The numbers
- Length of the cycle
- 29daysIt is called the synodic month
- Length of one phase
- 7daysIt can vary a little
- Why the length changes
- The Moon is not always the same distance away
How it happens, step by step
New moon
The Moon is between us and the Sun. Its lit face points the other way, and from here we see nothing.
First quarter
A week later we see it from the side: half of the disk is lit and the other half stays dark.
Full moon
The Earth is between the Sun and the Moon. From here we see the whole lit face, all at once.
Last quarter
Another week and we see it from the other side: half a disk again, but the opposite half.
It starts over
The crescent gets thinner until it vanishes and the cycle begins again. In all, about twenty-nine days.
To get an idea
If the phases were the Earth's shadow, the Moon would change shape only rarely, not every night.
The whole cycle lasts about twenty-nine days, and that is where the word month comes from.
Fun facts
These are not in the book: there was no room on paper.
Each of the four in-between phases lasts about seven and a half days on average.
The length of a phase can change by about a tenth, because the distance to the Moon changes.
The farthest point of the Moon's orbit is called apogee, the closest one perigee.
At new moon the Moon is in the daytime sky together with the Sun, and that is why we cannot see it.
From the other half of the world, the crescent looks turned the opposite way from how we see it.
Seen from the Moon, the Earth would show phases too, but opposite to ours.
Test yourself
The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.
Question 1 of 5★ is the one from your book
What causes the phases of the Moon?
Question 2 of 5
How much of the Moon is always lit?
Question 3 of 5
When is there a new moon?
Question 4 of 5
How long does the full cycle of phases last?
Question 5 of 5
Why does the length of a phase vary a little?
Riddles
I take about twenty-nine days to come back to where I started, and from that count came a word you use every month. What am I?
Sometimes I am a thin crescent, sometimes a whole disk, sometimes I am not there at all. And yet I never really change shape. What am I?
Try it yourself
Turn off the lights, switch on a lamp on one side and hold a ball. Turning around with the ball in front of you, you will see all the phases.
Notice which side the bulge of the crescent is on: it always points to where the Sun is, even when the Sun has already set.
The words
- apogee
- The point where the Moon is farthest from the Earth.
Sources: Wikidata (Q1425504), Wikipedia (Fasi lunari).