The phases of the Moon

North vs south

Australia is home, and I hope it will always be, but the new moon looks wrong to me. When the Moon starts to wax, it makes me feel a little uncomfortable, as if something is wrong. But then I’m from the UK and, although I have been here over 20 years, I still sometimes miss the dawn chorus, and in summer I miss the deep greens and lushness you get in a country where it rains a lot. Australia is home, but sometimes when I look at the new moon, I feel like a visitor.

What’s the STEM?

The terms waning, waxing, gibbous, full moon and new moon are terms we use to describe the Moon. We also might use the word phase. All of these can be explained with a little imagination and some playful experimenting. Adults may find it interesting that a new moon crescent in Australia forms, as you look up, on the left-hand edge. In the northern hemisphere is appears on the right.

What do you need?

6 cm or bigger Styrofoam or playdough/plasticine balls, lamp without a lamp shade, sharp pencils or pop sticks

Setting up

Put the lamp in the centre of the room on a low table or chair (so that the globe is roughly at shoulder level when you’re kneeling), push the sticks into the balls (you can secure them with sticky tape) and dim all lights (except the lamp in the centre).

Where to start?

You may have guessed it: The lamp represents the Sun, the ball stands in for the Moon and you are playing Earth in this scenario.

Kneel facing the lamp and hold the ball out in front of you so that it is between you and the lamp and you cannot see any parts of the ball that are lit up by the lamp. This starting point represents a new moon. A new moon is when the side of the Moon facing you is in complete darkness. For some cultures, it symbolises new beginnings. You can ask the children one by one or in small groups to stand behind you so that they can experience the phases of the Moon from your (Earth’s) perspective.

Now move the Moon around yourself clockwise with your arm stretched out and look at the way the light hits it. The left-hand edge is lit and looks like a crescent that fattens up as the Moon moves further away from the Sun. This is the Moon waxing, which is easy to remember as ‘waxing means maxing’. At 90 degrees to the lamp, half the moon will be lit. Then, is become gibbous (which is just a way of saying more than half of it is lit up – these astronomers!) until it eventually becomes a full moon. Then, as you continue around, it starts to wane.

You might want to model this yourself with the children peering over your shoulder and then encourage them to have a go at being the Earth themselves.

Interesting facts for adults

In this set-up, half the ball is always lit up by the lamp. The lit-up part only appears to be changing shapes as you can’t always see this entire lit-up half when you move the ball around. This is very similar to how the phases of the Moon work in real life, which is why we can use this model to show a simplified version of the phases of the Moon. Did you know that it takes the Moon 29.53 days to go through all its phases?

You may have noticed that you, as the Earth, don’t move around the Sun in the scenario above. In real life, the Earth is moving too, but completes only a small part of its orbit around the Sun during the time that it takes the Moon to orbit around the Earth. We have not included this movement as the double rotation would be difficult to coordinate and is not relevant.

Another important difference is that the Sun, Moon and Earth are not perfectly aligned but the Moon’s orbit is slightly tilted. This is important to understand as perfect alignment would cause a total solar eclipse every time the Moon was between the Earth and the Sun and a total lunar eclipse when the Earth is between the Sun and the Moon. In reality, these events are much rarer. For this set-up, you only need to keep in mind that you need to slightly lower your head if you’re accidentally causing an eclipse with your shadow!

The illustration above is showing the Moon phases as you would experience them from a northern hemisphere perspective. How do you think this is different in Australia? You could start observing the Moon every night. Maybe keep a Moon diary and draw a sketch of what you see every four days or so. Then compare it to the illustration. As a bonus question you can ask yourself what a crescent moon would look like from the equator.