As the Southern Cross moves across the night sky, learn how to use it as a clock.
Safety: This activity asks you to head outdoors at night. Ask an adult for permission, and make sure they know where you are. Check the weather forecast and dress for the conditions.
You will need
- Clock or watch
- Whiteboard marker
- Pencil and paper
- Magnetic compass (optional)
- A clear night, with no clouds
What to do
- At 9.00 pm, go outside and look for the Southern Cross in the sky.
- If you can’t find the Southern Cross, use a compass to find which direction is south. Look for a pair of bright stars quite close together – these are called the pointers. If you imagine a line joining the pointers, the Southern Cross is nearby on that line.
- When you have found the Southern Cross, record its position. You could take a photo of it, or draw a picture. If you can see it through a window, you could trace the cross onto the window with a whiteboard marker. Is it straight up and down, or is it at an angle?
- At midnight, go to the same spot and look for the Southern Cross. Record its position again.
- Go inside and look at your pictures. Did the Southern Cross move?
- Use a protractor to measure the angle of the Southern Cross in both your photos. Did it rotate? If so, by how much? The time between your two photos was three hours. Can you calculate how long it takes for the Southern Cross to do a full 360° rotation?
- Set your alarm early, and get up before sunrise. How far has the Southern Cross moved? Does that match your calculations?
- Put a reminder in your calendar in three months time. At 8.00 pm, look for the Southern Cross. Is the angle the same?
Over the course of a night, the stars appear to rotate. But stars are billions of kilometres away, and they would have to move impossibly fast to cross the sky in one night. What’s actually happening is that the Earth is rotating.
Night and day are caused by the Earth rotating. When your part of the Earth is pointing at the Sun, it is daytime. When you’re in the Earth’s shadow, it is night.
At midnight in summer, the Sun is directly underfoot and overhead you have a view of one direction in space. Over the course of a year, the Earth orbits around the Sun. So at midnight three months later in autumn, the Sun is still directly underfoot, but the stars directly overhead are different. The change from day to day seems tiny, but over a whole year it adds up to one whole rotation.
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