Fifth Class · Science · 2 minutes
Why does the Moon change shape?
The Sun lights half the Moon. We see different amounts of it.
Key terms
- Phase
- The shape of the lit part of the Moon that we see from Earth.
- New moon
- The phase when the Moon is between the Earth and the Sun and its lit side faces away from us.
- Full moon
- The phase when the Moon is on the far side of the Earth and we see the whole lit half.
- Waxing
- Growing; the Moon is waxing from new moon towards full moon.
- Waning
- Shrinking; the Moon is waning from full moon back to new moon.
- Crescent and gibbous
- Crescent is a thin curve of the lit part, and gibbous is more than half lit but not yet full.
Transcript
Some nights, the Moon is a thin, silver curve. Other nights, it's a big, bright circle. So why does the Moon seem to change shape?
Here's the secret. The Moon doesn't make any light of its own. It's a ball of grey rock, and what we see is sunlight, bouncing off it.
The Sun always lights up half of the Moon, the half that faces the Sun. The other half is in darkness. That's the Moon's night side, not the Earth's shadow.
Meanwhile, the Moon is travelling around the Earth. From one new moon to the next takes about twenty-nine and a half days, roughly a month. In fact, the word month comes from Moon!
Now, let's watch from two places at once. On the left, we're looking down from high above the Earth. On the right is what you'd see from here in Ireland.
When the Moon is between the Earth and the Sun, its lit side faces away from us. That's a new moon. Next comes a waxing crescent. Waxing means growing. Then first quarter, with half the face lit, and waxing gibbous.
When the Moon is on the far side of the Earth, we see the whole lit half. Full moon! Then it wanes, or shrinks: waning gibbous, last quarter, waning crescent, and back to new moon again.
So, the Moon reflects light from the Sun. The Sun always lights half of it. And as the Moon goes around the Earth, we see different amounts of that lit half. Tonight, look up, and see if you can name the phase!
Sources
Every fact in the video, and where it comes from.
- 1science.nasa.gov/moon/moon-phases
- The Moon does not make its own light; moonlight is reflected sunlight.
- The Sun always lights half of the Moon; we see different amounts of that lit half as the Moon orbits Earth.
- Phase order: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third (last) quarter, waning crescent.
- Phases are not caused by Earth's shadow (that is a lunar eclipse); the dark part is the Moon's own night side.
- In the Northern Hemisphere (Ireland) a waxing Moon is lit on the right-hand side, a waning Moon on the left.
- 2eclipse.gsfc.nasa.gov/phase/phase2001gmt.html
- From one new moon to the next (synodic month) takes about 29.5 days (mean 29.53 days).
- 3etymonline.com/word/month
- The word 'month' comes from Old English monath, related to mona (moon).
Image and sound credits
- “Full Moon (5836482263)” by NASA Goddard Space Flight Center (Public domain)
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