Phases of the Moon

Why does the Moon change shape — and why is it never really a different shape at all?

Half the Moon is always lit. What changes is how much of the lit half faces us. Follow the monthly cycle, learn the names, and find out why the Moon is up in the daytime too.

5 depths≈ 205 min of reading75 practice questions

The ladder

Choose how deep to go

Every depth is open. Start where it feels interesting — you can always move back.

Discoverabout 30 minThe shape that changes — except it never doesMeet the Moon's monthly cycle: borrowed sunlight, a ball that is always half lit, and eight named phases. Learn to tell waxing from waning tonight, find out why the Moon is up in the daytime, and kill the biggest myth in astronomy — that the phases are Earth's shadow.Owner-accepted · not line-by-line reviewed
Understandabout 40 minReading the Moon: one angle explains everythingTurn the phase picture into a tool. Learn to go from the Sun-Earth-Moon angle to the shape, the fraction lit and the rise and set times; find out why craters show best at quarter moon, what earthshine is, and why the Moon keeps one face towards Earth.Owner-accepted · not line-by-line reviewed
Investigateabout 45 minPut the Moon on trialStop reading and start checking. Build a working model of the phases with a ball and a lamp, keep a month-long moon diary, measure the fifty-minute daily lag against your own rooftop, hunt earthshine, and predict a festival moonrise well enough to announce it.Owner-accepted · not line-by-line reviewed
Go deeperabout 45 minThe chase, the wobble and the brakeGo past the rules to the reasoning: derive the synodic month from two orbital speeds, see why a tithi stretches and shrinks, work out how often adhik maas is needed, derive eclipse rarity from the 5.1-degree tilt, and follow the torque that locked the Moon and is now pushing it away.Owner-accepted · not line-by-line reviewed
Extendabout 45 minTo the wobble, the far side and the far futurePush past the settled parts of the topic: measure libration for yourself, trace the far side from Luna 3 to Chandrayaan-3, work out why total eclipses have an expiry date, compare world calendars, and take on puzzles and open questions nobody has fully answered.Owner-accepted · not line-by-line reviewed

Practise and apply

Question bank · 75 questions

9 sections from Foundation to Challenge, each with hints and a worked solution — or play a 10-question round for points and stars.

Foundation 19Core 24Stretch 18Challenge 14

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Connected ideas

Follow a connection when you’re curious — some lead into completely different areas.

Phases of the Moon

Builds onanother area
Light
The Moon has no light of its own: we see the half of it the Sun is lighting.
Builds onanother area
Gravity
Gravity is what keeps the Moon in the orbit that produces the monthly cycle of phases.
Helps you understand
Eclipses
Eclipses can only happen at new moon or full moon — the two phases where the three bodies line up.
Related to
Tides
Spring and neap tides follow the phases: the biggest tides come at new and full moon.
Usesanother area
Exploration: reasons and consequences
Before clocks and satellites, the Moon and stars were how a navigator knew where they were.

Words to know

40 words are explained in these lessons

All vocabulary →

PhaseNew moonCrescentFirst quarterGibbousFull moonLast quarterWaxingWaningTerminatorEarthshineLunar month+28 more