Moon phases

Big Idea

● The Moon does not make its own light. It looks bright because it reflects sunlight.

● The Moon’s phases happen because the Moon is a sphere orbiting Earth, and we see different

amounts of its sunlit half at different times.

● Important: half of the Moon is ALWAYS lit by the Sun — phases are about what we can see from

Earth.

Key Terms

● Phase: the shape of the Moon’s bright part that we see from Earth.

● Orbit: the path the Moon follows as it travels around Earth.

● Waxing: the lit part we see is getting bigger (growing).

● Waning: the lit part we see is getting smaller (shrinking).

● Terminator: the curved line that separates the Moon’s day side (lit) from night side (dark).

● Earthshine: a faint glow on the dark part of a thin crescent Moon, caused by sunlight reflecting off

Earth and lighting the Moon.

● Gibbous: a bulging, hunchback‑shaped phase when more than half of the Moon is lit.

Why the Moon Can Look Bright Even Though It’s Dark Rock

● The Moon’s surface is actually quite dark (low reflectivity / low albedo), similar to dark materials

like asphalt.

● It looks bright to us because it is lit by the Sun and the night sky around it is much darker.

How Long Is a Moon Phase Cycle?

● The Moon goes through a full set of phases about once a month.

● From New Moon to New Moon is about 29.5 days (this is called the synodic month).

Moon Phases in Order (One Full Cycle)

1. New Moon – Moon is near the Sun in the sky; the side facing Earth is mostly dark, so it’s hard to

see.

2. Waxing Crescent – a thin crescent appears and grows thicker each day.

3. First Quarter – the Moon looks half-lit, but it’s called ‘first quarter’ because it’s ¼ of the way

through its orbit cycle.

4. Waxing Gibbous – more than half lit and still growing toward full.

5. Full Moon – Earth is between the Sun and Moon, so we see the Moon’s fully lit half.

6. Waning Gibbous – still more than half lit, but shrinking after full.

7. Third Quarter – looks half-lit again; called ‘third quarter’ because it’s ¾ of the way through the

cycle.

8. Waning Crescent – a thin crescent that keeps shrinking until New Moon again.

When You Can See Each Phase (Rise/Set Pattern)

● New Moon: rises around sunrise and sets around sunset (it stays near the Sun, so it’s difficult to

spot).

● First Quarter: often visible in the afternoon and evening (roughly rises around noon, sets around

midnight).

● Full Moon: rises around sunset and sets around sunrise (up all night).

● Third Quarter: often visible late night into morning (roughly rises around midnight, sets around

noon).

Common Misconceptions (Fix These!)

● Myth: The Moon is only up at night. Truth: The Moon is up in the daytime about as often as at night

— it depends on the phase.

● Myth: Phases are caused by Earth’s shadow. Truth: Phases are caused by viewing angle (Earth’s

shadow causes a lunar eclipse, which is different).

● Myth: The Moon has a permanent ‘dark side.’ Truth: the far side is not always dark — it gets

sunlight too.

Extra Facts From the Video

● From the Moon, Earth would show phases too — and they are opposite: when we see New Moon,

someone on the Moon would see a ‘Full Earth,’ and vice versa.

● Earthshine: during a thin crescent Moon, the ‘dark’ part can glow faintly because bright Earthlight

is shining on it.

Why We Usually See the Same Face of the Moon

● The Moon rotates once in the same time it takes to orbit Earth (this is called tidal locking).

● That’s why we mostly see the same side (the near side) from Earth.