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Phases of the Moon

Find out how to explain lunar phases.
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In this film, we’re going to explore how and why the moon changes shape and moves across the sky. But first of all, we need to establish our direction to find the moon. Now here, at the observatory in Greenwich, I can use our planetarium cone to find my direction. So ahead of me is north, and behind me is south. And this is the way that the cone is aligned. And that means that over my right is east, and west is over here, on my left. Now the moon appears to be in the southeast. And we can see a waxing crescent moon. This means that the right-hand side of the moon is lit up.
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Now the reason why the moon is visible at all is because it reflects sunlight. So if the right hand side of the moon is lit up, that must mean that the sun must be to the right-hand side over in the west. Now the best way to find the sun without directly looking at it and damaging your eyesight is to look for your shadow. And I can see that my shadow is over here on the right. So the sun must be in the complete opposite direction over here in the west.
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Now the reason why we see the moon move across the sky– we see it rise over the east, we see its reach its highest point in the south, and it sets in the west, just like the sun– is because of the Earth’s rotation. And that whole motion takes about 24 hours. But another effect that we see is the fact that the moon undergoes phases. It changes shape. And that’s because the moon is also in orbit around the Earth. And that process takes 27.3 days. So let’s go inside and explore how we get the phases of the moon. OK. So here we have a model to represent the sun, the Earth, and the moon.
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So over here on my left, I have a spotlight that represents the sun. Over on my right, I have a globe to represent the earth. And in my hands, I have a smaller ball to represent the moon. And in the classroom, you can use basketballs, or footballs, or any type of ball to represent the Earth and the moon. And if you want, you can even use your pupils to demonstrate the Earth-Moon system. And for your light source, you can use an ordinary torch like this one. OK. So let’s start off with the moon in this position, in between the Earth and the sun. Now, from the perspective of the Earth, you can see the unlit side of the moon.
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And we call this phase a new moon. Now if I take it round in an anticlockwise fashion like this, you should be able to see from the Earth, the right-hand side of the moon lit up. And we call this a first quarter phase because I’ve covered one quarter of the moon’s orbit. Now if I take it round over here, we can see a full moon. And I have actually lifted the moon slightly. And this is what happens in space. The moon’s orbit is tilted, so it is able to receive sunlight. And from the perspective of the Earth, you would see the whole of the moon lit up. And we call this a full moon.
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I keep taking it round, so we’ve now covered three quarters of its orbit. And from the Earth, you would see the left-hand side of the moon lit up. We call this phase a last quarter. And we keep going round in anticlockwise fashion back to the beginning again where we have a new moon.
How do we get different phases of the Moon?
We find (and our teacher forum seem to agree) that it is a very difficult topic to tackle unless you have visual resources to help you. With that in mind we created a video that you can use with your students to help explain the phases of the Moon.
Here are some important facts and figures to remember:
  • The Moon rises and sets just like the Sun – this is due to the Earth’s spin.
  • The Moon reflects the light of the Sun, making it visible.
  • The Moon changes phase because it is in orbit around the Earth. This series from one full moon to the next is called the lunar phase cycle.
  • The Earth orbits the Sun every 365.25 days.
  • The lunar orbit around the Earth takes 27.3 days.
  • The lunar phase cycle lasts 29.5 days. It is longer than the lunar orbit because the Earth is continuously moving relative to the Sun and so the Moon takes a few extra days to get into the right position again for us to observe the same initial phase again.
  • We see more or less of the reflected light on the Moon as it makes its way around the Earth. The dark regions are not in the Earth’s shadow but are instead parts of the Moon facing away from the Sun.
  • The Moon’s orbit is tilted by 5 degrees relative to the orbital plane of the Earth around the Sun. It moves up and down slowly with each orbit. This is why we see a bright Full Moon approximately once a month but don’t see a solar eclipse as often.
  • We always see one side of the Moon – this is because the Moon spins at the same rate as its orbit – once every 27.3 days. It turns on its axis just enough for us to never see the far side from Earth.
  • A solar eclipse happens when the Moon lies directly between the Earth and the Sun. The Moon’s shadow is small on the Earth, only people who are directly beneath the shadow witness a total solar eclipse. The phase here is New Moon.
  • A lunar eclipse happens when the Moon moves into the Earth’s shadow (opposite to the Sun). It glows red due to bending of red sunlight by the Earth’s atmosphere onto the face of the Moon. The phase here is Full Moon.
Download our phase of the Moon activity for your class. We recommend this activity for pupils aged 9+. Phases of the Moon classroom resource.
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