Lesson focus: Moon phases come from viewing geometry; eclipseWhen one object in space blocks the light from another, such as the Moon blocking the Sun in a solar eclipse. require unusually close alignment.
Sequence phases, explain synchronous rotationSpinning around, like the Earth spinning once every day. and distinguish solar/lunar eclipseWhen the Earth comes between the Sun and the Moon, and the Earth's shadow covers the Moon..
Build on earlier learning
The EarthApply light, rotation and orbit geometry to the Earth–Moon system.
You will be successful when you can…
- Sequence eight common phases.
- Explain phases by illuminated fraction.
- Explain synchronous rotation.
- Distinguish solar and lunar eclipses.
Words that carry the science
- lunar phase
- Apparent shape of the Moon’s sunlit portion seen from Earth.
- new moon
- Phase when sunlit half faces mostly away from Earth.
- full moon
- Phase when sunlit half faces mostly toward Earth.
- solar eclipse
- Moon blocks sunlight from part of Earth.
- lunar eclipse
- Moon enters Earth’s shadow.
Keep phases, rotation and eclipses separate
01Phase sequence
New → waxing crescent → first quarter → waxing gibbous → full → waning gibbous → last quarter → waning crescent.
02Half is always lit
The Sun illuminates half of Moon; orbit changes how much of that half we see. Earth’s shadow causes lunar eclipseWhen the Earth comes between the Sun and the Moon, and the Earth's shadow covers the Moon., not ordinary phases.
The Moon does not make its own light: it reflectionWhen light or sound bounces off a surface, like light from a mirror. sunlight. Half of the Moon is always lit. As the Moon goes round Earth, we see different amounts of that lit half, so its shape seems to change. These shapes are called phasesThe different shapes of the lit-up part of the Moon that we see from Earth, from new Moon to full Moon..
03Rotation and alignment
Moon rotates once per orbit, so nearly same face points toward Earth. Its orbit is tilted, so most new/full moonWhen we can see the whole lit-up side of the Moon from Earth. miss exact eclipse alignment. An eclipseWhen one object in space blocks the light from another, such as the Moon blocking the Sun in a solar eclipse. only happens when the Sun, Earth and Moon are in an exact straight line. The Moon’s path is slightly tilted, so most months it passes just above or below that line.
Phases of the Moon
The Sun always lights half of the Moon. As the Moon orbits the Earth, we see different amounts of that lit half.
Controls
From the UK the lit part grows on the right (waxing), then shrinks to the left (waning). One cycle takes about 29.5 days.
Explain first quarter
At first quarter, Moon appears half lit.
- Sun still illuminates half the Moon.
- Moon is roughly 90° around orbit from new-moon direction.
- Earth sees half of the illuminated hemisphere.
- The name describes orbital progress, not illuminated fraction.
How to make the reasoning visible: Always distinguish whole Moon, sunlit half and visible fraction.
Phase and eclipse map
Match geometry to observation.
| Position/event | What Earth sees | Alignment |
|---|---|---|
| New moon | Mostly dark disk | Moon roughly Sun side |
| First quarter | Half lit | About 90° from Sun direction |
| Full moon | Fully lit face | Earth roughly between |
| Solar eclipse | Sun partly/fully blocked | New-moon alignment |
| Lunar eclipse | Moon darkens/reddens | Full-moon alignment |
Why not an eclipse every month?
Why does Moon rise later each day?
Use the map. During a solar eclipse, which object is in the middle: the Sun, the Earth or the Moon?
Check your understanding
Answer from memory, then check against the suggested answer.
What is lunar phase?
What is new moon?
What is full moon?
Why can we see the Moon?
About how long is the cycle of the Moon’s phases?
A learner says phases are Earth’s shadow. Correct them.
Why must Moon rotate to keep same face toward Earth?
Why does the Moon look like it changes shape during a month?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Answer the parts to explain why the Moon has phases (seems to change shape) and why there is not an eclipse every month.
Does the Moon make its own light? Where does its light come from?
(1)How much of the Moon is lit by the Sun at any time?
(1)What changes as the Moon goes round Earth?
(1)What line-up is needed for an eclipse?
(1)Why does this not happen every month?
(1)Total for this question: 5 marks
Command word: Answer the phases part first, then the eclipse part.