Activity: Finding South

Last week, when I was out taking photos of the Lunar Eclipse, I realised two things:

1. Even in May, Canberra can be pretty cold.
2. We haven't done an astronomy activity for a while.

Now that we are heading into winter, night is falling earlier, so it seems like a good time to take a look at the sky. I thought I might show one of the simplest, but most useful astronomy activities I know: how to find South using the Southern Cross.

You will need

• A clear view of the southern sky at night.

Sorry if you're reading this in the Northern Hemisphere, but this activity will only work in the Southern Hemisphere.

What to do

It may help to look at the photos on the web site for this one.

The Southern Cross is a constellation of five stars. They are arranged just like the stars on the Australian and New Zealand flags. Tonight (14/5/04), the Southern Cross will be tilted to the left in the early evening, straight up and down at around 9:45 pm and tilted over to the right by midnight.

Look for five stars arranged like the stars on the Australian flag. The star near the middle is fainter than the others and you may not be able to see it if you live in a city. If you hold your hand at arms length, the length of the Southern Cross is about the same as the width of three fingers.

The Southern Cross and Pointers. Roll your mouse over the image to highlight them.

There are a couple of places in the sky where the stars look like they could be the Southern Cross. To be sure you are looking at the Southern Cross, look for two bright stars nearby. If you draw a line through them, it should go near the top star in the Southern Cross. These stars are Alpha and Beta Centauri. They are part of the constellation Centaurus, the Centaur. The two stars together are often called "The Pointers", because they point towards the Southern Cross.

With a bit of practice, you'll find it easy to recognise the Southern Cross. Once you can find it, there are at least three ways to find south. All of these methods rely on finding a point on the sky called the "South Celestial Pole", then finding the point on the horizon directly below it. They give slightly different results, but they are all accurate enough for everyday (or everynight) use.

• Length of the cross.
  1. Imagine a line that goes from the top star in the cross and out through the bottom.
  2. Measure along this line 4.5 times the length of the Southern Cross. The point on the sky where you end up is the South Celestial Pole
     • I find this easiest if I hold my arm out straight, hold my thumb and forefinger apart the length of the cross and use this to measure the distance.
  3. Point at the South Celestial Pole with one finger.
  4. Drop your arm straight down until you are pointing at the horizon.
  5. You are now pointing due south.
• Through the pointers.
  1. Draw an imaginary line through the Southern Cross, from top to bottom.
  2. Draw another imaginary line going straight through the middle of the pointers (at right angles to a line joining the two pointers).
  3. Point at the spot on the sky where the lines cross. This is the South Celestial Pole.
  4. Drop your arm straight down until you are pointing at the horizon.
  5. You are now pointing due south.
• Halfway to Achenar.
  1. Imagine a line that goes from the top star in the cross and out through the bottom.
  2. Follow this line along until you reach a reasonably bright star (slightly brighter than the brightest star in the Southern Cross). This star is called Achenar.
  3. With one hand, point at the top star in the Southern Cross. Use the other hand to point at Achenar.
  4. Bring your hands together. You are now pointing at the South Celestial Pole.
  5. Drop your arm straight down until you are pointing at the horizon.
  6. You are now pointing due south.

Once you are facing south, north is behind you, east is on your left and west is on your right.

What's happening?

If you look at the stars for long enough, they appear to move slowly across the sky. In fact, although stars can be moving very quickly through space, they are so far away that to us they appear practically still. The apparent movement of the stars in the night sky is because the Earth is spinning.

If you were to stand on the South pole at night and look straight up, the stars would appear to move in a circle, around a point in the sky directly above you. That point is the South Celestial Pole (SCP).

If you move away from the South Pole, the stars will still appear to move in a circle around the same point in the sky, but it won't be above you any more. That's just because the Earth is round, so as you move around it, the angle between the ground and the SCP changes. When you are at the South Pole, it's right above you, when you are at the Equator it's right down on the horizon. I live in Canberra, which is 35 degrees away from the equator, so the SCP appears to be 35 degrees above the horizon.

No matter what angle it makes with the ground, since the SCP is the point in the sky directly above the South Pole, it will always be due south from you. So if you can find it, you just need to drop a line straight down and you can find south.

In the Northern Hemisphere, there is a star called Polaris that just happens to be close to the North Celestial Pole. This makes it easy for people in the northern hemisphere to find north - they just find Polaris. There is no handy star near the South Celestial Pole, so in the southern hemisphere we need to learn other ways of finding the South Celestial Pole. The ways we have seen, using the Southern Cross, are a pretty effective way of doing it. Each of them is slightly inaccurate, but they are close enough for everyday purposes.

Applications

• People have navigated by the stars for centuries. As well as being able to find North and South, sailors can also work out their latitude (how far North or South they are) using the stars. For example, if the SCP was 30 degrees above the horizon, then the sailors would know they were 30 degrees south of the Equator.