Create a free account now and get 3 full lessons, completely free. Get my 3 free lessons

Worldwide

Notes from the field ·

Why Do We Have Day and Night? A Ball-and-Flashlight Model

By Ahmed, founder

“Why is it dark at night?” is among the first science questions children ask, and it has one of the clearest classroom models: a ball, a flashlight and a sticker.

The model

You need: a ball (a foam ball or globe), a flashlight, a small sticker or a dot of clay, and a dark room.

  1. Stick a small “person” on the ball. Mark your own town if you’re using a globe.
  2. Shine the flashlight on the ball from about 50 cm away. The flashlight is the Sun.
  3. Turn the ball slowly anti-clockwise (looking down from above the North Pole), the direction Earth spins.
  4. Watch the sticker as it moves into the light (sunrise), faces the light (noon), moves out of the light (sunset) and faces away (midnight).

What the model shows

  • Half the ball is always lit and half is dark, just as half of Earth always faces the Sun.
  • Day and night happen because Earth spins. One full turn takes about 24 hours.
  • The Sun doesn’t go around us. We rotate past it.

Time zones

Put two stickers on the ball: one at your home and one on the other side. When yours is in the light, the other is dark. At the same moment, it’s breakfast time in one place and bedtime in another, which is why we have time zones. When it’s morning in Calgary, it’s already evening in Beijing.

Why does the Sun seem to move?

Because we’re on the spinning ball. When you ride a carousel, the world seems to move past you. Children often find it easier with a cue: ask them to hold a ball still while they turn around, and notice what moves.

Common mix-ups

  • “The Sun goes around Earth.” It looks that way. Earth turns.
  • “Night happens when the Sun goes behind the Moon or a cloud.” Night is Earth turning away.
  • “Earth’s shadow causes night.” Night is the part of Earth facing away from the Sun, not a shadow cast by something else.

Extension

  • Predict how time zones would look on a globe.
  • Why are days different lengths at different times of year? That’s about tilt. See Why are summer days longer?

The full lesson is Why do we have day and night?, the first in Sun, Earth and Moon. Sign up for three lessons free.