Why the shape changes

The Moon's shape never actually changes: only what we see changes. The Moon has no light of its own; it shines by reflecting sunlight. The half facing the Sun is lit; the half facing away is dark. Picture a ball painted black on one side and yellow on the other: half is always lit.

The Moon goes round the Earth, and we only ever see the half that faces us. But that half isn't always the lit half. Sometimes all of the lit half faces us; sometimes only part of it; and on new Moon day only the dark half faces us, so we see nothing.

See it for yourself (Activity 11.2). Push a stick into a small soft ball: that's the Moon. At night, in a dark open place, ask a teacher or guardian to shine a torch at you from about 3 m away: that's the Sun. Your head is the Earth.

Hold the ball at arm's length, a little above your head, towards the torch (position E). Is the side facing you lit? No. Now turn slowly round anticlockwise, arm out, watching the ball all the way.

The lit part you can see changes as you turn, and the line between light and dark looks curved, just like on the real Moon.

From A to C to E is waning; from E to G and back to A is waxing (Fig. 11.5). The real Moon goes all the way round in about a month.

The Earth turns once a day, much faster than the Moon goes round (nearly a month), so people all over the Earth see almost the same phase on any one day. And, as Activity 11.1 showed, the Moon is closest to the Sun in the sky at new Moon and farthest at full Moon.

One thing phases are not: they are not caused by the Earth's shadow falling on the Moon. That's a common mistake. They come from the changing positions of the Sun, Moon and Earth as the Moon goes round. The Earth's shadow on the Moon causes a lunar eclipse. Lunar eclipses can happen only at full Moon, solar eclipses only at new Moon, and they don't happen every month because the Moon's orbit is slightly tilted compared with the Earth's orbit round the Sun.