About a fortnight ago I received a text message at about 10 am from my mum.

Why is the moon still up there? Huh?
Texts from Mum

I don’t mind at all answering science questions like this one, but in my hasty text message reply I didn’t really do the moon – or my mum’s understanding of why she sometimes sees it during the day – justice. Sorry Mum. I was busy at the time. It’s a common misconception that we only see the moon at night and many primary school teachers will encounter students who hold this misconception (among others) in their classroom.

But I’ll answer Mum’s question, then we’ll move on to talking about the curriculum.

So why is the moon still up there? Huh?

Mum was looking at a waning quarter moon. These rise very late at night (early in the morning) in the dark and set very late in the morning (early in the afternoon) in the daylight. But let’s go back to the beginning.

The Earth orbits the sun, and the moon orbits the Earth. It takes the moon a little under 28 days to orbit the Earth. But the Earth rotates under the moon about 29 and a half times during this orbit… think about that for a little while, it will take a moment to think through.

So each day at the same time, the moon’s position is about 12.5 degrees  (~1/29 of a circle) from the point at which it was at the same time yesterday. Another way of thinking about this is that it rises a little less than an hour (~1/29 of a day) later each day.

So at the first point in its cycle, the moon will rise at about 6 pm, and set a little less than 12 hours later at about 6 am. A moon that does this is on the opposite side of the Earth from the sun; the half that is facing the sun and reflecting the light from the sun is also facing us. We can see the whole of the moon’s face lit up. This is called a full moon! We always see a full moon at night.

Each day following the full moon, the visible light from the moon will wane just a little each night, and the moon will rise a little later and set a little later too. This is because the moon is travelling a little further in its orbit each night. You can see the moon rise and set times for your area and dates at timeanddate.com.

Roughly a week after the full moon, the moon is at a right angle to imaginary line between the Earth and sun. When we look up at it we see only half of the face of the moon that is receiving and reflecting the sun’s light. It looks like a half moon to us, and it rises at approximately midnight and sets at approximately midday. So, Mum, that’s the moon you could see, and why you could still see the moon during the day.

Approximately a week later, the moon is between the Earth and the sun. The side that faces the sun can’t be seen at all from Earth, so we don’t receive any of its reflected light. This doesn’t matter too much anyway, because it rises at (have a guess….) about 6 am and sets at about 6 pm; it’s a new moon and it’s in our skies during the day.

Sometimes, in the days just before a new moon (when the moon phase is known as waning crescent, and it’s shrinking), or in the days just after a new moon (when the moon phase is known as a waxing crescent, and it’s getting bigger), we can see the faint crescent in our sky during the day. This is really hard because the sun’s light is so strong that it is hard to see anything else in the sky, particularly if an object is close to the sun.

Each day following a new moon, it appears as though the moon is waxing; growing bigger. As ever, it rises and sets a little later each day. About a week after the new moon, the moon is rising at about midday, and setting at about midnight. Once again we can only see about half of the face of the moon that is receiving and reflecting the sun’s light. But in the late afternoon we might see this half moon overhead.

And the cycle continues, each day moving a little further in its orbit, and from our viewpoint rising a little later and setting a little later. You can see that in the table below.

May 2014 Moon Rise and Set Times (Brisbane)
You can see the the moon rises a little later and sets a little later each day. You can also see (roughly) how illuminated the moon is as it passes over head.

So hang on… If each month the new moon is between the Earth and the sun, and each fortnight later, the Earth is between the moon and the sun, how come we don’t get a lunar eclipse and a solar eclipse every month?

This is a tricky bit.

The plane in which the moon orbits the Earth is not parallel with the plane in which the Earth orbits the sun. The plane of orbit of the moon around the Earth is tilted at about a 5 degree angle to the plane of the orbit of the Earth around the sun… Confused? Here’s a picture.

The plane of orbit of the moon around the Earth is tilted 5 degrees to the plane of orbit of the Earth around the sun.
The plane of orbit of the moon around the Earth is tilted 5 degrees to the plane of orbit of the Earth around the sun. Credit: Gary Osborn.

This tilt is just enough to offset the alignment so that most of the time we don’t experience an eclipse. The sun, Earth and moon have to be in almost-perfect alignment for an eclipse to occur. But isn’t it stunning when it does?

(Side note: Brian Cox’s Wonders of the Solar System, Wonders of the Universe, and especially Wonders of Life are absolutely stunning documentaries. I cannot recommend them enough.)

Still having trouble? It’s tricky to use a picture or an animation that just looks at the moon. Some of them look like the sun is traveling around the moon, when really, the moon is orbiting the Earth which is orbiting the sun. There are some great visualisations here for you to explore.

And here’s a playlist of videos about the moon that I’ve curated. If you can suggest a good video about the moon, please let me know!

The moon in the Australian Curriculum: Science

The moon and related phenomena are great topics for exploration in primary and middle years science lessons and units, and we have several opportunities to do so in the Australian Curriculum: Science, particularly the Earth and Space Sciences content of the Science Understanding strand.

Year 1: Observable changes occur in the sky and landscape (ACSSU019) – for example, recording short and longer term patterns of events that occur on Earth and in the sky, such as the appearance of the moon and stars at night, the weather and the seasons

Note how the misconception that the moon appears only at night can perhaps be reinforced by this statement! The night sky and relationships between the sun, Earth and moon appear as topics again in Year 3.

Year 3: Earth’s rotation on its axis causes regular changes, including night and day (ACSSU048) – modelling the relative sizes and movement of the sun, Earth and moon

In Year 7, there’s a lot of content that directly describes understandings of phenomena relating to the moon:

Year 7: Predictable phenomena on Earth, including seasons and eclipses, are caused by the relative positions of the sun, Earth and the moon (ACSSU115) – for example: 
  • investigating natural phenomena such as lunar and solar eclipses, seasons and phases of the moon
  • comparing times for the rotation of Earth, the sun and moon, and comparing the times for the orbits of Earth and the moon
  • modelling the relative movements of the Earth, sun and moon and how natural phenomena such as solar and lunar eclipses and phases of the moon occur

Strangely, the tides don’t appear explicitly as a topic at all in the Australian Curriculum: Science. But that doesn’t mean you can’t teach it!

Any questions or comments? I would love to hear from you, particularly if there’s anyway I can improve my explanations, or if you have an experience with teaching this topic that you can share!

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