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

Friday

Our constant companion
// The lit half seen from changing angles
⏱ about 20 min

Friday: Our Constant Companion

Rocket holds the orange on its pencil and walks a slow circle around Raven, always facing her.

"What do you notice?" Raven asks. "You never see the back of my head," Rocket says. "I turn once while I go around once."

"That is what the Moon does," Raven says. "Same face toward Earth, always."

Nova hovers above them. "NASA calls it synchronous rotation," she says. "The Moon spins once for every orbit."

"So there is a far side we never see," Rocket says. "Is it always dark?"

Nova hums. "You already know the answer. Think about the orange in the lamp light."

"Half lit, always," Rocket says. "The far side gets sunlight too. It is just far, not dark."

Why we always see the same face

The Moon rotates on its axis in exactly the same time it takes to orbit Earth. OpenStax calls this synchronous rotation.

As a result, the Moon always keeps the same face turned toward Earth. People on Earth only ever see one side.

You can act it out by walking around a friend while always facing them. You spin once for every lap.

Some people call the far side the dark side. OpenStax says that is a misunderstanding: the Sun rises and sets on all sides of the Moon.

The Moon in your life

NASA says the Moon is an average of 384,400 kilometers from Earth. About 30 Earth-sized planets could fit in between.

Its radius is about 1,740 kilometers, so it is less than a third of the width of Earth.

Sometimes a crescent Moon shows its dark part glowing dimly. NASA calls this earthshine: sunlight reflected off Earth onto the Moon.

It takes about 27.3 days for the Moon to orbit Earth. But one new Moon to the next takes about 29.5 days, because Earth moves too.

Next week you use the same lamp model to explain eclipses, the rare times the shadows do line up.

Number from NASAWhat it describes
384,400 kilometersAverage distance from Earth to the Moon
About 30Earths that would fit between Earth and the Moon
About 1,740 kilometersThe Moon's radius
27.3 daysOne orbit of the Moon around Earth
29.5 daysOne cycle of phases, new Moon to new Moon
EXPLAIN IT
  • Read the question.
  • Tap your answer.
Why do we always see the same face of the Moon?
Is the far side of the Moon always dark?
What is earthshine?
Week reviewTrue or false?
The Moon glows with reflected sunlight.?
The Sun always lights half of the Moon.?
Earth's shadow causes the monthly phases.?
The full Moon rises around sunset.?
One cycle of phases takes about 29.5 days.?
WHY THIS EXERCISEThese five ideas explain the cyclic pattern of lunar phases with one simple model.
OUR WEEK, IN ORDER
  • ?We learned why we always see the same face of the Moon.
  • ?We made the phases with a lamp, our head as Earth and a ball as the Moon.
  • ?We learned the Moon glows with reflected sunlight and is always half lit.
  • ?We put the eight phases in order from new to waning crescent.
  • ?We read NASA's table of when each phase rises and sets.
WHY THIS EXERCISEThe week moved from a puzzling shape to a model that predicts every shape and rising time.
About how many days does the Moon take to orbit Earth once? Type the number.
WHY THIS EXERCISETwo different numbers, 27.3 and 29.5, come from the Moon moving while Earth also moves.
Try it
Walk around a chair while always facing it. Count how many times you turn all the way around. It is once per lap.
Now walk around it without turning, always facing the same wall. Notice the chair sees every side of you.
Draw Earth and the Moon with the Moon at four points on its orbit. Mark the same face of the Moon toward Earth each time.

Strong week, Moon watcher. Tomorrow is Sky Night: the family starts the Moon log together.

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