Earth and Space · Grade 6
Why does the Moon change shape — is it the Earth's shadow?
No. The Sun lights exactly half the Moon at every moment. What changes is how much of that lit half we can see from here, and that depends on where the Moon is in its orbit around us. The Earth's shadow does fall on the Moon sometimes — but that is a lunar eclipse, a few times a year, not every month.
Learning objectives
- Draw the positions of Sun, Earth and Moon that produce a new moon, a full moon and a half moon
- Say which phase the Moon will be in about a week after a given phase
- Explain why we always see the same face of the Moon from Earth
- Tell the difference between a phase of the Moon and a shadow falling on it
Always half lit
The Moon is a ball and the Sun lights it from one direction. So at any moment exactly half of it is lit — exactly like the Earth itself.
The lit half never changes size. What changes is the angle we are looking from, and therefore how much of it we can see.
You can show this in a dark room with a ball and a lamp. Hold the ball towards the lamp and you see only its dark side; walk around it and you see a crescent, then a half, and finally a full disc.
The cycle
When the Moon is between the Earth and the Sun, its lit side faces away from us. We see almost nothing — that is the new Moon.
When it is on the far side, with the Earth between it and the Sun, we see the lit side in full — a full Moon.
Between those come halves and crescents. The whole cycle takes about twenty-nine and a half days, so roughly a week separates one main phase from the next.
Why always the same side
The Moon does spin on its axis — but in exactly the time it takes to travel once around us. One spin per orbit.
The result is that the same side always faces us. That does not mean the Moon has a permanently dark side: the far side gets sunlight just as the near side does, simply not while we are looking.
When we have a full Moon, it is night on the far side. When we have a new Moon, the far side is in full daylight. It is the far side, not the dark side — a mistake a great many songs have kept alive.
Worked examples
Tonight is a full Moon. What will it look like in about a week?
- The whole cycle is about twenty-nine and a half days
- A week is roughly a quarter of the cycle
- A quarter on from full
Answer: A half Moon, on the way to new
Where is the Moon at new Moon?
- At new Moon we can barely see it
- So its lit side must face away from us
- Which means the Sun is behind it from our point of view
Answer: Between the Earth and the Sun
How much of the Moon is lit when we see a thin crescent from here?
- The Sun always lights half a ball
- The phase depends only on the viewing angle
Answer: Exactly half, as always. We just see very little of it
Common mistakes
- Explaining the phases as the Earth's shadow
- That shadow is a lunar eclipse, and it happens a few times a year. The phases happen every month and come from where the Moon is in its orbit.
- Talking about 'the dark side of the Moon'
- The far side gets light exactly as the near side does. When we have a new Moon, it is full daylight there. It is far, not dark.
- Thinking the Moon does not spin
- It does, exactly once per orbit. That precise match is what keeps the same side facing us.
What to remember
- Half the Moon is always lit; only our view of it changes.
- New: Moon between Earth and Sun. Full: Earth in the middle.
- The cycle is about twenty-nine and a half days.
- Far side, not dark side.