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Earth and Space 6-8 / Week 05 / Sky Night
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Week 05 · Gravity Holds the System

Sky Night

Sky Night: The falling Moon
// One pull keeps the whole solar system together
⏱ about 20 min

Sky Night: The Falling Moon

The crew gathers on the porch step at dusk with a grown-up, warm jackets zipped. The Moon is a bright half circle.

"It is falling right now," Rocket tells everyone. "Falling around us. It just keeps missing."

Raven opens the Moon log. "What do you notice compared to last week's sketch? The lit side has grown."

A younger cousin asks why the Moon does not float away. Rocket grabs a ball to explain.

Nova hovers low, her light pointed at the ground, never at anyone's eyes. "Show them the two arrows," she says.

Rocket draws a sideways arrow and a downward arrow in the dirt. "Sideways motion plus a pull toward Earth equals an orbit."

  1. The solar system is the Sun and a collection of objects held in orbit by the Sun's gravitational pull.
  2. Anything with mass has gravity. More mass means more pull. The pull weakens with distance but never reaches zero.
  3. The Moon falls around Earth, not to it. Sideways motion plus a pull toward the center makes an orbit.
  4. A marble in a bowl models an orbit well, but friction makes it slow and fall in. Space has almost nothing to rub against.
  5. A NASA table shows that mass and distance together set surface gravity. Your weight changes from world to world; your mass does not.
  6. The Sun is one of hundreds of billions of stars whose combined gravity holds the Milky Way together.
◇ SKY NIGHT: MOON CHECK AND THE ORBIT GAME
With a grown-up, step outside to your usual Sky Deck spot after dark. Find the Moon if the sky is clear.
Sketch its shape in the Moon log with the date. Compare it with your sketches from the last three weeks.
Then play the orbit game. One person stands still as Earth. Another walks in a circle holding a ball on a string.
Let the string go slack for a moment. Which way does the ball want to go? Explain to your family why the Moon does not do that.
Cloudy night backup: play the orbit game indoors with a soft ball. Then roll a marble in the bowl for the family to watch.
For a grown-up
Please go outside together. Use a flashlight for the path and point it at the ground, never at anyone's eyes.
Stay in a known spot in the yard, on the porch or at a window. No one crosses a road or climbs for a better view.
If you swing a ball on a string, do it in an open space with a soft ball and plenty of room.
Safety first
Night observation only with a grown-up, from the yard, a porch or a window.
Swing a soft ball on a string only in open space, away from faces and windows. Marbles stay away from younger children.
SKY NIGHT CHECK
  • Read the question.
  • Tap your answer.
When the string goes slack, which way does the ball want to go?
What plays the part of the string for the real Moon?
Draw the Moon as you saw it tonight, or as you expect it from your log. Add the two arrows: sideways motion and the pull toward Earth.

Well done, crew. Next week you spin a disk of dust and gas and watch a solar system form.

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