Observations

# Finding south using the Southern Cross – an essential skill

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!

## 51 responses to “Finding south using the Southern Cross – an essential skill”

• Will the longitud axis of the Cross plus its 4.5 times distance to the drop to the horizon rotate around the CSP clockwise? How many degrees in a night?

• Andrew Jacob says:

Pepe, Yes, and it rotates at about 15-degrees per hour about the SCP.

• Bill Watt says:

How do you find S when the cross is below the horizon. In the N we can use Cassiopia which is opposite the dipper. Is there something opposite the cross in the S hemisphere?

Also, with all the questions about 4 vs 4.5, I thot of a way to discribe the process of finding S just with the cross; a way that might resonate with a lot of folks, especially kids. Imagine the Cross is a kite with a tail that extends straight away from the kite and is 4.5 times the length of the kite itself. The S celestial pole is at the end of the tail.

• Andrew Jacob says:

Bill, From much of Australia and ll of New Zealand the Cross never sets, it is circumpolar. Northwards of a latitude of about 33-degrees south the Cross does begin to set but not for long. Even from the far north of the continent it is only missing for a few hours of the day. In place of it you could use the Magellanic Clouds – they form (roughly) an equilateral triangle with the southern celestial pole. As long as you remember that the triangle points away from the bright stars Canopus and Achernar it works. From a light-polluted area you could use the same approach (less accurate) with Canopus and Achernar in place of the Clouds, but you would need a little familiarity with the sky to point the triangle the correct way!

The tail of a kite? I’ll give that a go next time I run a Night Tour and see how it goes.

• Phil says:

The confusion here may be that no-one has yet stated whether the 4.5 length includes the actual length of the head and base stars of the cross – plus 3.5 times that length. or whether it is 4.5 times along from the base star. Could someone clarify that?

• Andrew Jacob says:

Phil, It’s 4.5 times if you start from the brightest star (the closest one to the pole) of the Cross.

• Greg says:

If the pic is to scale, then it would include the cross (and a further 3.5 lengths) as you suggest Phil. This method also marries up with the Scout method in finding the South Celestial Pole.

• Greg says:

Scratch that, it appears this pic is not to scale and the correct count does not include the cross itself (ie. Correct measurement is 4.5 lengths starting from the base)…
Having said that, this method for me when practiced just now using the stars, seems to give me a South line that is further east of the Scout method (which utilises the Pointers).

• Marvin says:

Thx for the info. I’ll try to attempt locating the Southern Cross from Moorea. Zero light pollution…

• sam says:

why has the southern cross seem to have rotated about 30 – 40 degrees in a anticlockwise position. i.e. are we moving in the galaxy or why has it changed.

• Andrew Jacob says:

Sam, Keep watching it for a few hours and you will see it ‘rotate’ as the Earth spins on its axis.

• Hannah says:

How do I locate it. I’m from wa so I’m so confused

• Andrew Jacob says:

Hannah, From WA finding the Cross is just the same as from anywhere else in Australia. Often its best if another person shows you. So you might want to visit a local observatory (Perth Observatory) or a local amateur astronomy club/society.

• Understanding basic Scout’s and Guide’s way to find your direction by the Stars is easy to verify!

After locating the Southern Cross and the 2 pointers – bisect the 2 pointers and where this line intersects the long arm of the Cross itself is the south celestial Pole. From that point you drop a line directly to the Horizon and that is due South.

Being privileged to be caretakers and wardens at the EyreBird Observatory in the old telegraph station at Eyre back in 2006, observations when the Southern Cross turns over about the south celestial pole still prove to be correct in finding South

• Celesty says:

Hi! The south you mean is the same thing as south celestial pole?

• Andrew Jacob says:

They both lie in the same direction. But “south” is parallel to the ground and “south celestial pole” is up in the sky.

• Jim says:

Thanks, I understand how to find south using the Cross at ground level but can you tell me how a 17th century sailor could establish the altitude of the CSP? It’s just a point in the sky isn’t it – it’s not like its a star and I believe they needed it to calculate longitude didn’t they?
Regards
Jim

• Nick Lomb says:

Hello Jim. There are a number of methods that 17th century navigators used to establish the height of the South Celestial Pole or latitude. One method was to measure the altitude of a star near the SCP when it was at its highest point and then 12 hours later at its lowest point. The mean of the two measurements would give the latitude. Of course, if the navigator knew from a star catalogue how many degrees the observed star was from the SCP then only one observation was necessary. In the 17th century navigators could not determine their longitude, that is, how far east or west they were from a particular point like Greenwich. One technique they used to find, say, a small island, was to sail to the correct latitude and then guess whether the island was to their east or west and sail along the latitude until they hit the island. A wrong guess could be disastrous as they would miss the island and possibly run out of water and fresh food.

• Jim says:

Thank you Nick, them using a nearby star or a catalogue explains it. I was struggling with how they could take a sight on an invisible point in the sky!

• chris says:

Thank you for letting people know how to use the southern cross to find south. Obviously they will then also know where north is.

Trust the southern cross.It will take you home..

• Toby says:

This is so cool but it’s still confusing

• Robyn says:

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

• Nick Lomb says:

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.

• Michael Edleston says:

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

• Nick Lomb says:

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.

• Michael Edleston says:

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

• 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.

• dave says:

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

• 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.

• Bushman says:

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!

• Reidtillery says:

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

• Zetellafella says:

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.

• Anthony in Malawi says:

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…

• Andrew Gee says:

I suspect Rigil Kent and Hagar are called pointers because they point to the Southern Cross. (Actually, they point just a tad north of Crux, but it is close enough.)

Why are the pointers useful? Crux is not very big. From top to bottom (Gacrux to Acrux), it is only about six degrees. As a frame of reference, the pointer stars in Ursa Major (Merak and Dubhe) are only five degrees apart. Nor is it all that bright. In hazy conditions or bright skies it could be easy to miss. The pointer stars are much brighter and much easier to see. They grab your attention.

Another reason to look for the pointer stars is they keep you from getting confused with another grouping nearby called the False Cross (Delta Velorum, Kappa Velorum, Epsilon Carinae, and Iota Carinae). Not that the False Cross is of no use. You can use the first two (δ Vel and κ Vel) to get within one degree of true south.

Andrew Gee

• Tigersnake says:

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.

• Anonymous says:

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.

• Anthony in Malawi says:

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!

• Anonymous says:

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

• Tony Johnston says:

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

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

• Anthony in Malawi says:

Ah, good!   It does work.

• Lukas says:

> 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.

• Anonymous says:

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

• nam95 says:

“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.

Cheers nam95

• A. says:

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.

• Monte North says:

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.

• 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.

• Peter van Roekel says:

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?

• Bob Howe says:

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

• Lesley Ware says:

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