Why does the Moon change shape? An interactive moon phases simulation

The Moon does not change shape. The Sun always lights half of it, and as the Moon goes round the Earth we see more or less of that lit half. It is hard to see from one picture, because you need two views at once. This moon phases model puts them side by side: the Moon going round the Earth seen from space, and what you see when you look up at that moment. Move one and the other moves with it, so the class can see why the lit side is always the side the Sun is on.

The eight phases of the Moon, in order

1. New moon. The lit half of the Moon faces away from us, so we cannot see it at all.
2. Waxing crescent. A thin slice of the lit half faces us. Waxing means it grows a little every night.
3. First quarter. Half of the lit half faces us, so we see half a Moon, lit on the side the Sun is on.
4. Waxing gibbous. Most of the lit half faces us now, and it is still growing.
5. Full moon. The whole lit half faces us. The Sun is behind us, on the other side of the Earth.
6. Waning gibbous. It starts to shrink. Waning means a little less of it is lit each night.
7. Last quarter. Half a Moon again, but now the other side is lit, because the Sun is on that side.
8. Waning crescent. Only a thin slice is left. A few days later it is a new moon again, about a month after the last.

What you can do with it

Step through it: eight phases, one sentence each. Press a number on the Moon's path to go to that phase. Drag the Moon round the Earth and watch the view from the ground change with it. Play it, or slow it down. Switch to South of the equator to see the Moon the way it looks from there. Hide the labels and ask the class to name the phase. Share copies a link that opens it on the phase you are on.

Are the Moon's phases caused by the Earth's shadow?

No. The phases come from where the Moon is in its orbit, which changes how much of its sunlit half we can see. The Earth's shadow only touches the Moon in a lunar eclipse. It is a very common mix-up. In the view from space the class can see the Sun lighting half of the Moon the whole way round. At full moon it looks as if the Moon should sit in the Earth's shadow, but the real Moon's path is tilted, so it usually passes above or below it. The model does not show eclipses.

How can you tell if the Moon is waxing or waning?

North of the equator, a Moon lit on the right is waxing (growing) and a Moon lit on the left is waning (shrinking). South of the equator it is the other way round. Press South of equator and the view from the ground flips, which is how the Moon looks from Australia, New Zealand or South Africa. Either way, the lit side points towards the Sun. A first quarter looks like half a Moon because the name counts the journey, not the shape: the Moon is a quarter of the way round the Earth.

Why can you see the Moon in the daytime?

Because the Moon is up in the daytime sky about as often as it is in the night sky, and it is bright enough to see against the blue. The view from the ground gives a rough time of day for each phase. A first quarter Moon is high at about sunset, so it is already up in the afternoon, and a full moon is high at about midnight.

Where it fits

The words are pitched at ages 7 to 11: Years 3 to 6 in England, 2nd to 5th grade in the United States. In England it fits Year 5 Earth and space, which asks pupils to "describe the movement of the moon relative to the Earth". In the United States it supports NGSS 1-ESS1-1, using observations of the Sun, Moon and stars to describe patterns that can be predicted, and it is the kind of Earth, Sun and Moon model that MS-ESS1-1 asks for when a class explains the phases. The Moon and Earth pictures are from NASA.

Questions

More simulations

The other eleven, from blood flow through the heart to transport in plants, are on the Science Simulations page. For models the class can spin, from skeletons to dinosaurs, try 3D Explorer.