern Cross, the method involved determining the point of intersection of the line drawn along the major axis of the Cross with a line drawn drawn as the perpendicular bisector of the two pointers & south was determined by dropping a line from the point of that intersection to the horizon.

    Since learning that method I’ve been very happy to accept that as a reliable technique & have often praised whatever ancient mariner it was who first worked out that combination of stars from all of the others up there.

    More recently, I’ve been told that south can be determined more simply by extend the major axis by four times its length & now I read here that the method involves measuring four & a half times the length of the major axis.

    This leaves me with the following two questions:

    1/ The “four” & the “four & a half” methods can’t both be correct – which is it?
    2/ The method that relies purely on multiples of the length of the major axis doesn’t appear to be able to compensate for the changing orientation of the Cross in the night sky. Given that the Cross markedly changes its orientation with time, how does the simpler method compensate for this?

    Regards,
    Michael

    Reply

    • June 21, 2013 at 12:33 pm, Nick Lomb said:

      Hello Michael. Four and a half times the length of the major axis of the Cross is a better approximation to the exact value than four times. And yes, the method does compensate for the changing orientation of the Cross during a night or during the year. As the Cross rotates about the South Celestial Pole its major axis always points towards it so that the above diagram applies whatever the orientation.

      Reply

      • June 24, 2013 at 1:04 pm, Michael Edleston said:

        Thanks for your response but I’m still a little in the dark: you mentioned the “exact value” but I’m none the wiser on the method to determine the “exact value” (position?) of South.

        Does the method I described return the “exact value” or is there some other method?

        Regards,
        Michael

        Reply

        • June 24, 2013 at 1:24 pm, Nick Lomb said:

          Hello again Michael. Regarding the method described above, the angular distance from Acrux, the closest star of the Cross to the south celestial pole, is 26°50′ and the distance between Acrux and Gacrux, the star of the Cross furthest from the pole, is 5°59′. Hence the number of multiples of the angular length of the long axis of the Cross needed to reach the pole is 26°50′/5°59′ = 4.485. As you can see the value 4.5 is a good approximation of the more exact value.

          Regarding the other method that you mention involving the perpendicular bisector of the two pointer stars, please see the later blog post http://www.sydneyobservatory.com.au/2013/finding-south-using-the-southern-cross/.

          I hope that this clarifies things for you.

          Reply

  4. January 15, 2013 at 8:43 pm, dave said:

    Using the pointer method, how do u know where to go perpendicular from the line created through the pointers? cheers

    Reply

    • January 16, 2013 at 3:13 pm, Nick Lomb said:

      Hello Dave. When using the Pointers to find south, draw a line perpendicular to the line joining the two stars and about halfway between them. Where that line meets the line formed by the two most widely separated stars in the Southern Cross is the south point in the sky (the South Celestial Pole).

      I hope that this helps.

      Reply

  5. July 09, 2012 at 6:39 pm, Bushman said:

    If you’re lost in the bush this method will definitely help you find south.  You’ll still be lost, but at least you’ll know where south is!

    Reply

  6. June 08, 2012 at 10:22 pm, Reidtillery said:

    If you want to see this for yourself, check it out on the Neave planetarium, an on line planetarium you can operate yourself. 

    Reply

  7. April 22, 2012 at 9:02 am, Zetellafella said:

    Anthony in Malawi 4 step’s is how I was taught to do it. Looking at the night sky around Woomera South Australia in 1983! Lived in the outback from 1982 to mid 1986.

    Reply

    • May 05, 2012 at 12:55 am, Anthony in Malawi said:

      Thank you.  I think this is why “The Pointers” are called “The Pointers”.
      My back door in Malawi faces south, and I watch the strange rotations of the Southern Cross every night.   Luckily, according to my method, south is always south, just to the left of a small tree and in line with my compass bearing.
      I think I might include this is a novel I am writing…

      Reply

  8. October 15, 2011 at 2:12 am, Tigersnake said:

    It is important to realise that the cross rotates 180 degrees during the night   
    around the south celestial pole.  Many people think the cross simply points south, but if you don’t understand that it rotates with the sky you would walk in a big half circle.  As you say, the pole is 4 1/2 times the length of the cross away from it, and I find an easy way is to look for the Small Magellan Cloud (if you can see it) and find the point half way between it and the Cross.  This is pretty close to spot-on.

    Reply

    • October 18, 2011 at 5:50 am, Anonymous said:

      Good point Tigersnake, but to use it you need to be lucky enough to be in an area sufficiently far from bright lights so that the Magellanic clouds are visible.

      Reply

  9. April 27, 2011 at 5:06 pm, Anthony in Malawi said:

    Not the easiest way! — how do you estimate 4.5x the length of the Southern Cross?
    Better way:
    a. Draw an imaginary line through the Southern Cross, as shown above
    b. Draw an imaginary line through the two Pointers
    c. Draw an imaginary line perpendicular to this
    d. The point where lines a and c intersect is the South Celestial Pole
    e. Drop perpendicular as above
    Honestly, this is much more practical!

    Reply

    • May 04, 2011 at 2:48 am, Anonymous said:

      Thanks Anthony. Yes, that is an excellent alternative way of finding south.

      Reply

    • August 25, 2011 at 10:41 am, Tony Johnston said:

      That’s the way they taught us in Army survival training!

      Reply

    • April 22, 2012 at 1:22 am, Del Wratten said:

      That is exactly what my Dad told me when I was a kid. 

      Reply

      • May 05, 2012 at 12:48 am, Anthony in Malawi said:

        Ah, good!   It does work.

        Reply

      • March 17, 2013 at 7:15 am, Lukas said:

        > this is also the way I was taught in the navy. Draw the 2 lines (right angel center to pointers and through the length of the cross) and where they meet, straight down to horrison is south. It compensates for the stars’ rotation. by following method 1 at this very moment in Cape Town in South Africa, south is 3.5 times southern cross’ length according to my compass and not 4.5.

        Reply

  10. April 18, 2011 at 6:12 am, Anonymous said:

    Thank you A and nam95 for pointing out the discrepancy. The text has been corrected.

    Reply

  11. April 17, 2011 at 3:32 pm, nam95 said:

    “Extend the line four and a half times the length of the cross.
    4. This will bring you to the point in the sky called the South Celestial Pole.
    From “museumvictoria.com.au”…….

    I for many years have successfully found South using the “Crux” in the Victorian high country -fortunately not often as my compass was on a cord around my neck-but there have been a few occasions.
    Aren’t these two contradictory?

    Cheers nam95

    Reply

  12. March 16, 2011 at 10:52 am, A. said:

    There is a discrepancy between the diagram (4 1/2) and the writing 3 1/2. Note big difference if covering a few kilometres, but considerable when needing to cover a few hundred. Otherwise, thank you.

    Reply

  13. December 20, 2008 at 12:19 pm, Monte North said:

    Hi Guys, Thanks for the info, it really helped when I was teaching ”Space” for Year 8 students, last term. Nick, maybe a diagram of the second method using the “Pointers” bisected could be included on your web page.

    Reply

  14. November 07, 2008 at 11:16 am, Nick Lomb said:

    Hello Peter. One of my colleagues gave me a bad rap over the knuckles about this post. The distance from Acrux to the South Celestial Pole is four and a half times the length of the Southern Cross and not three and a half as stated above. Hopefully, you will find that the diagram will work out with correct number.

    Reply

  15. November 06, 2008 at 6:34 pm, Peter van Roekel said:

    I have known both these ways and always thought they agreed. On looking at the diagram for the 3 1/2 distance, that does not work out. Diagrammatic error?

    Reply

  16. August 02, 2008 at 4:37 pm, Bob Howe said:

    Another way is to extend the main (long) axis of the cross as one line and a second line is created as a perpendicular bisector of the line joining the two pointers. Where these two “imaginery” lines intersect is South. I find that this is easier than figuring out 3 1/2 times a distance.

    Cheers

    Bob – ex RAAF Navigator

    Reply

  17. July 25, 2008 at 9:23 pm, Lesley Ware said:

    Thank you so much Nick. Wunnerful! Now, when we are out boating with our boating friends, we won’t get lost – you’ve also settled a long-time debate.

    Great website, and even better service!

    Kind regards,
    Lesley

    Reply

Search

About

The 'Observations' blog is run by the staff of Sydney Observatory which is located at Observatory Hill, The Rocks, in Sydney, Australia.

This site is for discussion purposes only and does not represent the official views of Sydney Observatory. Any views expressed on this website are those of the individual post author only. Sydney Observatory accepts no liability for the content of this site.

Please direct any correspondence about the content of the blog to:
observatory [at] phm.gov.au
and about web matters to:
web [at] phm.gov.au.