The Sun lights exactly half the Moon at all times, just as it lights half of Earth.
Step 1: Something to Notice
Watch first. The explanation comes later.
Over a month the Moon goes from invisible, to a thin crescent, to half, to full, and back again — always in the same order, always taking about 29 days.
What is actually changing about the Moon?
Step 2: Find Out
Watch the top-down view and the view from the ground side by side.
Step 1 — Predict
Across a whole month, how much of the Moon is lit by the Sun at any moment?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
What changes is our view
As the Moon orbits us, we see different amounts of that lit half. That is the phase.
About 29 days
One full cycle of phases takes about 29.5 days.
Waxing and waning
Waxing means the lit part is growing. Waning means it is shrinking.
When the shadow does matter
Earth's shadow does sometimes fall on the Moon. That is a lunar eclipse, and it dims the whole Moon at once.
Half the Moon is always lit
The Sun lights one half of the Moon at all times. Phases are about how much of that lit half faces Earth, which changes as the Moon orbits. Nothing is covering it up.
Phases are not the Earth's shadow
If the Earth's shadow caused phases there would be one every month, and the shadow would be curved the wrong way. The Earth's shadow on the Moon is a lunar eclipse, which is a different and rarer event.
Step 4: A Common Mistake
Lots of people think
“The Moon's phases are Earth's shadow falling across the Moon.”
Step 5: Why We Always See the Same Face
The same idea somewhere new.
The Moon takes the same time to spin once as it takes to orbit Earth once. That is called tidal locking, and it means the same side always faces us — there is no permanently dark side, only a far side that gets just as much sunlight.
Step 6: Think It Through
Practice makes it stick.
The Calendar
Problem 1 of 1
A full moon happens tonight. About how many days until the next full moon?
days
Reading the Phases
1 of 6
What percentage of the Moon is lit by the Sun right now?
2 of 6
At new moon, where is the Moon?
3 of 6
Put the phases in order, starting from new moon.
- 1First Quarter
- 2Full Moon
- 3Last Quarter
- 4New Moon
4 of 6
Which statements are true?
5 of 6
A lunar eclipse can only happen at which phase? Enter 1 for new moon, 2 for full moon.
6 of 6
Match each phase to what is happening.
Tap a card on the left to start.
Step 7: Quick Check
Show what you know.
Question 1 of 1
What actually changes during the monthly cycle of phases?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
A classmate says the phases of the Moon happen because Earth's shadow covers part of the Moon. Use the model to explain why this cannot be right.
Words to useorbitlit halfshadowlunar eclipsephase
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- Half the Moon is lit at all times.
- The phase is how much of that lit half faces Earth.
- One full cycle takes about 29.5 days.
- Earth's shadow causes lunar eclipses, not phases.