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Phases of the Moon

Science · Year 5–Year 6

Tap the Moon in its orbit, and see the phase from here.

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About the moon phases

Two views of the same thing. On the left, the Moon at eight points in its orbit, looking down from above; on the right, the Moon as you would see it from here that night. Tap a Moon in the orbit and the view changes, or tap Next to walk round the month.

Every one of the eight Moons is lit on the same side, and that is the whole mechanism rather than a sentence underneath it. The Sun is off to the left and its light arrives in parallel, so nothing about the Moon changes through the month: half of it is always lit and half of it is always dark. What changes is where we are looking at it from.

The phases are not the Earth's shadow. That is a lunar eclipse, and it happens two or three times a year rather than every month. The picture is drawn from ABOVE the orbit for exactly this reason: drawn edge on, which is how most diagrams do it, the Moon appears to pass behind the Earth every four weeks and the diagram itself says the wrong thing.

Everything on the board is worked out from one number, the angle at the Earth between the Sun and the Moon. How much of the disc is lit, which side it is lit on and how far through the month it is all come from that, so no two of the eight phases can disagree, and the lit shape drawn is measured against the arithmetic in a test.

A crescent has one curved edge and one flatter one, because the line between light and dark is a circle seen at an angle. It is not a disc with a bite out of it, which is how a class draws it, and a bite is what an eclipse would look like.

How to use it on your whiteboard

  1. 1Ask the class what shape the Moon was last night.
  2. 2Tap a Moon in the orbit to see the phase from Earth, or tap Next to move round the month.
  3. 3Ask why the sliver is on the right at the start of the month and on the left at the end.
  4. 4Tap Why? for the line about the Earth's shadow.
  5. 5Watch the Day readout: the whole cycle takes 29 and a half days.

Questions

Is the Moon's phase the Earth's shadow on it?
No. Half the Moon is lit by the Sun all the time, and the phase is how much of that lit half we can see from where we are standing. The Earth's shadow falling on the Moon is a lunar eclipse, which happens two or three times a year, not every month.
Why is a quarter moon a half of a disc?
Because the quarter is a quarter of the way round the ORBIT, not a quarter of the disc. At first quarter the Moon has gone a quarter of the way round from new moon, and from here we see exactly half of its lit side.
What do waxing and waning mean?
Waxing is growing and waning is shrinking. A waxing moon is lit on the right and getting bigger each night; a waning moon is lit on the left and getting smaller.
Is it the same from Australia?
The phases happen at the same time, but they look flipped: an observer in the southern hemisphere sees the waxing moon lit on the left. This board draws the view from here.