Rocket sticks an orange on a pencil and holds it at arm's length. "I am Earth. This is the Moon. The lamp is the Sun."
Raven turns the room lights off. The lamp glows alone on the table. "What do you notice on the orange?"
"Half lit, half dark," Rocket says. "Always."
"Now turn slowly," Nova says, hovering by the ceiling. "Keep the orange above your shoulder so your head does not block the light."
Rocket turns. The lit part of the orange grows from a sliver to a half to a full circle, then shrinks again.
"The phases!" he shouts, and spins too fast. The orange wobbles off the pencil.
Nova hums. "Slow down. Science waits for the orange."
OpenStax Astronomy describes this exact experiment. Stand about 6 feet from a bright light in a dark room and hold a small round object.
Your head represents Earth, the light represents the Sun and the ball the Moon. Move the ball around your head.
Make sure you do not cause an eclipse by blocking the light with your head. You will see phases just like the Moon's on the ball.
| Where you face | Phase name | Lit part you see | Sketch it |
|---|---|---|---|
| Toward the lamp | |||
| Lamp on your right | |||
| Back to the lamp | |||
| Lamp on your left |
The lamp never changed. The ball was always half lit. Only your angle changed, and the shape you saw changed with it.
That is the whole explanation of the phases. The Moon orbits Earth, so our viewing angle shifts a little every day.
The model also shows the common mistake. Your head never cast a shadow on the ball unless you blocked the light on purpose.
What the model gets wrong: the real Moon is about 30 Earth widths away, far farther than an arm's length.
| What the model shows | True or false? |
|---|---|
| The lamp moved to make the phases. | ? |
| The ball was half lit at every position. | ? |
| Your head's shadow made the crescent shapes. | ? |
| Full Moon happened when your back was to the lamp. | ? |
A model that explains the shapes. Tomorrow you read NASA's table of when each phase rises and sets.