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Earth and Space 6-8 / Week 02 / Monday
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Week 02 · Moon Phases

Monday

The Moon changes shape
// The lit half seen from changing angles
⏱ about 20 min

Monday: The Moon Changes Shape

Rocket points at the pale afternoon sky above the Sky Deck. "The Moon is out. In the daytime. And it is half a Moon."

Raven squints up. "What do you notice about the lit side?"

"It faces the Sun," Rocket says slowly. "The bright edge points right at where the Sun is."

Nova hovers beside them, her light off in the daylight. "Half a Moon," she repeats. "Is the Moon really half there?"

"No," Rocket says. "It is a whole ball. We just cannot see the rest."

"Then what hides it?" Raven asks. Rocket opens his mouth, then closes it.

Nova hums. "Good. Not knowing is where the week starts."

Rocket, Raven and Nova at the Sky Deck window in daylight. Rocket points up at a half Moon in a pale blue sky while Raven squints beside him with the Sky Log. Nova the companion drone hovers between them with her light off. A ball and a small orange sit on the sill.

Reflected sunlight

After the Sun, the Moon is the brightest object in our sky. But OpenStax Astronomy says it does not shine under its own power.

The Moon glows with reflected sunlight. NASA puts it plainly: what we call moonlight is actually sunlight reflecting off the Moon.

The Sun always lights up one half of the Moon, the half that faces the Sun. The other half is dark.

Like Earth, the Moon has a day side and a night side. Which side is lit changes as the Moon moves around Earth.

A cycle of shapes

Follow the Moon for a month and you will see a cycle of phases. It starts dark and grows more lit over about two weeks.

After the Moon's disk becomes fully bright, it begins to fade, returning to dark about two weeks later.

Many people think the phases come from Earth's shadow. They do not. The phases come from the angle we see the lit half from.

NASA says the cycle repeats about once a month, every 29.5 days. Your Thursday table from week 1 said 29.5 Earth days too.

FactWhat it means for the shape we see
The Moon makes no light of its ownOnly the half facing the Sun is bright
The Moon orbits Earth about once a monthOur viewing angle changes a little every day
The lit half always faces the SunThe bright edge of the Moon points toward the Sun
The cycle repeats every 29.5 daysThe same shape comes back about a month later
WHY IT GLOWS
  • Read the question.
  • Tap your answer.
Where does moonlight come from?
How much of the Moon does the Sun light at any moment?
What causes the phases?
StatementTrue or false?
The Moon shines with its own light.?
The bright edge of the Moon points toward the Sun.?
Earth's shadow causes the monthly phases.?
The cycle of phases repeats about every 29.5 days.?
WHY THIS EXERCISEThe standard asks you to explain the cyclic pattern of lunar phases with a model.
About how many days does one cycle of Moon phases take? Type the number.
WHY THIS EXERCISEThis number sets the pace of the Moon log you start this week.
Try it
Look for the Moon today, in daylight or at dusk, from the Sky Deck. If you find it, note which side is lit.
Point one arm at the Moon and one at the Sun's glow, never at the Sun itself. Notice the lit edge faces your Sun arm.
For a grown-up
This week your student models Moon phases with a lamp and their own head as Earth. A small ball or an orange on a pencil is the Moon.
From Sky Night on, the student keeps a nightly Moon log: a quick sketch of the Moon's shape. Please go out with them.
The facts this week come from NASA at science.nasa.gov and spaceplace.nasa.gov, and from OpenStax Astronomy at openstax.org.
Draw the Moon as you last saw it. Shade the dark part. Add an arrow showing which way the Sun was.

Good start. Tomorrow you learn the eight phases in order and why the Moon rises later each day.