But these “Phases” as they are called--these changes in the shape and brightness of the Moon--are brought about by her own movements, as she travels round the Earth. She does not actually alter her shape: but she does actually alter her place in the sky, so that we get different views of her from week to week. The four weeks of the Moon’s phases are called a “Lunar Month.” You can make clear to your mind how the changes come about, by acting them out with a lamp and a big ball. There must be no other light in the room. Stand first with your back to the lamp and the ball in your hand, held out at arm’s length: so that your head is _nearly between the lamp and_ _the ball_. Not quite between, so as to shade the ball. Hold the ball just a little higher than your head: and the lamplight will fall upon that side of it which is towards your face. Then you have Full Moon. The lamp is the Sun: your head is our Earth: the ball is the Moon. You see how the lamp lights up the half of the ball which is towards yourself. Next turn round with your face to the lamp, and hold the ball at arm’s length _between your head and the lamp_, only a little higher or lower--not quite in the line between so as to hide the lamp from you. The lamp-light now falls on the other side of the ball; and the dull unlighted side is towards your face. This is New Moon. Once again the lamp is the Sun, your head is the Earth, and the ball is the Moon. You see how the lamp lights that half of the ball which is turned away from you. The real New Moon in the sky is invisible. Here you can see the dark side of the ball because the lamplight creeps round it. Still even here you will find a difference between the bright and the shaded parts. Then, if you hold the ball at arm’s length half-way round on one side of your head, you will see how matters are at the Quarters. The lamp still shines full on one side of the ball, but only half of the brighter side is towards you, and half of the darker side. In the real Moon the shaded quarter would be hidden, and only the bright quarter would be visible. [Illustration: _The Moon. Third quarter. 16¾ days old._] This “quarter” we call “a Half Moon.” It is a quarter of the whole Moon, taking the Moon all round; but it is a half of the bright side, which makes our Full Moon. All that we really know about the Moon’s surface is what we see on one side of her. The other side is never turned towards us. No man on this Earth has ever seen it. A man living on that side of the Moon which we can see might look at all parts of the Earth in turn. As the Earth spins on her axis she turns each side towards the Moon, one after another, in only twenty-four hours. But, although the moon spins, we see only and always one side of her.